timeline unit interactions of - mrs. erin schumacher ...€”combine in different patterns to form...

T I M E L I N E

U N I T Interactions ofMatter

Unit 5348

5n this unit you willstudy the inter-

actions through whichmatter can change itsidentity. You will learnhow atoms bond withone another to formcompounds and howdifferent types ofbonds account fordifferences in com-pounds. You will alsolearn how atoms joinin different combina-tions to form newsubstances throughchemical reactions.Finally, you will learnabout the propertiesof several categoriesof compounds. Thistimeline includessome of the eventsleading to the currentunderstanding ofthese interactions of matter.

I1828

Urea, a compound foundin urine, is produced in alaboratory. Until this time,

chemists had believedthat compounds createdby living organisms could

not be produced in the laboratory.

1858German chemistFriedrich August

Kekulé suggests thatcarbon forms four

chemical bonds andcan form longchains of car-bon bonded

to itself.

1964Dr. Martin Luther King, Jr.,

American civil rights leader, isawarded the Nobel Peace Prize.

1969The Nimbus III weather satellite

is launched by the UnitedStates, representing the first

civilian use of nuclear batteries.

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1979Public fear about nuclear

power grows after an accidentoccurs at the Three Mile Island

nuclear power station, inPennsylvania.

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Copyright © by Holt, Rinehart and Winston. All rights reserved.

1898The United States defeats Spain in

the Spanish-American War.

Marie Curie, Pierre Curie, and Henri Becquerel are

awarded the Nobel Prize inphysics for the discovery

of radioactivity.

1996Evidence of organic compounds in

a meteorite leads scientists tospeculate that life may haveexisted on Mars more than

3.6 billion years ago. 2001The first total

solar eclipse ofthe millenium

occurs onJune 21.

1867Swedish chemist Alfred Nobeldevelops dynamite. Dynamite’sexplosive power is a result ofthe decomposition reaction

of nitroglycerin.! ! ! ! ! ! ! ! ! !

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!1942

The first nuclear chain reaction iscarried out in a squash court

under the football stadium at theUniversity of Chicago.

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1903

349Interactions of MatterCopyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 14350

14CH

AP

TE

R

In 1987, pilots Richard Rutan and JeanaYeager performed a record-breaking feat.They flew the Voyager aircraft, shown above,around the world without refueling. The triplasted just over 9 days. In order to carryenough fuel for the trip, the plane had tobe as lightweight as possible. The designersknew that using fewer bolts than usual toattach parts would make the airplane lighter.But without the bolts, what would hold theparts together? The designers decided toreplace the bolts with glue!

Not just any glue would do. They usedsuperglue. When superglue is applied, itcombines with water from the air to formchemical bonds. A chemical bond is a forceof attraction that holds atoms together. Theparticles of superglue squeeze into the ma-terials being glued. The materials sticktogether as if they were one material.Superglue is so strong that the weight of atwo-ton elephant cannot separate two metalplates glued together with just a few drops!

Along with hundreds of household uses,superglue also has many uses in industryand medicine. For example, to make shoesstronger and lighter, manufacturers canreplace some of the stitching with super-glue. To repair a cracked tooth, dentists canapply superglue to hold the tooth together.

Chemical Bonding

Strange but True!

Superglue was discovered by a scientist in the early 1950s who wastrying to develop a new plastic for thecockpit bubble of a jet plane.


Chemical Bonding 351

From Glue to GoopParticles of glue can bond to other particles andhold objects together. Different types of bondscreate differences in the properties of substances.In this activity, you will see how the formation ofbonds causes an interesting change in the prop-erties of a very common material—white glue.

Procedure1. Fill a small paper cup 1/4 full of white glue.

Observe the properties of the glue, and recordyour observations in your ScienceLog.

Chemical bonding is responsible for theways all materials behave—the properties ofmaterials. In this chapter, you will learnabout the different types of bonds that holdatoms together and how those bonds affectthe properties of the materials.

2. Fill a second small paper cup 1/4 full of boraxsolution.

3. Pour the borax solution into the cup containingthe white glue, and stir well using a plasticspoon.

4. When it becomes too thick to stir, remove thematerial from the cup and knead it with yourfingers. Observe the properties of the material,and record your observations in your ScienceLog.

Analysis5. Compare the properties of the glue with those

of the new material.

6. The properties of the new materialresulted from the bonds between theborax and the particles in the glue. Iftoo little borax were used, in what way

would the properties of the materialhave been different?

In your ScienceLog, try to answer thefollowing questions based on what youalready know:

1. What is a chemical bond?

2. How are ionic bonds different fromcovalent bonds?

3. How are the properties of metalsrelated to the type of bonds in them?


Chapter 14352

N E W T E R M Schemical bondingchemical bondtheoryvalence electrons

O B J E CT I V E S! Describe chemical bonding.! Identify the number of valence

electrons in an atom.! Predict whether an atom is likely

to form bonds.

Section1 Electrons and

Chemical BondingHave you ever stopped toconsider that by usingjust the 26 letters of thealphabet, you make allof the words you useevery day? Even thoughthe number of letters islimited, their ability to be combinedin different ways allows you to makean enormous number of words.

Now look around the room.Everything around you—desks, chalk,paper, even your friends—is made ofatoms of elements. How can so manysubstances be formed from about 100 elements? In the sameway that words can be formed by combining letters, differ-ent substances can be formed by combining atoms.

Atoms Combine Through Chemical BondingThe atoms of just three elements—carbon, hydrogen, andoxygen—combine in different patterns to form the sub-stances sugar, alcohol, and citric acid. Chemical bonding isthe joining of atoms to form new substances. The prop-erties of these new substances are different from those ofthe original elements. A force of attraction that holds twoatoms together is called a chemical bond. As you will see,chemical bonds involve the electrons in the atoms.

Atoms and the chemical bonds that connect them cannotbe observed with your eyes. During the past 150 years, scien-tists have performed many experiments that have led to thedevelopment of a theory of chemical bonding. Remember thata theory is a unifying explanation for a broad range of hypoth-eses and observations that have been supported by testing.The use of models helps people to discuss the theory of howand why atoms form chemical bonds.

Electron Number and OrganizationTo understand how atoms form chemical bonds, you first needto know how many electrons are in a particular atom and howthe electrons in an atom are organized.

In your ScienceLog, writedown the term chemicalbonding. Then write down asmany different words as youcan that are formed from theletters in these two words.

across the sciencesC O N N E C T I O N

Why are the amino acids that are chemically bondedtogether to form your proteinsall left-handed? Read aboutone cosmic explanation onpage 370.


The number of electrons in an atom can be determinedfrom the atomic number of the element. Remember that theatomic number is the number of protons in an atom. Becauseatoms have no charge, the atomic number also represents thenumber of electrons in the atom. Equal numbers of positivelycharged protons and negatively charged electrons are neededto make the overall charge of the atom zero.

The electrons are organized in levels inan atom. These levels are usually calledenergy levels. The levels farther fromthe nucleus contain electrons thathave more energy than levels closerto the nucleus. The arrangementof electrons within the energylevels of a chlorine atom is shown in Figure 1.

Outer-Level Electrons Are the Key to Bonding As you justsaw in Figure 1, a chlorine atom has a total of 17 electrons.When a chlorine atom bonds to another atom, not all of theseelectrons are used to create the bond. Most atoms form bondsusing only the electrons in their outermost energy level. Theelectrons in the outermost energy level of an atom are calledvalence (VAY luhns) electrons. Thus, a chlorine atom has 7valence electrons. You can see the valence electrons for atomsof some other elements in Figure 2.


Figure 1 Electron Arrangementin an Atom

Figure 2 Valence electrons are the electrons in theoutermost energy level of an atom.

a

b

OxygenElectron total: 8First level: 2 electronsSecond level: 6 electrons

SodiumElectron total: 11First level: 2 electronsSecond level: 8 electronsThird level: 1 electron

Electrons will enter the secondenergy level only after the firstlevel is full. The second energylevel can hold up to 8 electrons.

The second energylevel is the outermostlevel, so an oxygenatom has 6 valenceelectrons.

The third energy levelis the outermost level,so a sodium atom has1 valence electron.

c

The first energy level isclosest to the nucleus andcan hold up to 2 electrons.

The third energy level in thismodel of a chlorine atomcontains only 7 electrons,for a total of 17 electrons inthe atom. This outer level ofthe atom is not full.


Valence Electrons and the Periodic Table You can deter-mine the number of valence electrons in Figure 2 because youhave a model to look at. But what if you didn’t have a model?You have a tool that helps you determine the number of valenceelectrons for some elements—the periodic table!

Remember that elements in a group often have similarproperties, including the number of electrons in the outer-most energy level of their atoms. The number of valence elec-trons for many elements is related to the group number, asshown in Figure 3.

To Bond or Not to BondAtoms do not all bond in the same manner. In fact, someatoms rarely bond at all! The number of electrons in the outer-most energy level of an atom determines whether an atomwill form bonds.

Atoms of the noble, or inert, gases (Group 18) do not nor-mally form chemical bonds. As you just learned, atoms ofGroup 18 elements (except helium) have 8 valence electrons.Therefore, having 8 valence electrons must be a special con-dition. In fact, atoms that have 8 electrons in their outermostenergy level do not normally form new bonds. The outermostenergy level of an atom is considered to be full if it contains8 electrons.

Chapter 14354

Determine the number ofvalence electrons in each ofthe following atoms: lithium(Li), beryllium (Be), aluminum(Al), carbon (C), nitrogen (N),sulfur (S), bromine (Br), andkrypton (Kr).

Figure 3 Determining the Number of Valence Electrons

Atoms of elements in Groups 13–18have 10 fewer valence electrons thantheir group number. However, heliumatoms have only 2 valence electrons.

Atoms of elements in Groups 1and 2 have the same numberof valence electrons as theirgroup number.

Atoms of elements in Groups 3–12 donot have a general rule relating theirvalence electrons to their group number.


Atoms Bond to Have a Filled Outermost Level An atomthat has fewer than 8 valence electrons is more reactive, or morelikely to form bonds, than an atom with 8 valence electrons.Atoms bond by gaining, losing, or sharing electrons in order tohave a filled outermost energy level with 8 valence electrons.Figure 4 describes the ways in which atoms can achieve a filledoutermost energy level.

A Full Set—with Two? Not all atoms need 8 valence elec-trons for a filled outermost energy level. Helium atoms needonly two valence electrons. With only two electrons in theentire atom, the first energy level (which is also the outermostenergy level) is full. Atoms of the elements hydrogen andlithium form bonds with other atoms in order to have thesame number of electrons as helium atoms have.


Figure 4 These atomsachieve a full set of valenceelectrons in different ways.

1. What is a chemical bond?

2. What are valence electrons?

3. How many valence electrons does a silicon atom have?

4. Predict how atoms with 5 valence electrons will achievea full set of valence electrons.

5. Interpreting Graphics At right is a diagram of a fluorineatom. Will fluorine form bonds? Explain.

REVIEW

SulfurAn atom of sulfur has 6 valence electrons. It canhave 8 valence electrons by sharing 2 electronswith or gaining 2 electrons from other atoms tofill its outermost energy level.

MagnesiumAn atom of magnesium has 2 valence electrons. It can have a full outer level by losing 2 electrons.The second energy level becomes the outermostenergy level and contains a full set of 8 electrons.

Fluorine


Chapter 14356

N E W T E R M Sionic bond covalent bondions moleculecrystal lattice metallic bond

O B J E CT I V E S! Describe ionic, covalent, and

metallic bonding.! Describe the properties

associated with substancescontaining each type of bond.

Section2 Types of Chemical Bonds

As you have learned, atoms bond by gaining, losing, or shar-ing electrons. Once bonded, most atoms have a filled outer-most energy level containing eight valence electrons. Atomsare less reactive when they have a filled outermost energylevel. The way in which atoms interact through their valenceelectrons determines the type of bond that forms. The threetypes of bonds are ionic (ie AHN ik), covalent (KOH VAY luhnt),and metallic. In this section, you will study each type of bond,starting with ionic bonds.

Ionic BondsSeashells, table salt, and plaster of Paris, shown in Figure 5,have much in common. They are all hard, brittle solids at room temperature, they all have high melting points,and they all contain ionic bonds. An ionic bond is theforce of attraction between oppositely charged ions. Ions

are charged particles that form duringchemical changes when one or more

valence electrons transfer from oneatom to another.

Remember that in an atom, thenumber of electrons equals the

number of protons, so the negativecharges and positive charges cancel each

other. Therefore, atoms are neutral. Atransfer of electrons between atomschanges the number of electrons in eachatom, while the number of protons staysthe same. The negative charges and posi-tive charges no longer cancel out, and theatoms become ions. Although an atomcannot gain (or lose) electrons withoutanother atom nearby to lose (or gain) elec-trons, it is often easier to study the for-mation of ions one at a time.

Atoms That Lose Electrons Form Positive Ions Ionic bondsform during chemical reactions when atoms that have astronger attraction for electrons pull electrons away from otheratoms. The atoms that lose electrons form ions that have fewerelectrons than protons. The positive charges outnumber thenegative charges in the ions. Thus, the ions that are formedwhen atoms lose electrons have an overall positive charge.

Figure 5 Calcium carbonate inseashells, sodium chloride intable salt, and calcium sulfateused to make plaster of Paris casts all contain ionic bonds.


Atoms of most metals have few electrons in their outerenergy level. When metal atoms bond with other atoms, themetal atoms tend to lose these valence electrons and form posi-tive ions. For example, look at the model in Figure 6. An atomof sodium has one valence electron. When a sodium atomloses this electron to another atom, it becomes a sodium ion.A sodium ion has a charge of 1+ because it contains 1 moreproton than electrons. To show the difference between a sodiumatom and a sodium ion, the chemical symbol for the ion iswritten as Na+. Notice that the charge is written to the upperright of the chemical symbol. Figure 6 also shows a model forthe formation of an aluminum ion.

The Energy of Losing Electrons When an atom loses elec-trons, energy is needed to overcome the attraction betweenthe electrons and the protons in the atom’s nucleus. Removingelectrons from atoms of metals requires only a small amountof energy, so metal atoms lose electrons easily. In fact, theenergy needed to remove electrons from atoms of elements inGroups 1 and 2 is so low that these elements react very eas-ily and can be found only as ions in nature. On the otherhand, removing electrons from atoms of nonmetals requires alarge amount of energy. Rather than give up electrons, theseatoms gain electrons when they form ionic bonds.


Figure 6 Forming Positive Ions

Sodium atom (Na)11+ protons11– electrons0 charge

Sodium ion (Na+)11+ protons10– electrons1+ charge

Aluminum atom (Al)13+ protons13– electrons0 charge

Aluminum ion (Al3+)13+ protons10– electrons3+ charge

Self-CheckLook at the periodictable, and determinewhich noble gas has thesame electron arrange-ment as a sodium ion.(See page 596 to checkyour answer.)

Here’s How It Works: During chemical reactions, a sodium atom can lose its 1 electron in the thirdenergy level to another atom. The filled secondlevel becomes the outermost level, so the resultingsodium ion has 8 valence electrons.

Here’s How It Works: During chemical reactions,an aluminum atom can lose its 3 electrons in thethird energy level to another atom. The filled sec-ond level becomes the outermost level, so theresulting aluminum ion has 8 valence electrons.


Atoms That Gain Electrons Form Negative Ions Atoms thatgain electrons from other atoms during chemical reactions formions that have more electrons than protons. The negative chargesoutnumber the positive charges, giving each of these ions anoverall negative charge.

The outermost energy level of nonmetal atoms is almostfull. Only a few electrons are needed to fill the outer level, soatoms of nonmetals tend to gain electrons from other atoms.For example, look at the model in Figure 7. An atom of chlo-rine has 7 valence electrons. When a chlorine atom gains 1electron to complete its outer level, it becomes an ion with a1– charge called a chloride ion. The symbol for the chloride ionis Cl–. Notice that the name of the negative ion formed fromchlorine has the ending -ide. This ending is used for the namesof the negative ions formed when atoms gain electrons. Figure7 also shows a model of how an oxide ion is formed.

The Energy of Gaining Electrons Atoms of most nonmetalsfill their outermost energy level by gaining electrons. Energyis given off by most nonmetal atoms during this process. Themore easily an atom gains an electron, the more energy anatom gives off. Atoms of the Group 17 elements (the halogens)give off the most energy when they gain an electron. The halo-gens, such as fluorine and chlorine, are extremely reactive non-metals because they release a large amount of energy.

Chapter 14358

Figure 7 Forming Negative Ions

Charge!Calculating the charge of anion is the same as addingintegers (positive or negativewhole numbers or zero) withopposite signs. You write thenumber of protons as a posi-tive integer and the number of electrons as a negative inte-ger and then add the integers.Calculate the charge of an ionthat contains 16 protons and18 electrons. Write the ion’ssymbol and name.

MATH BREAK

Oxygen atom (O)8+ protons8– electrons0 charge

Oxide ion (O2–)8+ protons

10– electrons2– charge

Chlorine atom (Cl)17+ protons17– electrons0 charge

Chloride ion (Cl–)17+ protons18– electrons1– charge

Here’s How It Works: During chemical reactions, a chlorine atom gains 1 electron in the third en-ergy level from another atom. A chloride ion isformed with 8 valence electrons. Thus, its outer-most energy level is filled.

Here’s How It Works: During chemical reactions,an oxygen atom gains 2 electrons in the secondenergy level from another atom. An oxide ion isformed with 8 valence electrons. Thus, its outer-most energy level is filled.


Ions Bond to Form a Crystal Lattice When a metal reactswith a nonmetal, the same number of electrons is lost by themetal atoms as is gained by the nonmetal atoms. Even thoughthe ions that bond are charged, the compound formed is neu-tral. The charges of the positive ions and the negative ionscancel each other through ionic bonding. An ionic bond is anexample of a special kind of attraction, called electrostatic attrac-tion, that causes opposite electrical charges to stick together.Another example is static cling, as shown in Figure 8.

The ions that make up an ionic compound are bonded ina repeating three-dimensional pattern called a crystal lattice(KRI stuhl LAT is). In ionic compounds, such as table salt, theions in the crystal lattice are arranged as alternating positiveand negative ions, forming a solid. Each ion is bordered onevery side by an ion with the opposite charge. The model inFigure 9 shows a small part of a crystal lattice. The arrange-ment of bonded ions in a crystal lattice determines the shapeof the crystals of an ionic compound.

The strong force of attraction between bonded ions in acrystal lattice gives ionic compounds certain properties, includ-ing a high melting point and boiling point. Ionic compoundstend to be brittle solids at room temperature. Ionic compoundsusually break apart when hit with a hammer because as theions move, ions with like charges line up and repel one another.


Figure 8 Like ionic bonds, staticcling is the result of the attrac-tion between opposite charges.

1. How does an atom become a negative ion?

2. What are two properties of ionic compounds?

3. Applying Concepts Which group of elements lose 2valence electrons when their atoms form ionic bonds?What charge would the ions formed have?

REVIEW

Figure 9 This model of the crystal lattice of sodium chloride,

or table salt, shows a three-dimensional view

of the bonded ions.


Covalent BondsMost materials you encounter every day, such as water,sugar, and carbon dioxide, are held together by bondsthat do not involve ions. These substances tend to havelow melting and boiling points, and some of these sub-stances are brittle in the solid state. The type of bondsfound in these substances, including the substances shownin Figure 10, are covalent bonds.

A covalent bond is the force of attraction between thenuclei of atoms and the electrons shared by the atoms.Based on experiments and observations, the current theo-ry about covalent bonding is that it occurs between atomsthat require a large amount of energy to remove an elec-tron. When two atoms of nonmetals bond, too muchenergy is required for either atom to lose an electron, sono ions are formed. Rather than transferring electronsto complete their outermost energy levels, two nonmetalatoms bond by sharing electrons with one another, asshown in the model in Figure 11.

Covalently Bonded Atoms Make Up Molecules The parti-cles of substances containing covalent bonds differ from thosecontaining ionic bonds. Ionic compounds consist of ions organ-ized in a crystal. Covalent compounds consist of individualparticles called molecules (MAHL i KYOOLZ). A molecule is aneutral group of atoms held together by covalent bonds. Eachmolecule is separate from other molecules of the substance. InFigure 11, you saw a model of a hydrogen molecule, which iscomposed of two hydrogen atoms covalently bonded. However,most molecules are composed of atoms of two or more el-ements. The models in Figure 12 show two ways to representthe covalent bonds in a molecule.

Chapter 14360

Figure 10 Covalentbonds join the atomsthat make up this plastic ball, the rubbercovering on the paddle,the cotton fibers inclothes, and even manyof the substances thatmake up the humanbody!

Figure 11 By sharing electrons ina covalent bond, each hydrogenatom (the smallest atom known)has a full outermost energy levelcontaining two electrons.

Make models of molecules outof marshmallows on page 574

of the LabBook.

The shared electrons spend most of their timebetween the nuclei of the atoms.

The protons and the shared electrons attractone another. This attraction is the basis of thecovalent bond that holds the atoms together.


Making Electron-Dot DiagramsAn electron-dot diagram is a model that shows onlythe valence electrons in an atom. Electron-dot dia-grams are helpful when predicting how atomsmight bond. You draw an electron-dot diagram bywriting the symbol of the element and placing the

correct number of dots around it. This type of modelcan help you to better understand bonding by show-ing the number of valence electrons and how atomsshare electrons to fill their outermost energy levels,as shown below.


Figure 12 Covalent Bonds in a Water Molecule

Self-Check1. How many dots does the electron-dot diagram of

a sulfur atom have?

2. How is a covalent bond different from an ionicbond? (See page 596 to check your answers.)

Carbon atoms have 4valence electrons, so 4dots are placed aroundthe symbol. A carbonatom needs 4 moreelectrons for a filledoutermost energy level.

Oxygen atoms have 6valence electrons, so 6dots are placed aroundthe symbol. An oxygenatom needs only 2 moreelectrons for a filled out-ermost energy level.

The noble gas kryptonhas a full set of 8valence electrons in itsatoms. Thus, krypton isnonreactive because itsatoms do not need anymore electrons.

This electron-dot dia-gram represents hydro-gen gas, the samesubstance shown in themodel in Figure 11.

C•••

•

•••• ••O

••

•••• ••Kr ••H H

••••

••••O HH

Another way to showcovalent bonds is to draw an electron-dot diagram.An electron-dot diagramshows only the outermostlevel of electrons for eachatom. But you can still seehow electrons are sharedbetween the atoms.

Each hydrogen atom sharesits 1 electron with the oxy-gen atom. This allows eachhydrogen to have a filledouter level with 2 electrons.

Through covalent bonding,the oxygen atom sharesone of its electrons witheach of the two hydrogenatoms. As a result, it has a filled outermost energylevel with 8 electrons.


A Molecule Is the Smallest Particle of a CovalentCompound An atom is the smallest particle intowhich an element can be divided and still be thesame substance. Likewise, a molecule is the small-est particle into which a covalently bonded com-

pound can be divided and still be thesame compound. Figure 13 illustrates

how a sample of water is made upof many individual molecules

of water (shown as three-dimensional models). If youcould divide water over andover, you would eventuallyend up with a single mol-ecule of water. However, ifyou separated the hydrogenand oxygen atoms that makeup a water molecule, youwould no longer have water.

The Simplest Molecules All molecules are composed of atleast two covalently bonded atoms. The simplest molecules,known as diatomic molecules, consist of two atoms bondedtogether. Some elements are called diatomic elements becausethey are found in nature as diatomic molecules composed oftwo atoms of the element. Hydrogen is a diatomic element,as you saw in Figure 11. Oxygen, nitrogen, and the halogensfluorine, chlorine, bromine, and iodine are also diatomic. Bysharing electrons, both atoms of a diatomic molecule can filltheir outer energy level, as shown in Figure 14.

Chapter 14362

Try your hand at drawingelectron-dot diagrams for a molecule of chlorine (adiatomic molecule) and amolecule of ammonia (onenitrogen atom bonded withthree hydrogen atoms).

Figure 14 Models of a Diatomic Fluorine Molecule

This is a three-dimensionalmodel of a fluorine molecule.

Figure 13 The water in this fishbowl is made up of many tiny water molecules.Each molecule is the smallestparticle that still has thechemical properties of water.

Two covalently bonded fluorine atoms have filledoutermost energy levels.The pair of electronsshared by the atoms arecounted as valence elec-trons for each atom.


More-Complex Molecules Diatomic molecules are the sim-plest—and some of the most important—of all molecules. Youcould not live without diatomic oxygen molecules. But otherimportant molecules are much more complex. Gasoline, plas-tic, and even proteins in the cells of your body are examplesof complex molecules. Carbon atoms are the basis of many ofthese complex molecules. Each carbon atom needs to make 4covalent bonds to have 8 valence electrons. These bonds canbe with atoms of other elements or with other carbon atoms,as shown in the model in Figure 15.

Metallic BondsLook at the unusual metal sculp-ture shown in Figure 16. Notice thatsome metal pieces have been flat-tened, while other metal pieceshave been shaped into wires. Howcould the artist change the shapeof the metal into all of these dif-ferent forms without breaking themetal into pieces? A metal can beshaped because of the presence ofa special type of bond called ametallic bond. A metallic bond isthe force of attraction between a positively charged metal ion and the electrons in a metal.(Remember that metal atoms tendto lose electrons and form posi-tively charged ions.)

life scienceC O N N E C T I O N

Proteins perform many func-tions throughout your body,such as digesting your food,building components of yourcells, and transporting nutri-ents to each cell. A single pro-tein can have a chain of 7,000atoms of carbon and nitrogenwith atoms of other elementscovalently bonded to it.

363Chemical Bonding

Figure 16 The different shapes of metalin this sculpture are possible because ofthe bonds that hold the metal together.

Figure 15 A granola bar contains the covalent compound sucrose,or table sugar. A molecule of sucrose is composed of carbon atoms (green spheres), hydrogen atoms (blue spheres), and oxygen atoms (red spheres) joined by covalent bonds.


Chapter 14364

Gold can be pounded out tomake a foil only a few atomsthick. A piece of gold thesize of the head of a pin canbe beaten into a thin “leaf”that would cover this page!

Bending with Bonds1. Straighten out a wire

paper clip. Recordthe result in yourScienceLog.

2. Bend a piece of chalk.Record the result in yourScienceLog.

3. Chalk is composed of cal-cium carbonate, a com-pound containing ionicbonds. What type of bondsare present in the paperclip?

4. In your ScienceLog, explainwhy you could change theshape of the paper clip butcould not bend the chalkwithout breaking it.

Electrons Move Throughout a Metal Our scientific under-standing of the bonding in metals is that the metal atoms getso close to one another that their outermost energy levels over-lap. This allows their valence electrons to move throughout themetal from the energy level of one atom to the energy levelsof the atoms nearby. The atoms form a crystal much like theions associated with ionic bonding. However, the negativecharges (electrons) in the metal are free to move about. You canthink of a metal as being made up of positive metal ions withenough valence electrons “swimming” about to keep the ionstogether and to cancel the charge of the ions, as shown inFigure 17. The ions are held together because metallic bondsextend throughout the metal in all directions.

Explaining Metallic Properties You encounter metallic prop-erties every day, such as when you turn on a lamp or wrapleftovers in aluminum foil. The abilities to conduct electricalenergy and to be flattened and shaped without breaking aretwo properties of metals that result from metallic bonding.

When you turn on a lamp, electrons move within the wire.These moving electrons are the valence electrons in the metal.Because these electrons are not connected to any one atom inthe wire, they can move freely within the wire.

Metals have a fairly high density because the metal atomsare closely packed. But because the atoms can be rearranged,metals can be shaped into useful forms. The properties of ductility (the ability to be drawn into wires) and malleability(the ability to be hammered into sheets) describe a metal’sability to be reshaped. For example, copper is made into wiresfor use in electrical cords. Aluminum can be pounded intothin sheets and made into aluminum foil and cans.

Figure 17 The moving electrons are attracted to the metal ions, forming metallic bonds.

The positive metal ions are infixed positions in the metal.

Negative electrons arefree to move about.


•••C•• •H •

HH

When the shape of a piece of metal is changed, the metalions shift position in the crystal. You might expect the metalto break apart as the ions push away from one another.However, even in their new positions, the positive ions aresurrounded by and attracted to the electrons, as shown inFigure 18. On the other hand, ionic compounds do break apartwhen hit with a hammer because neither the positive ions northe negative ions are free to move.

Figure 18 The shape of a metal can bechanged without breaking because metallicbonds occur in many directions.

The moving electrons maintainthe metallic bonds no matter howthe shape of the metal changes.

The repulsion between the positivelycharged metal ions increases as theions are pushed closer to one another.


1. What happens to electrons in covalent bonding?

2. What type of element is most likely to form covalent bonds?

3. What is a metallic bond?

4. What is one difference between a metallic bond and acovalent bond?

5. Interpreting Graphics The electron-dot diagram at rightis not yet complete. Which atom needs to form anothercovalent bond? How do you know?

REVIEW

A lthough they are not very glamorous, metal staplesare very useful in holding things such as sheets of

paper together. Explain how the metallic bonds in a stapleallow it to change shape so that it can function properly.


Chapter Highlights

Chapter 14366

SECTION 1 SECTION 2

Vocabularychemical bonding (p. 352)chemical bond (p. 352)theory (p. 352)valence electrons (p. 353)

Section Notes• Chemical bonding is the

joining of atoms to form newsubstances. A chemical bondis a force of attraction thatholds two atoms together.

• Valence electrons are theelectrons in the outermostenergy level of an atom.These electrons are used toform bonds.

• Most atoms form bonds bygaining, losing, or sharingelectrons until they have 8valence electrons. Atoms ofhydrogen, lithium, and he-lium need only 2 electrons tofill their outermost level.

Vocabularyionic bond (p. 356)ions (p. 356)crystal lattice (p. 359)covalent bond (p. 360)molecule (p. 360)metallic bond (p. 363)

Section Notes• In ionic bonding, electrons

are transferred between twoatoms. The atom that loseselectrons becomes a positiveion. The atom that gainselectrons becomes a negativeion. The force of attractionbetween these oppositelycharged ions is an ionicbond.

• Ionic bonding usually occursbetween atoms of metals andatoms of nonmetals.

Skills CheckVisual UnderstandingDETERMINING VALENCE ELECTRONSKnowing the number of valence electrons in anatom is important in predicting how it will bondwith other atoms. Review Figure 3 onpage 354 to learn how an element’slocation on the periodic tablehelps you determine the numberof valence electrons in an atom.

FORMING IONS Turn back toFigures 6 and 7 on pages 357–358to review how ions are formedwhen atoms lose or gain electrons.

Math ConceptsCALCULATING CHARGE To calculate the charge of an ion, you must add integers withopposite signs. The total positive charge of theion (the number of protons) is written as apositive integer. The total negative charge ofthe ion (the number of electrons) is written as a negative integer. For example, the charge ofan ion containing 11 protons and 10 electronswould be calculated as follows:

(11!) ! (10") # 1!


367Chemical Bonding

SECTION 2

• Energy is needed to removeelectrons from metal atomsto form positive ions. Energyis released when most non-metal atoms gain electrons to form negative ions.

• In covalent bonding, elec-trons are shared by twoatoms. The force of attractionbetween the nuclei of theatoms and the shared elec-trons is a covalent bond.

• Covalent bonding usuallyoccurs between atoms ofnonmetals.

• Electron-dot diagrams are asimple way to represent thevalence electrons in an atom.

• Covalently bonded atomsform a particle called a mol-ecule. A molecule is thesmallest particle of a com-pound with the chemicalproperties of the compound.

• Diatomic elements are theonly elements found innature as diatomic moleculesconsisting of two atoms ofthe same element covalentlybonded together.

• In metallic bonding, theoutermost energy levels ofmetal atoms overlap, allow-ing the valence electrons tomove throughout the metal.The force of attractionbetween a positive metal ion and the electrons in themetal is a metallic bond.

• Many properties of metals,such as conductivity, ductil-ity, and malleability, resultfrom the freely moving elec-trons in the metal.

LabsCovalent Marshmallows (p. 574)

Visit the National Science Teachers Association on-line Website for Internet resources related to this chapter. Just type inthe sciLINKS number for more information about the topic:

TOPIC: The Electron sciLINKS NUMBER: HSTP305TOPIC: The Periodic Table sciLINKS NUMBER: HSTP310TOPIC: Types of Chemical Bonds sciLINKS NUMBER: HSTP315TOPIC: Properties of Metals sciLINKS NUMBER: HSTP320

Visit the HRW Web site for a variety oflearning tools related to this chapter. Just type in the keyword:

KEYWORD: HSTBND

GO TO: go.hrw.com GO TO: www.scilinks.org


Chapter ReviewUSING VOCABULARY

To complete the following sentences, choosethe correct term from each pair of terms listedbelow.

1. The force of attraction that holds twoatoms together is a ____. (crystal lattice orchemical bond)

2. Charged particles that form when atomstransfer electrons are ____. (molecules or ions)

3. The force of attraction between the nucleiof atoms and shared electrons is a(n) ____.(ionic bond or covalent bond)

4. Electrons free to move throughout amaterial are associated with a(n) ____.(ionic bond or metallic bond)

5. Shared electrons are associated with a____. (covalent bond or metallic bond)

UNDERSTANDING CONCEPTS

Multiple Choice

6. Which element has a full outermostenergy level containing only twoelectrons?a. oxygen (O) c. fluorine (F)b. hydrogen (H) d.helium (He)

7. Which of the following describes whathappens when an atom becomes an ionwith a 2– charge?a. The atom gains 2 protons.b. The atom loses 2

protons.c. The atom gains

2 electrons.d.The atom loses

2 electrons.

Chapter 14368

8. The propertiesof ductility andmalleability areassociated with which type of bonds?a. ionicb. covalentc. metallicd.none of the above

9. In which area of the periodic table do youfind elements whose atoms easily gainelectrons?a. across the top two rowsb. across the bottom rowc. on the right sided.on the left side

10. What type of element tends to lose elec-trons when it forms bonds?a. metalb. metalloidc. nonmetald.noble gas

11. Which pair of atoms can form an ionicbond?a. sodium (Na) and potassium (K)b. potassium (K) and fluorine (F)c. fluorine (F) and chlorine (Cl) d. sodium (Na) and neon (Ne)

Short Answer

12. List two properties of covalent compounds.

13. Explain why an iron ion is attracted to asulfide ion but not to a zinc ion.

14. Using your knowledge of valence elec-trons, explain the main reason so manydifferent molecules are made from carbonatoms.

15. Compare the three types of bonds basedon what happens to the valence electronsof the atoms.


Concept Mapping

16. Use the followingterms to create aconcept map:chemical bonds, ionic bonds, covalentbonds, metallic bonds,molecule, ions.

CRITICAL THINKING AND PROBLEM SOLVING

17. Predict the type of bond each of the fol-lowing pairs of atoms would form:a. zinc (Zn) and zinc (Zn)b. oxygen (O) and nitrogen (N)c. phosphorus (P) and oxygen (O)d.magnesium (Mg) and chlorine (Cl)

18. Draw electron-dot diagrams for each ofthe following atoms, and state how manybonds it will have to make to fill its outerenergy level.a. sulfur (S)b. nitrogen (N)c. neon (Ne)d. iodine (I)e. silicon (Si)

19. Does the substance being hit in the photobelow contain ionic or metallic bonds?Explain.

MATH IN SCIENCE

20. For each atom below, write the number ofelectrons it must gain or lose to have 8valence electrons. Then calculate thecharge of the ion that would form.a. calcium (Ca) c. bromine (Br)b. phosphorus (P) d. sulfur (S)

INTERPRETING GRAPHICS

Look at the picture of the wooden pencilbelow, and answer the following questions.

21. In which part of the pencil are metallicbonds found?

22. List three materials composed of mol-ecules with covalent bonds.

23. Identify two differences between theproperties of the metallically bondedmaterial and one of the covalentlybonded materials.


Take a minute to review your answersto the ScienceLog questions on page351. Have your answers changed? Ifnecessary, revise your answers based onwhat you have learned since you beganthis chapter.


P H Y S I C A L S C I E N C E • E A R T H S C I E N C E

Left-Handed Molecules

370

Some researchers think that light from a newlyforming star 1,500 light-years away (1 light-year is equal to about 9.6 trillion kilometers)may hold the answer to an Earthly riddle thathas been puzzling scientists for over 100 years!

We Are All Lefties!In 1848, Louis Pasteur discovered that carbon-containing molecules come in left-handed andright-handed forms. Each of the molecules isan exact mirror image of the other, just aseach of your hands is a mirror image of theother. These molecules are made of the sameelements, but they differ in the elements’arrangement in space.

Shortly after Pasteur’s discovery,researchers stumbled across an interesting but unexplained phenomenon—all organisms,including humans, are made almost entirely of left-handed molecules! Chemists werepuzzled by this observation because whenthey made amino acids in the laboratory, theamino acids came out in equal numbers ofright- and left-handed forms. Scientists also

found that organisms cannot even use theright-handed form of the amino acids to make proteins! For years, scientists have triedto explain this. Why are biological moleculesusually left-handed and not right-handed?

Cosmic ExplanationAstronomers recently discovered that a newlyforming star in the constellation Orion emits aunique type of infrared light. Infrared light hasa wavelength longer than the wavelenth of visi-ble light. The wave particles of this light spiralthrough space like a corkscrew. This light spi-rals in only one direction. Researchers suspectthat this light might give clues to why allorganisms are lefties.

Laboratory experiments show that depend-ing on the direction of the ultraviolet lightspirals, either left-handed or right-handedmolecules are destroyed. Scientists wonder if a similar type of light may have been presentwhen life was beginning on Earth. Such lightmay have destroyed most right-handed mol-ecules, which explains why life’s molecules are left-handed.

Skeptics argue that the infrared light hasless energy than the ultraviolet light used in the laboratory experiments and thus is not avalid comparison. Some researchers, however,hypothesize that both infrared and ultravioletlight may be emitted from the newly formingstar that is 1,500 light-years away.

Find Out More! The French chemist Pasteur discovered left-handed and right-handed molecules in tartaricacid. Do some research to find out more aboutPasteur and his discovery. Share your findingswith the class.

" Molecules, such as the carbon moleculesshown above, often come in two mirror-image forms, just as hands do.


371

Here’s Looking At Ya’!

To some people, just the thought of puttingsmall pieces of plastic in their eyes isuncomfortable. But for millions of others,

those little pieces of plastic, known as contactlenses, are a part of daily life. So what would youthink about putting a piece of glass in your eyeinstead? Strangely enough, the humble beginningof the contact lens began with doing just that—inserting a glass lens right in the eye! Ouch!

Molded GlassEarly developers of contactlenses had only glass touse until plastics were dis-covered. In 1929, aHungarian physiciannamed Joseph Dalloscame up with a way tomake a mold of thehuman eye. This was acritical step in the devel-opment of contact lenses.He used these molds tomake a glass lens that fol-lowed the shape of theeye rather than laying flatagainst it. In combinationwith the eye’s natural lens,light was refocused toimprove a person’s eye-sight. As you can probablyguess, glass lenses weren’tvery comfortable.

Still Too HardSeven years later, an American optometrist,William Feinbloom, introduced contact lensesmade of hard plastic. Plastic was a newly devel-oped material made from long, stable chains ofcarbon, hydrogen, and oxygen molecules calledpolymers. But polymers required a lot of work to make. Chemists heated short chains, forcing

them to chemically bond to form a longer, more-stable polymer. The whole process was alsoexpensive. To make matters worse, the hard-plastic lenses made from polymers weren’t muchmore comfortable than the glass lenses.

How About Spinning Plastic Gel?In an effort to solve the comfort problem,Czech chemists Otto Wichterle and DrahoslavLim invented a water-absorbing plastic gel. Thelenses made from this gel were soft and pli-

able, and they allowed airto pass through the lensto the eye. These charac-teristics made the lensesmuch more comfortableto wear than the glasslenses.

Wichterle solved thecost problem by develop-ing a simple and inexpen-sive process to make theplastic-gel lenses. In thisprocess, called spin cast-ing, liquid plastic is addedto a spinning mold of aneye. When the plasticforms the correct shape, itis treated with ultravioletand infrared light, whichhardens the plastic. Bothplastic gel and spin cast-ing were patented in

1963, becoming the foundation for the contactlenses people wear today.

Look Toward the Future! What do you think contact lenses might belike in 20 years? Let your imagination run wild.Sometimes the strangest ideas are the bestseeds of new inventions!

" Does the thought of puttingsomething in your eye makeyou squirm?


Imagine . . .

Chapter 15372

15CH

AP

TE

R

A car slams into a wall at 97 km/h(60 mph). Although both occu-pants are wearing seat belts, onesuffers a crushing blow to thehead as he strikes the dashboard.The other occupant suffers onlyminor bruises thanks to the pres-ence of an air bag. Fortunately, noone was really injured because thiswas just a crash test using dummies.The results of this test could lead to thedesign of better air bags.

The key to an air bag’s success during acrash is the speed at which it inflates. Insidethe bag is a gas generator that contains thecompounds sodium azide, potassium nitrate,and silicon dioxide. At the moment of a crash,an electronic sensor in the vehicle detectsthe sudden decrease in speed. The sensorsends a small electric current to the gas gen-erator, providing the activation energy, or theenergy needed to start the reaction, to thechemicals in the gas generator.

The rate, or speed, at which the reactionoccurs is very fast. In 1/25 of a second—less than the blink of an eye—the gas formedin the reaction inflates the bag. The air bagfills upward and outward. By filling the spacebetween a person and the car’s dashboard,the air bag protects him or her from injury.

ChemicalReactions

Designers of air bags must understand alot about chemical reactions. In this chap-ter, you will learn about the different typesof chemical reactions. You will learn the cluesthat will help you identify when achemical reaction is taking place. Youwill also learn about the factorsthat affect the rateof a reaction.

The reaction betweenvinegar and bakingsoda produces a gas.However, the reactionis too slow for use inan air bag.


Reaction Ready The reactions that occur in an air bag produce thegas that fills the bag. In fact, the production of agas is often a sign that a chemical reaction is takingplace. In this activity, you will observe a reaction and identify signs that indicate that a reaction istaking place.

Procedure1. In a large, sealable plastic bag, place one plas-

tic spoonful of baking soda and two spoonfulsof calcium chloride.

2. Fill a plastic film canister two-thirds full withwater.

3. Carefully place the canister in the bag withoutspilling the water. Squeeze the air out of thebag, and seal it tightly.

4. Tip the canister over, and mix the contents of the bag.

Analysis5. Observe the contents of the bag. Record

your observations in your ScienceLog.

6. What evidence did you see that a chemical reaction was taking place?


1. What clues can help you identify achemical reaction?

2. What are some types of chemicalreactions?

3. How can you change the rate of achemical reaction?

Chemical Reactions 373Copyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 15374

Forming New Substances Each fall, an amazing transformation takes place. Leaves changecolor, as shown in Figure 1. Vibrant reds, oranges, and yellowsthat had been hidden by green all year are seen as the tem-

peratures get cooler and the hours of sunlightbecome fewer. What is happening to cause this

change? Leaves have a green color as a result ofa compound called chlorophyll (KLOR uh FIL).

Each fall, the chlorophyll undergoes a chemicalchange and forms simpler substances that haveno color. The green color of the chlorophyllno longer hides them, so the red, orange, andyellow colors in the leaves can be seen.

Chemical Reactions The chemical change that occurs as chlorophyll breaks downinto simpler substances is one example of a chemical reaction.A chemical reaction is the process by which one or more sub-stances undergo change to produce one or more different sub-stances. These new substances have different chemical andphysical properties from the original substances. Many of thechanges you are familiar with are chemical reactions, includ-ing the ones shown in Figure 2.

N E W T E R M Schemical reactionchemical formulasubscriptchemical equationreactantsproductscoefficientlaw of conservation of mass

O B J E CT I V E S! Identify the clues that indicate

a chemical reaction might betaking place.

! Interpret and write simplechemical formulas.

! Interpret and write simplebalanced chemical equations.

! Explain how a balancedequation illustrates the law ofconservation of mass.

Section1

Figure 1 The change of color in thefall is a result of chemical changesin the leaves.

Figure 2 Examples of Chemical Reactions

The substances that make up baking powderundergo a chemical reaction when mixed with water.One new substance that forms is carbon dioxide gas,which causes the bubbles in this muffin.

Once ignited, gasoline reacts with oxygen gas inthe air. The new substances that form, carbondioxide and water, push against the pistons inthe engine to keep the car moving.


Clues to Chemical Reactions How can you tell when achemical reaction is taking place? There are several clues thatindicate when a reaction might be occurring. The more ofthese clues you observe, the more likely it is that the changeis a chemical reaction. Several of these clues are described below.

Chemical Reactions 375

Gas FormationThe formation of gas bubbles is a clue that a chemicalreaction might be taking place. For example, bubbles ofcarbon dioxide are produced when hydrochloric acid isplaced on a piece of limestone. Hydrogen gas is producedwhen a metal reacts with an acid.

Color ChangeChlorine bleach is great for removingthe color from stains on white clothes.But don’t spill it on your jeans. Thebleach reacts with the blue dye on thefabric, causing the color of the materialto change.

Some Clues to Chemical Reactions

Energy ChangeEnergy is released during some chemicalreactions. A fire heats a room and provideslight. Electrical energy is released whenchemicals in a battery react. During someother chemical reactions, energy is absorbed.Chemicals on photographic film react whenthey absorb energy from light shining on the film.

Solid FormationSometimes a solid formswhen two solutions react. A solid formed in a solutionas a result of a chemicalreaction is called a precipitate(pruh SIP uh TAYT). Here yousee potassium chromate solu-tion being added to a silvernitrate solution. The dark redsolid is a precipitate of silverchromate.


Breaking and Making Bonds New substances are formedin a chemical reaction because chemical bonds in the startingsubstances break, atoms rearrange, and new bonds form tomake the new substances. Look at the model in Figure 3 tounderstand how this process occurs.

Chemical FormulasRemember that a chemical symbol is a shorthand method ofidentifying an element. A chemical formula is a shorthandnotation for a compound or a diatomic element using chemi-cal symbols and numbers. A chemical formula indicates thechemical makeup by showing how many of each kind of atomis present in a molecule.

The chemical formula for water, H2O, tells you that a watermolecule is composed of two atoms of hydrogen and one atomof oxygen. The small number 2 in the formula is a subscript.A subscript is a number written below and to the right of achemical symbol in a formula. When no subscript is writtenafter a symbol, as with the oxygen in water’s formula, onlyone atom of that element is present. Figure 4 shows two morechemical formulas and what they mean.

Chapter 15376

Figure 3Reaction of Hydrogen and Chlorine

Breaking Bonds The elements hydrogen andchlorine are diatomic, meaning they are composedof molecules that consist of two atoms bondedtogether. For these molecules to react, the bondsjoining the atoms must break.

Making Bonds Molecules of the new substance,hydrogen chloride, are formed as new bonds aremade between hydrogen atoms and chlorineatoms.

Figure 4 A chemical formula shows the number of atoms of each element present.

Oxygen is a diatomic element.Each molecule of oxygen gas is composed of two atoms ofoxygen bonded together.

Counting AtomsSome chemical formulascontain two or more chem-ical symbols enclosed byparentheses. When countingatoms in these formulas,multiply everything inside theparentheses by the subscriptas though they were part of amathematical equation. Forexample, Ca(NO3)2 contains:

1 calcium atom 2 nitrogen atoms (2 ! 1)6 oxygen atoms (2 ! 3)

Now It’s Your TurnDetermine the number ofatoms of each element in theformulas Mg(OH)2 andAl2(SO4)3.

MATH BREAK

O2Every molecule of glucose (thesugar formed by plants duringphotosynthesis) is composedof six atoms of carbon, twelveatoms of hydrogen, and sixatoms of oxygen.

C6H12O6


Writing Formulas for Covalent Compounds You can oftenwrite a chemical formula if you know the name of the sub-stance. Remember that covalent compounds are usuallycomposed of two nonmetals. The names of covalent com-pounds use prefixes to tell you how many atoms of eachelement are in the formula. A prefix is a syllable or sylla-bles joined to the beginning of a word. Each prefix used ina chemical name represents a number, as shown in the tableat right. Figure 5 demonstrates how to write a chemical for-mula from the name of a covalent compound.

Writing Formulas for Ionic Compounds If the name of acompound contains the name of a metal and a nonmetal, thecompound is probably ionic. To write the formula for an ioniccompound, you must make sure the compound’s overall chargeis zero. In other words, the formula must have subscripts thatcause the charges of the ions to cancel out. (Remember thatthe charge of many ions can be determined by looking at theperiodic table.) Figure 6 demonstrates how to write a chemi-cal formula from the name of an ionic compound.


Figure 5 The formulas of these covalent compounds can bewritten using the prefixes in their names.

Carbon dioxide Dinitrogen monoxide

Figure 6 The formula of an ionic compound is written byusing enough of each ion to make the overall charge zero.

One sodium ion and onechloride ion have an overallcharge of (1") " (1#) = 0

One magnesium ion and twochloride ions have an overall charge of (2") " 2(1#) = 0

Self-CheckHow many atoms ofeach element make upNa2SO4? (See page 596to check your answer.)

Determine whether each ofthe following compounds iscovalent or ionic, and writethe chemical formula foreach: sulfur trioxide, calciumfluoride, phosphorus penta-chloride, dinitrogen trioxide,and lithium oxide.

Prefixes Used in Chemical Names

mono- 1 hexa- 6

di- 2 hepta- 7

tri- 3 octa- 8

tetra- 4 nona- 9

penta- 5 deca- 10

The lack of a prefixindicates 1carbon atom.

CO2

The prefix di- indicates2 oxygenatoms.

The prefix di- indicates2 nitrogenatoms.

The prefixmono-indicates 1oxygen atom.

N2O

Sodium chloride Magnesium chloride

A sodiumion has a 1" charge.

NaCl

A chlorideion has a 1# charge.

A magnesiumion has a 2" charge.

A chlorideion has a 1# charge.

MgCl2


Chemical EquationsA composer writing a piece of music, like the one in Figure 7,must communicate to the musician what notes to play, how long to play each note, and in what style each noteshould be played. The composer does not use words todescribe what must happen. Instead, he or she uses musi-cal symbols to communicate in a way that can be easilyunderstood by anyone in the world who can read music.

Similarly, people who work with chemical reactions needto communicate information about reactions clearly to otherpeople throughout the world. Describing reactions usinglong descriptive sentences would require translations intoother languages. Chemists have developed a method ofdescribing reactions that is short and easily understood byanyone in the world who understands chemical formulas.A chemical equation is a shorthand description of a chem-ical reaction using chemical formulas and symbols. Becauseeach element’s chemical symbol is understood around theworld, a chemical equation needs no translation.

Reactants Yield Products Consider theexample of carbon reacting with oxygento yield carbon dioxide, as shown inFigure 8. The starting materials in a chemi-cal reaction are reactants (ree AKT UHNTS).The substances formed from a reaction areproducts. In this example, carbon and oxy-gen are reactants, and carbon dioxide isthe product formed. The parts of thechemical equation for this reaction aredescribed in Figure 9.

Figure 8 Charcoal is usedto cook food on a barbecue.When carbon in charcoalreacts with oxygen in theair, the primary product iscarbon dioxide, as shown in the chemical equation in Figure 9.

Chapter 15378

Figure 9 The Parts of a Chemical Equation

Figure 7 The symbols on thismusic are understood around theworld—just like chemical symbols!

A plus sign separates the for-mulas of two or more reac-tants or products from oneanother.

C+O2 CO2

The arrow, also called theyields sign, separates the for-mulas of the reactants fromthe formulas of the products.

The formulas of the reactantsare written before the arrow.

The formulas of the productsare written after the arrow.

!


The symbol or formula for each substance in the reactionmust be written correctly. For a compound, determine if it isa covalent compound or an ionic compound, and write theappropriate formula. For an element, use the proper chemicalsymbol, and be sure to use a subscript of 2 for the diatomicelements. (The seven diatomic elements are hydrogen, nitro-gen, oxygen, fluorine, chlorine, bromine, and iodine.) An equa-tion with an incorrect chemical symbol or formula will notaccurately describe the reaction. In fact, even a simple mis-take in a symbol or formula can make a huge difference, asshown in Figure 10.

An Equation Must Be Balanced In a chemical reaction,every atom in the reactants becomes part of the products.Atoms are never lost or gained in a chemical reaction. Whenwriting a chemical equation, you must show that the numberof atoms of each element in the reactants equals the numberof atoms of those elements in the products by writing abalanced equation.


Self-CheckWhen calcium bromide reacts with chlorine,bromine and calcium chloride are produced. Writean equation to describe this reaction. Identify eachsubstance as either a reactant or a product. (See page 596 to check your answers.)

Hydrogen gas, H2, is animportant fuel that may help reduce air pollution.Because water is the onlyproduct formed as hydro-gen burns, there is little airpollution from vehicles thatuse hydrogen as fuel.

The chemical symbol for theelement cobalt is Co. Cobalt is ahard, bluish gray metal.

The chemical formula for thecompound carbon monoxide isCO. Carbon monoxide is a col-orless, odorless, poisonous gas.

The chemical formula for thecompound carbon dioxide is CO2.Carbon dioxide is a colorless,odorless gas that you exhale.

Figure 10 The symbols and formulasshown here are similar, but confusingthem while writing an equation wouldcause you to indicate the wrongsubstance.


H2 + O2 H2O

Reactants Products

Writing a balanced equation requires the use of coefficients(KOH uh FISH uhnts). A coefficient is a number placed in frontof a chemical symbol or formula. When counting atoms, youmultiply a coefficient by the subscript of each of the elementsin the formula that follows it. Thus, 2CO2 represents 2 car-bon dioxide molecules containing a total of 2 carbon atomsand 4 oxygen atoms. Coefficients are used when balancingequations because the subscripts in the formulas cannot bechanged. Changing a subscript changes the formula so that itno longer represents the correct substance. Figure 11 showshow to use coefficients to balance an equation. After you learnhow to use coefficients, you can practice balancing chemicalequations by doing the MathBreak at left.

Chapter 15380

Figure 11 Follow these steps to write abalanced equation for H2 + O2 ! H2O.

Count the atoms ofeach element in thereactants and in theproducts. You cansee that there arefewer oxygen atomsin the products thanin the reactants.

1

Balancing ActWhen balancing a chemicalequation, you must placecoefficients in front of anentire chemical formula,never in the middle of a for-mula. Notice where the coef-ficients are in the balancedequation below:

F2 " 2KCl ! 2KF " Cl2Now It’s Your Turn

Write balanced equations forthe following:

HCl " Na2S ! H2S " NaClAl " Cl2 ! AlCl3

MATH BREAK

Become a better balancer ofchemical equations on page

576 of the LabBook.

!

H = 2 O = 2H = 2O = 1

!

H2 + O2 2H2O

Reactants Products

To balance the oxy-gen atoms, place thecoefficient 2 in frontof water’s formula.This gives you 2 oxy-gen atoms in boththe reactants and the products. Butnow there are toofew hydrogen atomsin the reactants.

2!

H = 2 O = 2H = 4O = 2

!

2H2 + O2 2H2O

Reactants

To balance thehydrogen atoms,place the coefficient2 in front of hydro-gen’s formula. Butjust to be sure youranswer is correct,always double-check your work!

3!

H = 4 O = 2

!

Products

H = 4O = 2


Mass Is Conserved—It’s a Law! The practice of balancingequations is a result of the work of a French chemist, AntoineLavoisier (luh vwa ZYAY). In the 1700s, Lavoisier performedexperiments in which he carefully measured and comparedthe masses of the substances involved in chemical reactions.He determined that the total mass of the reactants equaledthe total mass of the products. Lavoisier’s work led to the law of conservation of mass, which states that mass is neithercreated nor destroyed in ordinary chemical and physicalchanges. Thus, a chemical equation must show the same num-ber and kind of atom on both sides of the arrow. The law ofconservation of mass is demonstrated in Figure 12. You canexplore this law for yourself in the QuickLab at right.


1. List four clues that a chemical reaction is occurring.

2. How many atoms of each element make up 2Na3PO4?

3. Write the chemical formulas for carbon tetrachloride andcalcium bromide.

4. Explain how a balanced chemical equation illustrates thatmass is never lost or gained in a chemical reaction.

5. Applying Concepts Write the balanced chemical equa-tion for methane, CH4, reacting with oxygen gas to pro-duce water and carbon dioxide.

Figure 12 In this demonstration, magnesium in the flash-bulb of a camera reacts with oxygen. Notice that the massis the same before and after the reaction takes place.

REVIEW

Mass Conservation1. Place about 5 g

(1 tsp) of bakingsoda into a seal-able plastic bag.

2. Place about 5 mL (1 tsp) of vinegarinto a plastic filmcanister, and closethe lid.

3. Use a balance to deter-mine the masses of thebag with baking soda andthe canister with vinegar,and record both values inyour ScienceLog.

4. Place the canister into theplastic bag. Squeeze the airout of the bag, and tightlyseal the bag.

5. Carefully open the lid ofthe canister while it is inthe bag. Pour the vinegaronto the baking soda, andmix them.

6. When the reaction hasstopped, use the samebalance used in step 3 todetermine the total massof the bag and its contents.

7. Compare the mass of thematerials before and afterthe reaction.


Figure 13 The synthesis reactionthat occurs when magnesiumreacts with oxygen in the airforms the compound magnesiumoxide.

Chapter 15382

N E W T E R M Ssynthesis reactiondecomposition reactionsingle-replacement reactiondouble-replacement reaction

O B J E CT I V E S! Describe four types of chemical

reactions.! Classify a chemical equation as

one of the four types of chemi-cal reactions described here.

Section2 Types of Chemical Reactions

Imagine having to learn 50 chemical reactions. Sound tough?Well, there are thousands of known chemical reactions. Itwould be impossible to remember them all. But there is help!Remember that the elements are divided into categories basedon their properties. In a similar way, reactions can be classi-fied according to their similarities.

Many reactions can be grouped into one of four categories:synthesis (SIN thuh sis), decomposition, single replacement,and double replacement. By dividing reactions into these cat-egories, you can better understand the patterns of how reac-tants become products. As you learn about each type ofreaction, study the models provided to help you recognizeeach type of reaction.

Synthesis ReactionsA synthesis reaction is a reaction in which two ormore substances combine to form a single compound.For example, the synthesis reaction in which thecompound magnesium oxide is produced is seen inFigure 13. (This is the same reaction that occurs in the flashbulb in Figure 12.) One way to rememberwhat happens in each type of reaction is to imaginepeople at a dance. A synthesis reaction would bemodeled by two people joining to form a dancingcouple, as shown in Figure 14.

Figure 14 A model for the synthesisreaction of sodium reacting with chlorineto form sodium chloride is shown below.

2Na + Cl2 2NaCl

!+

!


Decomposition ReactionsA decomposition reaction is a reaction in which a singlecompound breaks down to form two or more simpler sub-stances. The decomposition of water is shown in Figure 15.Decomposition is the reverse of synthesis. The dance modelwould represent a decomposition reaction as a dancing cou-ple splitting up, as shown in Figure 16.

Single-Replacement ReactionsA single-replacement reaction is a reaction in which an el-ement takes the place of another element that is part of a com-pound. The products of single-replacement reactions are a newcompound and a different element. The dance model for single-replacement reactions is a person who cuts in on a coupledancing. A new couple is formed and a different person is leftalone, as shown in Figure 17.


Figure 15 Water can bedecomposed into theelements hydrogen andoxygen through electrolysis.

Figure 16 A model for the decomposition reaction of carbonicacid to form water and carbon dioxide is shown below.

Figure 17 A model for a single-replacement reaction of zincreacting with hydrochloric acid toform zinc chloride and hydrogenis shown below.

! +

! ++

Zn + 2HCl ZnCl2 + H2!

H2CO3 H2O + CO2!


Remember that some elements are more reactive than others.In a single-replacement reaction, a more-reactive element canreplace a less-reactive one from a compound. However, theopposite reaction does not occur, as shown in Figure 18.

Double-Replacement ReactionsA double-replacement reaction is a reaction in which ions intwo compounds switch places. One of the products of thisreaction is often a gas or a precipitate. A double-replacementreaction in the dance model would be two couples dancingand switching partners, as shown in Figure 19.

Figure 18 More-reactive elementsreplace less-reactive elements insingle-replacement reactions.

Figure 19 A model for thedouble-replacement reaction ofsodium chloride reacting with sil-ver nitrate to form sodium nitrateand the precipitate silver chlorideis shown below.

Chapter 15384

1. What type of reaction does each of the following equa-tions represent?a. FeS ! 2HCl FeCl2 ! H2Sb. NH4OH NH3 ! H2O

2. Which type of reaction always has an element and a com-pound as reactants?

3. Comparing Concepts Compare synthesis and decom-position reactions.

REVIEW

+ +

!

!

!

Cu + 2AgNO3 2Ag + Cu(NO3)2Copper is more reactive than silverand replaces it.

!

Ag + Cu(NO3)2 No reactionSilver is less reactive than copperand cannot replace it.

!

NaCl + AgNO3 NaNO3 + AgCl!



N E W T E R M Sexothermicendothermiclaw of conservation of energyactivation energycatalystinhibitor

O B J E CT I V E S! Compare exothermic and

endothermic reactions.! Explain activation energy.! Interpret an energy diagram.! Describe the factors that affect

the rate of a reaction.

Section3 Energy and Rates of

Chemical ReactionsYou just learned one method of classifying chemical reactions.In this section, you will learn how to classify reactions in termsof the energy associated with the reaction and learn how tochange the rate at which the reaction occurs.

Every Reaction Involves EnergyAll chemical reactions involve chemical energy. Remember thatduring a reaction, chemical bonds in the reactants break asthey absorb energy. As new bonds form in the products, energyis released. Energy is released or absorbed in the overall reac-tion depending on how the chemical energy of the reactantscompares with the chemical energy of the products.

Energy Is Released in Exothermic Reactions If the chemi-cal energy of the reactants is greater than the chemical energyof the products, the difference in energy is released during thereaction. A chemical reaction in which energy is released orremoved is called exothermic. Exo means “go out” or “exit,”and thermic means “heat” or “energy.” The energy can bereleased in several different forms, as shown in Figure 20. Theenergy released in an exothermic reaction is often written asa product in a chemical equation, as in this equation:

2Na ! Cl2 2NaCl ! energy

Figure 20 Types of Energy Released in Reactions

Energy in the form of light isreleased in the exothermicreaction taking place in thesenecklaces and light sticks.

Electrical energy is released inthe exothermic reaction takingplace in the dry cells in thisflashlight.

Energy that keeps you warmand lights your way is releasedin the exothermic reactiontaking place in a campfire.

!


Energy Is Absorbed in Endothermic Reactions If the chemi-cal energy of the reactants is less than the chemical energy ofthe products, the difference in energy is absorbed during thereaction. A chemical reaction in which energy is absorbed iscalled endothermic. Endo means “go in,” and thermic means“heat” or “energy.” The energy absorbed in an endothermicreaction is often written as a reactant in a chemical equation,as in this equation:

2H2O ! energy 2H2 ! O2

Energy Is Conserved—It’s a Law! You learned that mass isnever created or destroyed in chemical reactions. The sameholds true for energy. The law of conservation of energy statesthat energy can be neither created nor destroyed. The energyreleased in exothermic reactions was originally stored in thereactants. And the energy absorbed in endothermic reactionsdoes not just vanish. It is stored in the products that form. Ifyou could carefully measure all the energy in a reaction, youwould find that the total amount of energy (of all types) is thesame before and after the reaction.

Activation Energy Gets a Reaction Started A match canbe used to light a campfire—but only if the match is lit! Astrike-anywhere match, like the one shown in Figure 21, hasall the reactants it needs to be able to burn. And thoughthe chemicals on a match are intended to react and burn,

they will not ignite by themselves. Energy is neededto start the reaction. The minimum amount of

energy needed for substances to react is called activation energy.

The friction of striking a match heats the substances onthe match, breaking bonds in the reactants and allowing thenew bonds in the products to form. Chemical reactions requiresome energy to get started. An electric spark in a car’s engineprovides activation energy to begin the burning of gasoline.Light can also provide the activation energy for a reaction.You can better understand activation energy and the differ-ences between exothermic reactions and endothermic reac-tions by studying the diagrams in Figure 22.

386


Photosynthesis is an endother-mic process in which lightenergy from the sun is usedto produce glucose, a simplesugar. The equation thatdescribes photosynthesis is as follows:6CO2 ! 6H2O ! energy C6H12O6 ! 6O2

The cells in your body useglucose to get the energy theyneed through cellular respira-tion, an exothermic processdescribed by the reverse ofthe above reaction:C6H12O6 ! 6O2 6CO2 !

6H2O ! energy

Chapter 15

Matches rubbing together in a box could provide theactivation energy to light a strike-anywhere match.Safety matches, which mustbe struck on a strike plateon the box, were developedto prevent such accidents.

!

!

!

Figure 21 Rubbing the tip of this strike-anywherematch on a rough surface provides the energyneeded to get the chemicals to react.


Factors Affecting Rates of ReactionsYou can think of a reaction as occurring only if the particlesof reactants collide when they have enough energy to breakthe appropriate bonds. The rate of a reaction is a measure ofhow rapidly the reaction takes place. Four factors that affectthe rate of a reaction are temperature, concentration, surfacearea, and the presence of a catalyst or inhibitor.


Figure 22 Energy Diagrams

Ener

gy

Reaction progress

Reactants

Energy given off

Activation energy

Products

"

"

Ener

gy

Reactants

Energy absorbed

Activation energy

Products

Reaction progress

"

"

Fighting fires with slime? Readmore about it on page 394.

Exothermic Reaction Once begun, anexothermic reaction can continue to occur,as in a fire. The energy released as theproduct forms continues to supply the acti-vation energy needed for the substancesto react.

Endothermic Reaction An endothermicreaction requires a continuous supply ofenergy. Energy must be absorbed to pro-vide the activation energy needed for thesubstances to react.

ydrogen peroxide isHused as a disinfectantfor minor scrapes and cutsbecause it decomposes toproduce oxygen gas andwater, which help cleanse the wound. The decomposi-tion of hydrogen peroxide is

an exothermic reaction.Explain why hydrogenperoxide must be stored in a dark bottle to main-tain its freshness. (HINT:What type of energy would be blocked by this type of container?)


Temperature An increase in temperature increases the rateof a reaction. At higher temperatures, particles of reactantsmove faster, so they collide with each other more frequentlyand with more energy. More particles therefore have the acti-vation energy needed to react and can change into productsfaster. Thus, more particles react in a shorter time. You cansee this effect in Figure 23 and by doing the QuickLab at left.

Concentration Generally, increasing the concentration of reac-tants increases the rate of a reaction, as shown in Figure 24.Concentration is a measure of the amount of one substance dis-solved in another. Increasing the concentration increases thenumber of reactant particles present and decreases the distancebetween them. The reactant particles collide more often, somore particles react each second. Increasing the concentrationis similar to having more people in a room. The more peoplethat are in the room, the more frequently they will collideand interact.

Chapter 15388

Figure 23 The light stick on the right glows brighter than the one on the left because the higher temperaturecauses the rate of the reaction to increase.

Figure 24 The reaction on the right produces bubblesof hydrogen gas at a faster rate because theconcentration of hydrochloric acid used is higher.

Which Is Quicker?1. Fill a clear plastic

cup half-full withwarm water. Fill asecond clear plasticcup half-full withcold water.

2. Place one-quarter of aneffervescent tablet ineach of the two cups ofwater at the same time.

3. Observe the reaction, andrecord your observations inyour ScienceLog.

4. In which cup did the reac-tion occur at a greaterrate? What evidence sup-ports your answer?

Do you feel as though you arenot up to speed on controlling

the rate of a reaction? Thenhurry over to page 580 of

the LabBook.


Surface Area Increasing the surface area, or the amount ofexposed surface, of solid reactants increases the rate of a reac-tion. Grinding a solid into a powder exposes more particles ofthe reactant to other reactant particles. The number of colli-sions between reactant particles increases, increasing the rateof the reaction. You can see the effect of increasing the sur-face area in the QuickLab at right.

Catalysts and Inhibitors Some reactions would be too slowto be useful without a catalyst (KAT uh LIST). A catalyst is asubstance that speeds up a reaction without being permanentlychanged. A catalyst lowers the activation energy of a reaction.The lower energy needed to start the reaction allows the reac-tion to occur more rapidly. Most reactions in your body aresped up using catalysts called enzymes. Catalysts are even foundin cars, as seen in Figure 25.

An inhibitor is a substance that slows down or stops achemical reaction. Preservatives added to foods are inhibitorsthat slow down reactions in the bacteria or fungus that canspoil food. Many poisons are also inhibitors.


1. How does the rate of a reaction change when the tem-perature is decreased?

2. What is activation energy?

3. List four ways to increase the rate of a reaction.

4. Comparing Concepts Compare exothermic and endo-thermic reactions.

5. Interpreting Graphics Does the energy diagram at rightshow an exothermic or an endothermic reaction? Howcan you tell?

REVIEW

I’m Crushed!1. Fill two clear plastic

cups half-full withroom-temperaturewater.

2. Fold a sheet ofpaper around one-quarterof an effervescent tablet.Carefully crush the tablet.

3. Get another one-quarter ofan effervescent tablet.Carefully pour the crushedtablet into one cup, andplace the uncrushed tabletin the second cup.

4. Observe the reaction, andrecord your observations inyour ScienceLog.

5. In which cup did the reac-tion occur at a greaterrate? What evidence sup-ports your answer?

6. Explain why the water ineach cup must have thesame temperature.

Ener

gy

Reaction progress

"

"

Figure 25 This catalytic converter contains platinum and palladium—two catalysts used to treat automobile exhaust. They increase the rate of reactions that make the car’s exhaust less polluting.


Chapter Highlights

Chapter 15390

SECTION 1 SECTION 2

Vocabularychemical reaction (p. 374)chemical formula (p. 376)subscript (p. 376)chemical equation (p. 378)reactants (p. 378)products (p. 378)coefficient (p. 380)law of conservation

of mass (p. 381)

Section Notes• Chemical reactions form new

substances with differentproperties than the startingsubstances.

• Clues that a chemical reac-tion is taking place includeformation of a gas or solid, acolor change, and an energychange.

• A chemical formula tells thecomposition of a compoundusing chemical symbols andsubscripts. Subscripts aresmall numbers written belowand to the right of a symbolin a formula.

• Chemical formulas cansometimes be written fromthe names of covalentcompounds and ioniccompounds.

• A chemical equationdescribes a reaction usingformulas, symbols, andcoefficients.

• A balanced equation usescoefficients to illustrate thelaw of conservation of mass,that mass is neither creatednor destroyed during achemical reaction.

LabsFinding a Balance (p. 576)

Vocabularysynthesis reaction (p. 382)decomposition reaction (p. 383)single-replacement

reaction (p. 383)double-replacement

reaction (p. 384)

Section Notes• Many chemical reactions can

be classified as one of fourtypes by comparing reactantswith products.

• In synthesis reactions, thereactants form a singleproduct.

• In decomposition reactions,a single reactant breaks apartinto two or more simplerproducts.

Skills CheckMath ConceptsSUBSCRIPTS AND COEFFICIENTS A subscript isa number written below and to the right of achemical symbol when writing the chemicalformula of a compound. A coefficient is a num-ber written in front of a chemical formula in achemical equation. When you balance a chemi-cal equation, you cannot change the subscriptsin a formula; you can only add coefficients, asseen in the equation 2H2 ! O2 2H2O.

Visual UnderstandingREACTION TYPES It can be challenging to identifywhich type of reaction aparticular chemical equa-tion represents. Reviewfour reaction types bystudying Figures 14, 16, 17, and 19.

!


391Chemical Reactions

SECTION 3

• In single-replacementreactions, a more-reactiveelement takes the place of aless-reactive element in acompound. No reaction willoccur if a less-reactiveelement is placed with acompound containing amore-reactive element.

• In double-replacementreactions, ions in two com-pounds switch places. A gasor precipitate is oftenformed.

LabsPutting Elements Together (p. 578)

Vocabularyexothermic (p. 385)endothermic (p. 386)law of conservation

of energy (p. 386)activation energy (p. 386)catalyst (p. 389)inhibitor (p. 389)

Section Notes• Energy is released in exother-

mic reactions. The energyreleased can be written as aproduct in a chemicalequation.

• Energy is absorbed in endo-thermic reactions. Theenergy absorbed can bewritten as a reactant in achemical equation.

• The law of conservation ofenergy states that energy is neither created nordestroyed.

• Activation energy is theenergy needed to start achemical reaction.

• Energy diagrams indicatewhether a reaction isexothermic or endothermicby showing whether energyis given off or absorbed dur-ing the reaction.

• The rate of a chemical reac-tion is affected by tempera-ture, concentration, surfacearea, and the presence of acatalyst or inhibitor.

• Raising the temperature,increasing the concentration,increasing the surface area,and adding a catalyst canincrease the rate of areaction.

LabsCata-what? Catalyst! (p. 577)Speed Control (p. 580)


TOPIC: Chemical Reactions sciLINKS NUMBER: HSTP330TOPIC: Chemical Formulas sciLINKS NUMBER: HSTP335TOPIC: Chemical Equations sciLINKS NUMBER: HSTP340TOPIC: Exothermic and sciLINKS NUMBER: HSTP345

Endothermic Reactions


KEYWORD: HSTREA


SECTION 2



To complete the following sentences, choosethe correct term from each pair of terms listedbelow.

1. Adding a(n) ____ will slow down a chemi-cal reaction. (catalyst or inhibitor)

2. A chemical reaction that gives off light iscalled ____. (exothermic or endothermic)

3. A chemical reaction that forms one com-pound from two or more substances iscalled a ____. (synthesis reaction or decom-position reaction)

4. The 2 in the formula Ag2S is a ____.(subscript or coefficient)

5. The starting materials in a chemical reac-tion are ____. (reactants or products)


Multiple Choice

6. Balancing a chemical equation so that thesame number of atoms of each element is found in both the reactants and theproducts is an illustration ofa. activation energy.b. the law of conservation of energy.c. the law of conservation of mass.d.a double-replacement reaction.

7. What is the correct chemical formula forcalcium chloride?a. CaCl c. Ca2Clb. CaCl2 d.Ca2Cl2

8. In which type of reaction do ions in twocompounds switch places?a. synthesisb. decompositionc. single-replacementd.double-replacement

9. Which is an example of the use of activa-tion energy?a. plugging in an ironb. playing basketballc. holding a lit match to paperd.eating

10. Enzymes in your body act as catalysts.Thus, the role of enzymes is toa. increase the rate of chemical reactions.b. decrease the rate of chemical reactions.c. help you breathe.d. inhibit chemical reactions.

Short Answer

11. Classify each of the following reactions:a. Fe ! O2 Fe2O3b. Al ! CuSO4 Al2(SO4)3 ! Cuc. Ba(CN)2 ! H2SO4 BaSO4 ! HCN

12. Name two waysthat you couldincrease the rateof a chemicalreaction.

13. Acetic acid, a compound found invinegar, reacts with baking soda to produce carbon dioxide, water, andsodium acetate. Without writingan equation, identify the reactants and the products of this reaction.

Chapter 15392

!

!

!


Take a minute to review your answersto the ScienceLog questions on page373. Have your answers changed? Ifnecessary, revise your answers based on what you have learned since youbegan this chapter.

Concept Mapping

14. Use the followingterms to create aconcept map:chemical reaction,chemical equation,chemical formulas,reactants, products,coefficients, subscripts.


15. Your friend is very worried by rumors hehas heard about a substance called dihy-drogen monoxide. What could you say toyour friend to calm his fears? (Be sure towrite the formula of the substance.)

16. As long as proper safety precautions havebeen taken, why can explosives be trans-ported long distances without exploding?

MATH IN SCIENCE

17. Calculate the number of atoms of eachelement shown in each of the following:a. CaSO4b. 4NaOClc. Fe(NO3)2d.2Al2(CO3)3

18. Write balanced equations for the following:a. Fe ! O2 Fe2O3b. Al ! CuSO4 Al2(SO4)3 ! Cuc. Ba(CN)2 ! H2SO4 BaSO4 ! HCN

19. Write and balance chemical equationsfrom each of the following descriptions:a. Bromine reacts with sodium iodide to

form iodine and sodium bromide.b. Phosphorus reacts with oxygen gas to

form diphosphorus pentoxide.c. Lithium oxide decomposes to form

lithium and oxygen.


20. What evidence in the photo supports the claim that a chemical reaction istaking place?

21. Use the energy diagram below to answerthe questions that follow.

a. Which letter represents the energy ofthe products?

b. Which letter represents the activationenergy of the reaction?

c. Is energy given off or absorbed by thisreaction?

Ener

gy

Reaction progress

A

D

B

C

""

!

!

!

Chemical Reactions 393Copyright © by Holt, Rinehart and Winston. All rights reserved.

is applied, firefighters on the ground can gainvaluable time when a fire is slowed with a fireretardant. This extra time allows them to createa fire line that will ultimately stop the fire.

Neon Isn’t NecessaryOnce a fire is put out, the slimy red streaks lefton the blackened ground can be an eyesore. Tosolve the problem, scientists have created spe-cial dyes for the retardant. These dyes make thegoop neon colors when it is first applied, butafter a few days in the sun, the goop turns anatural brown shade!

What Do They Study?! Do some research to learn about a fire-fighter’s training. What classes and exams are firefighters required to pass? Howdo they maintain their certificationsonce they become firefighters?

Slime That Fire!Once a fire starts in the hard-to-reachmountains of the western United States,it is difficult to stop. Trees, grasses, andbrush can provide an overwhelming sup-ply of fuel. In order to stop a fire, fire-fighters make a fire line. This is an areawhere all the burnable materials areremoved from the ground. How wouldyou slow down a fire to give a groundcrew more time to build a fire line?Would you suggest dropping water froma plane? That is not a bad idea, but whatif you had something even better thanwater—like some slimy red goop?

Red Goop Goes the DistanceThe slimy red goop is actually a powerful fire retardant. The goop is a mixture of a pow-der and water that is loaded directly onto an old military plane. Carrying between 4,500 and11,000 L of the slime, the plane drops it all in front of the raging flames when the pilotpresses the button.

The amount of water added to the powderdepends on the location of the fire. If a fire is

burning over shrubs and grasses, morewater is needed. In this form the goop actu-

ally rains down to the ground through thetreetops. But if a fire is burning in tall trees,

less water is used so the slime will glob ontothe branches and ooze down very slowly.

Failed FlamesThe burning of trees, grass, and brush is anexothermic reaction. A fire retardant slows orstops this self-feeding reaction. A fire retardantincreases the activation energy for the materialsit is applied to. Although a lot depends on howhot the fire is when it hits the area treated withthe retardant and how much of the retardant

" This plane is dropping fire retardant on aforest fire.

394Copyright © by Holt, Rinehart and Winston. All rights reserved.

During a fire, fuel and oxygen combine in a chemical reactioncalled combustion. On the scene, Lt. Larry McKee questions

witnesses and firefighters about what they saw. He knows, forexample, that the color of the smoke can indicate certain chemicals.

McKee explains that fires usually burn “up and out, in a Vshape.“ To find where the V begins, he says, “We work from thearea with the least amount of damage to the one with the mostdamage. This normally leads us to the point of origin.“ Once theorigin has been determined, it's time to call in the dogs!

An Accelerant-Sniffing Canine“We have what we call an accelerant-sniffing canine. Our canine,Nikki, has been trained to detect approximately 11 different chemi-cals.“ When Nikki arrives on the scene, she sniffs for traces ofchemicals, called accelerants, that may have been used to start thefire. When she finds one, she immediately starts to dig at it. At thatpoint, McKee takes a sample from the area and sends it to the lab for analysis.

At the LabOnce at the laboratory, the sample is treated so that any accelerantsin it are dissolved in a liquid. A small amount of the liquid is theninjected into an instrument called a gas chromatograph. The instru-

Once a fire dies down, youmight see arson investigatorLt. Larry McKee on thescene. “After the fire is out, Ican investigate the fire sceneto determine where the firestarted and how it started. Ifit was intentionally set andI'm successful at putting thearson case together, I can geta conviction. That's verysatisfying,” says Lt. McKee.

A R S O N I N V E S T I G AT O R

ment heats the liquid, forming a mixture of gases. Thegases then are passed through a flame. As each gaspasses through the flame, it “causes a fluctuation in anelectronic signal, which creates our graphs.”

Solving the CaseIf the laboratory report indicates that a suspicious accel-erant has been found, McKee begins to search for arsonsuspects. By combining detective work with scientificevidence, fire investigators can successfully catch andconvict arsonists.

Fascinating Fire Facts ! The temperature of a house fire can reach 980°C! Atthat temperature, aluminum window frames melt, andfurniture goes up in flames. Do some research to dis-cover three more facts about fires. Create a display withtwo or more classmates to illustrate some of your facts.

" Nikki searches for traces of gasoline,kerosene, and other accelerants.


Strange but True . . .

396

ChemicalCompounds16C

HA

PT

ER

During World War II, the United States couldnot obtain natural rubber from Asian sup-pliers, who gathered it from rubber trees asshown below. Faced with a shortage of rawmaterial, American scientists searched forother materials to use in truck tires and sol-diers’ boots.

James Wright, an engineer at GeneralElectric, was working with silicone oil—aclear, gooey compound composed of siliconbonded to several other elements. By sub-stituting silicon for carbon, the main el-ement in rubber, Wright hoped to create anew compound with all the flexibility andbounce of rubber.

In 1943, Wright made a surprising dis-covery. He mixed boric acid with silicone oilin a test tube. Instead of forming the hardrubber material he sought,the compound remainedslightly gooey to the touch.Disappointed with theresults, Wright tossed a gobof the material from the testtube onto the floor. To hissurprise, the gob bouncedright back at him.

The new compound was very bouncy andcould be stretched and pulled. However, itwasn’t a good rubber substitute, so Wrightand other General Electric scientists contin-ued their search.

Seven years later, a toy seller named PeterHodgson packaged some of Wright’s creation

in small plastic “eggs” andpresented his new product at the 1950 International Toy Fair in New York. The material, called Silly Putty®,proved quite popular. Millionsof eggs containing Silly Puttyhave been sold to kids of allages since then.

Rubber and boric acid are substances with very dif-ferent properties. In thischapter, you will learn aboutthe properties that are usedto classify many differentcompounds.

Chapter 16

Natural rubber is collected froma rubber tree as it flows fromcuts made in the bark.


Ionic Versus Covalent Compounds are often classified based on similari-ties in their structure or properties. For some com-pounds, differences in properties are a result of thetype of chemical bonding present. In this activity,you will investigate the properties of two substancesand relate the properties to the type of bonding inthe compounds.

Procedure1. Place a small amount of paraffin wax into a test

tube. Place an equal amount of table salt intoa second test tube.

2. Fill a plastic-foam cup halfway with hot water.

3. Place the test tubes into the water. After3 minutes, remove the test tubes fromthe water. Observe the contents of thetest tubes, and record your observa-tions in your ScienceLog.

4. Add 10 mL of water to each testtube using a graduated cylinder.

5. Stir each test tube with astirring rod. Record yourobservations in yourScienceLog.

Analysis6. Summarize the properties you observed for

each type of compound.

7. Ionic bonding is present in many compoundsthat have a high melting point and that will

dissolve in water. Covalent bonding is pre-sent in many compounds that have a lowmelting point and that will not dissolve in water. Identify the type of bondingpresent in paraffin wax and in table salt.


1. What is the difference between ioniccompounds and covalent compounds?

2. What is an acid?

3. What elements make up ahydrocarbon?

Chemical Compounds 397Copyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 16398

N E W T E R M Sionic compoundscovalent compounds

O B J E CT I V E S! Describe the properties of ionic

and covalent compounds.! Classify compounds as ionic

or covalent based on their properties.

Section1 Ionic and Covalent

CompoundsThe world around you is made up of chemical compounds.Chemical compounds are pure substances composed of ionsor molecules. There are millions of different kinds of com-pounds, so you can imagine how classifying them might behelpful. One simple way to classify compounds is by group-ing them according to the type of bond they contain.

Figure 1 An ionic compound isformed when the metal sodiumreacts with the nonmetalchlorine. Sodium chloride isformed in the reaction, andenergy is released as light and thermal energy.

Figure 2 Ionic compounds tend to be brittle,and they will shatter when hit with a hammer.

Ionic CompoundsCompounds that contain ionic bonds arecalled ionic compounds. Remember thatan ionic bond is the force of attractionbetween two oppositely charged ions.Ionic compounds can be formed by thereaction of a metal with a nonmetal.Electrons are transferred from the metalatoms (which become positively chargedions) to the nonmetal atoms (whichbecome negatively charged ions). Forexample, when sodium reacts with chlo-rine, as shown in Figure 1, the ionic com-pound sodium chloride, or ordinary tablesalt, is formed.

Properties of Ionic Compounds Theforces acting between the ions that makeup ionic compounds give these com-pounds certain properties. Ionic com-pounds tend to be brittle, as shown inFigure 2. The ions that make up an ioniccompound are arranged in a repeatingthree-dimensional pattern called a crystallattice. The ions that make up the crystallattice are arranged as alternating positiveand negative ions. Each ion in the latticeis surrounded by ions of the oppositecharge, and each ion is bonded to the ionsaround it. When an ionic compound isstruck with a hammer, the pattern of ionsin the crystal lattice is shifted. Ions withthe same charge line up and repel oneanother, causing the crystal to shatter.


Because ionic bonds are verystrong, ionic compounds havehigh melting points and arealmost always solid at room tem-perature, as shown in Figure 3. Anionic compound will only melt at temperatures high enough toovercome the strong ionic bondsbetween the ions. Sodium chlo-ride, for instance, must be heatedto 801°C before it will melt. Thistemperature is much higher thanyou can produce in your kitchenor even your school laboratory.

Another property shared by many ionic compounds is thatthey dissolve easily in water. Molecules of water attract eachof the ions of an ionic compound and pull them away fromone another. The solution created when an ionic compounddissolves in water can conduct an electric current, as shownin Figure 4. The dissolved ions are able to move past one anotherand allow the electric current to exist in the solution. In con-trast, the ions in an undissolved crystal cannot move past oneanother, so a crystal of an ionic compound does not conductan electric current.

Covalent CompoundsCompounds composed of elements that are covalently bondedare called covalent compounds. Remember that a covalent bondis formed when two atoms share electrons. Gasoline, carbondioxide, water, and sugar are well-known examples of cova-lent compounds.

Chemical Compounds 399

Figure 3 Each of these ioniccompounds has a high meltingpoint and is solid at room temperature.

Nickel(II) oxidemelts at 1,984°C.

Magnesium oxidemelts at 2,800°C.

Potassium dichromatemelts at 398°C.

Pure water Salt water

Figure 4 The beaker of pure water on the left does not conductan electric current. As salt dissolves in the beaker of water on theright, an electric current can exist, and the bulb lights up.


Properties of Covalent Compounds The properties of cova-lent compounds are quite different from those of ionic com-pounds. Covalent compounds exist as independent particlescalled molecules. The forces of attraction between moleculesof covalent compounds are much weaker than the bondsbetween ions in a crystal lattice. Thus, covalent compoundshave lower melting points than ionic compounds.

You have probably heard the phrase “oil and water don’tmix.” Oil, such as that used in salad dressing, is composed ofcovalent compounds. Many covalent compounds do not dis-solve well in water. Water molecules have a stronger attrac-tion for one another than they have for the molecules of mostother covalent compounds. Thus, the molecules of the cova-lent compound get squeezed out as the water molecules pulltogether. Some covalent compounds do dissolve in water. Mostof these solutions contain uncharged molecules dissolved inwater and do not conduct an electric current, as shown inFigure 5. Some covalent compounds form ions when they dis-solve in water. Solutions of these compounds, including com-pounds called acids, do conduct an electric current. You willlearn more about acids in the next section.

Chapter 16400

1. List two properties of ionic compounds.

2. List two properties of covalent compounds.

3. Methane is a gas at room temperature. What type of com-pound is this most likely to be?

4. Comparing Concepts Compare ionic and covalentcompounds based on the type of particle that makesup each.

Figure 5 This solution of sugar, acovalent compound, in water doesnot conduct an electric currentbecause the individual moleculesof sugar are not charged.

Molecules of covalent com-pounds can have anywherefrom two atoms to hundredsor thousands of atoms!Small, lightweight mol-ecules, like water or carbondioxide, tend to form liquidsor gases at room tempera-ture. Heavier molecules,such as sugar or plastics,tend to form solids at roomtemperature.

REVIEW

Sugar water


Figure 7 Bubbles of hydrogengas are produced when zincmetal reacts with hydrochloricacid.


N E W T E R M Sacid pHbase salt

O B J E CT I V E S! Describe the properties and uses

of acids and bases.! Explain the difference between

strong acids and bases andweak acids and bases.

! Identify acids and bases usingthe pH scale.

! Describe the properties and usesof salts.

Section2 Acids, Bases, and Salts

Have you ever noticed a change in your teawhen you added lemon? When you squeezelemon juice into tea, the color of the teabecomes lighter, as shown in Figure 6.Lemon juice contains a substance calledan acid that changes the color of a sub-stance in the tea. The ability to changethe color of certain chemicals is oneproperty used to classify substances asacids or bases. A third category of sub-stances, called salts, will also be dis-cussed in this section. Salts are formedby the reaction of an acid with a base.

AcidsAn acid is any compound that increases the number of hydro-gen ions when dissolved in water, and whose solution tastessour and can change the color of certain compounds.

Properties of Acids If you have ever had orange juice, youhave experienced the sour taste of an acid. The taste of lemons,limes, and other citrus fruits is a result of citric acid. Taste,however, should NEVER be used as a test to identify an unknownchemical. Many acids are corrosive, meaning they destroy bodytissue and clothing, and many are also poisonous.

In Figure 7 you see the result of placing a piece of zincinto a hydrochloric acid solution. Acids react with some met-als to produce hydrogen gas. Adding an acid to baking sodaor limestone produces a different gas, carbon dioxide. Vinegarcontains acetic acid. When vinegar is added to baking soda,bubbles of carbon dioxide are produced.

Solutions of acids conduct an electric current because acidsbreak apart to form ions in water. Acids increase the numberof hydrogen ions, H+, in a solution. However, the hydrogen iondoes not normally exist alone. In a water solution, the hydro-gen ions strongly attract water molecules. Each hydrogen ionattaches to a water molecule to form a hydronium ion, H3O+.

NEVER touch or taste

a concentrated solution of astrong acid.

Figure 6 Acids, like thosefound in lemon juice, canchange the color of tea.


As mentioned earlier, a property of acids is their ability tochange the color of a substance. An indicator is a substancethat changes color in the presence of an acid or base. A com-monly used indicator is litmus. Paper strips containing litmusare available in both blue and red. When an acid is added toblue litmus paper, the color of thelitmus changes to red, as shown in Figure 8. (Red litmus paper isused to detect bases, as will be dis-cussed shortly.) Many plant mate-rials, such as red cabbage, containcompounds that are indicators.

Uses of Acids Acids are used in many areasof industry as well as in your home. Sulfuric acidis the most widely produced industrial chemicalin the world. It is used in the production of met-als, paper, paint, detergents, and fertilizers. It isalso used in car batteries, as shown in Figure 9.Nitric acid is used to make fertilizers, rubber, andplastics. Hydrochloric acid is used in the pro-duction of metals and to help keep swimmingpools free of algae. It is also found in your stom-ach, where it aids in digestion. Citric acid andascorbic acid (vitamin C) are found in orangejuice, while carbonic acid and phosphoric acidhelp give extra “bite” to soft drinks.

Strong Versus Weak As an acid dissolves in water, its mol-ecules break apart and produce hydrogen ions. When all themolecules of an acid break apart in water to produce hydrogenions, the acid is considered a strong acid. Strong acids includesulfuric acid, nitric acid, and hydrochloric acid.

When few molecules of an acid break apart in water toproduce hydrogen ions, the acid is considered a weak acid.Acetic acid, citric acid, carbonic acid, and phosphoric acid areall weak acids.

Chapter 16402

Some hydrangea plants actas indicators. Leaves on theplants change from pink toblue as the soil becomesmore acidic.

Figure 8 Vinegar turns blue litmuspaper red because it containsacetic acid.

Figure 9 The label on this carbattery warns you that sulfuricacid is found in the battery.


BasesA base is any compound that increases the number of hydrox-ide ions when dissolved in water, and whose solution tastesbitter, feels slippery, and can change the color of certaincompounds.

Properties of Bases If you have ever accidentally tasted soap,then you have experienced the bitter taste of a base. Soap alsodemonstrates that a base feels slippery. However, NEVER usetaste or touch as a test to identify an unknown chemical. Likeacids, many bases are corrosive. If you are using a base in anexperiment and your fingers begin to feel slippery, it mightmean that some of the base got on your hands. You shouldimmediately rinse your hands with large amounts of water.

Solutions of bases conduct an electric current because basesform ions in water. Bases increase the number of hydroxideions, OH!, in a solution. A hydroxide ion isactually a hydrogen atom and an oxy-gen atom bonded together. An extraelectron gives the ion a negative charge.

Like acids, bases change the colorof an indicator. Most indicators turn adifferent color for bases than they dofor acids. For example, bases willchange the color of red litmus paper toblue, as shown in Figure 10.

Uses of Bases Like acids, bases have many uses. Sodiumhydroxide is used to make soap and paper. You can find sodiumhydroxide in your home in oven cleaners and in products that unclog your drain, as shown in Figure 11. Remember, basescan harm your skin, so carefully follow the safety instructionswhen using these products. Calcium hydroxide is used to makecement, mortar, and plaster. Ammonia is found in many house-hold cleaners and is also used in the production of fertilizers.Magnesium hydroxide and alumi-num hydroxide are used inantacids to treat heartburn.

403

To determine how acidic orbasic a solution is, just use your head—of cabbage! Try it for yourself on page 582 of the LabBook.

NEVER touch or taste

a concentrated solution of astrong base.

Figure 10 Sodium hydroxide,a base, turns red litmuspaper blue.

Figure 11 This drain cleanercontains sodium hydroxide tohelp dissolve grease that can clog the drain.


Strong Versus Weak When all the molecules of a base breakapart in water to produce hydroxide ions, the base is called astrong base. Strong bases include sodium hydroxide, calciumhydroxide, and potassium hydroxide.

When only a few of the molecules of a base producehydroxide ions in water, the base is called a weak base.Ammonia, magnesium hydroxide, and aluminum hydroxideare all weak bases.

Acids and Bases Neutralize One AnotherIf you have ever suffered from an acid stomach, or heartburn,as shown in Figure 12, you might have taken an antacid.Antacids contain weak bases that soothe your heartburn byreacting with and neutralizing the acid in your stomach. Acidsand bases neutralize one another because the H" of the acidand the OH! of the base react to form water, H2O. Other ionsfrom the acid and base are also dissolved in the water. If thewater is evaporated, these ions join to form a compound calleda salt. You’ll learn more about salts later in this section.

The pH Scale Indicators such as litmus can identify whethera solution contains an acid or base. To describe how acidic orbasic a solution is, the pH scale is used. The pH of a solutionis a measure of the hydronium ion concentration in the solu-tion. By measuring the hydronium ion concentration, the pHis also a measure of the hydrogen ion concentration. On thescale, a solution that has a pH of 7 is neutral, meaning thatit is neither acidic nor basic. Pure water has a pH of 7. Basicsolutions have a pH greater than 7, and acidic solutions havea pH less than 7. Look at Figure 13 to see the pH values formany common materials.

Chapter 16404

Increasing acidity Increasing basicity

Lemonjuice

Softdrink Human

saliva

Tap water

Acid rain Clean rain

Human stomach contents

Seawater

Detergents Householdammonia

Milk

1 2 3 4 5 6 7 8 9 10 11 12 13

Figure 12 Have heartburn? Takean antacid! Antacid tablets con-tain a base that neutralizes theacid in your stomach.

Figure 13 pH Values of Common Materials

pHast Relief!1. Fill a small plastic

cup halfway with vinegar. Test the vinegar with redand blue litmus paper. Record your results in your ScienceLog.

2. Carefully crush an antacidtablet, and mix it with thevinegar. Test the mixturewith litmus paper. Recordyour results in yourScienceLog.

3. Compare the acidity of thesolution before and afterthe reaction.


Many indicators, including litmus, have only twocolors. This allows you to determine if a solution is acidic or basic, but it does not identify its pH.A mixture of different indicators can be used todetermine the pH of a solution. After deter-mining the colors for this mixture at differentpH values, the indicators can be used to deter-mine the pH of an unknown solution, as shownin Figure 14. Indicators can be used as paperstrips or solutions, and they are often used totest the pH of soil and of water in pools andaquariums. Another way to determine the acidity of a solution is to use an instrument called a pH meter, which can detect and measure hydrogenions electronically.

pH and the Environment Living things depend on havinga steady pH in their environment. Plants are known to havecertain preferred growing conditions. Some plants, such aspine trees, prefer acidic soil with a pH between 4 and 6. Otherplants, such as lettuce, require basic soil with a pH between8 and 9. Fish require water near pH 7. As you can see in Figure13, rainwater can have a pH as low as 3. This occurs in areaswhere compounds found in pollution react with water to makethe strong acids sulfuric acid and nitric acid. As this acid pre-cipitation collects in lakes, it can lower the pH to levels thatmay kill the fish and other organisms in the lake. To neu-tralize the acid and bring the pH closer to 7, a base can beadded to the lakes, as shown in Figure 15.


Self-CheckWhich is more acidic, a soft drink ormilk? (Hint: Refer to Figure 13 to findthe pH values of these drinks.) (Seepage 596 to check your answer.)


Human blood has a pH ofbetween 7.38 and 7.42. If thepH is above 7.8 or below 7,the body cannot functionproperly. Sudden changes inblood pH that are not quicklycorrected can be fatal.

Figure 14 The paper strip con-tains several indicators. The pHof a solution is determined bycomparing the color of the stripto the scale provided.

Figure 15 This helicopter is adding a base to anacidic lake. Neutralizing the acid in the lake mighthelp protect the organisms living in the lake.


SaltsWhen you hear the word salt, you probably think of the tablesalt you use to season your food. But the sodium chloridefound in your salt shaker is only one example of a large groupof compounds called salts. A salt is an ionic compound formedfrom the positive ion of a base and the negative ion of anacid. You may remember that a salt and water are producedwhen an acid neutralizes a base. However, salts can also beproduced in other reactions, as shown in Figure 16.

Uses of Salts Salts have many uses in industry and in yourhome. You already know that sodium chloride is used to sea-son foods. It is also used in the production of other com-pounds, including lye (sodium hydroxide), hydrochloric acid,and baking soda. The salt calcium sulfate is made into wall-board, or plasterboard, which is used in construction. Sodiumnitrate is one of many salts used as a preservative in foods.Calcium carbonate is a salt that makes up limestone, chalk,and seashells. Another use of salts is shown in Figure 17.

Figure 16 The salt potassiumchloride can be formed fromseveral different reactions.

Figure 17 Salts are used duringicy weather to help keep roadsfree of ice.

Chapter 16406

1. What ion is present in all acid solutions?

2. What are two ways scientists can measure pH?

3. What products are formed when an acid and base react?

4. Comparing Concepts Compare the properties of acids andbases.

5. Applying Concepts Would you expect the pH of a solutionof soap to be 4 or 9?

Write the formula of the saltproduced by each of the fol-lowing pairs of acids andbases:

1. HCl + LiOH

2. HBr + Ca(OH)2

REVIEW


Figure 18 These models, called structural formulas, are usedto show how atoms in a molecule are connected. Each linerepresents a pair of electrons shared in a covalent bond.


N E W T E R M Sorganic compoundsbiochemicals proteinscarbohydrates nucleic acidslipids hydrocarbons

O B J E CT I V E S! Explain why so many organic

compounds are possible.! Describe the characteristics of

carbohydrates, lipids, proteins,and nucleic acids and their func-tions in the body.

! Describe and identify saturated,unsaturated, and aromatichydrocarbons.

Section3 Organic Compounds

Of all the known compounds, more than 90 percent are mem-bers of a group of compounds called organic compounds.Organic compounds are covalent compounds composed ofcarbon-based molecules. Sugar, starch, oil, protein, nucleic acid,and even cotton and plastic are organic compounds. How canthere be so many different kinds of organic compounds? Thehuge variety of organic compounds is explained by examin-ing the carbon atom.

Each Carbon Atom Forms Four BondsCarbon atoms form the backbone of organic compounds.Because each carbon atom has four valence electrons (elec-trons in the outermost energy level of an atom), each atomcan make four bonds. Thus, a carbon atom can bond to one,two, or even three other carbon atoms and still have electronsremaining to bond to other atoms. Three types of carbon back-bones on which many organic compounds are based are shownin the models in Figure 18.

Some organic compounds have hundreds or even thou-sands of carbon atoms making up their backbone! Althoughthe elements hydrogen and oxygen, along with carbon, makeup many of the organic compounds, sulfur, nitrogen, andphosphorus are also important—especially in forming the mol-ecules that make up all living things.

C HHC HHCH

HH

C HHC HHCH

HH

CHC HHCH

HH

CC HH

HHHCH

HH

CC

CC

C

HH H

H

HH

HH

H

H

H H

C

Straight Chain All carbonatoms are connected oneafter another in a line.

Branched Chain The chain of carbonatoms continues in more than onedirection where a carbon atom bondsto three or more other carbon atoms.

Ring The chain of car-bon atoms forms a ring.


Figure 19 The sugar moleculesin the left image are simple car-bohydrates. The starch in theright image is a complex carbo-hydrate because it is composedof many sugar moleculesbonded together.

Biochemicals: The Compounds of LifeMany different compounds are found in living things. Somecompounds are composed of very small, simple moleculesthat are not based on carbon. These compounds, includingwater and salt, are considered inorganic. Organic compoundsmade by living things are called biochemicals. The moleculesof most biochemicals are very large. There are hundreds ofthousands of different biochemicals, which can be dividedinto four categories: carbohydrates, lipids, proteins, and nucleicacids. Each type of biochemical has important functions inliving organisms.

Carbohydrates Starch and cellulose are examples of carbo-hydrates. Carbohydrates are biochemicals that are composedof one or more simple sugars bonded together; they are usedas a source of energy and for energy storage. The energy youget from these biochemicals is stored in the form of chemi-cal bonds in the molecules. There are two types of carbohy-drates: simple carbohydrates and complex carbohydrates. A single sugar molecule, represented using a hexagon, or afew sugar molecules bonded together are examples of simplecarbohydrates, as illustrated in Figure 19. Glucose is a simplecarbohydrate produced by plants through photosynthesis.

When an organism has more sugar than it needs, its extrasugar may be stored for later use in the form of complexcarbohydrates, as shown in Figure 19. Molecules of complex

carbohydrates are composed of hundreds or even thou-sands of sugar molecules bonded together. Because

carbohydrates are used to provide you with energyyou need each day, you should include sources

of carbohydrates in your diet, such as the foodsshown in Figure 20.

Figure 20 Simple carbohydrates includesugars found in fruits and honey. Complexcarbohydrates, such as starches, arefound in bread, cereal, and pasta.


Lipids Fats, oils, waxes, and steroids are examples of lipids.Lipids are biochemicals that do not dissolve in water andhave many different functions, including storingenergy and making up cell membranes. Althoughtoo much fat in your diet can be unhealthy, somefat is extremely important to good health. Thefoods in Figure 21 are sources of lipids.

Lipids store excess energy in the body.Animals tend to store lipids primarily as fats,while plants store lipids as oils. When an organ-ism has used up most of its carbohydrates, it canobtain energy by breaking down lipids. Lipids arealso used to store vitamins in the body. Vitamins thatdo not dissolve in water will often dissolve in fat.

Lipids make up a structure called a cell membrane that sur-rounds each cell. Much of the cell membrane is formed frommolecules of phospholipids. The structure of phospholipid mol-ecules plays an important part in the phospholipid’s role inthe cell membrane. A phospholipid molecule has two regionswith very different properties. The tail of a phospholipid mol-ecule is a long, straight-chain carbon backbone composed onlyof carbon and hydrogen atoms. The tail is not attracted towater. In addition to carbon and hydrogen atoms, the headof a phospholipid molecule is composed of phosphorus, oxy-gen, and nitrogen atoms, which cause the head of the mol-ecule to be attracted to water. When phospholipids are inwater, the tails are forced together as water is attracted to theheads of the molecules. The result is the double layer of phos-pholipid molecules shown in the model in Figure 22. Thisarrangement of phospholipid molecules creates a barrier tohelp control the flow of chemicals into and out of the cell.


Figure 22 A cell membrane iscomposed primarily of two layersof phospholipid molecules.

Deposits of the lipid choles-terol in the body have beenlinked to health problemssuch as heart disease.However, cholesterol isneeded in nerve and braintissue as well as to makecertain hormones that regu-late body processes such as growth.

The head of each phospholipidmolecule is attracted to watereither inside or outside of the cell.

The tail of each phospholipidmolecule is pushed againstother tails because they arenot attracted to water.

Figure 21 Vegetable oil, meat,cheese, nuts, and milk aresources of lipids in your diet.


Chapter 16410

Proteins Most of the biochemicals found in living things areproteins. In fact, after water, proteins are the most abundantmolecules in your cells. Proteins are biochemicals that are

composed of amino acids; they have many different functions, including regulating chemical activities,

transporting and storing materials, and providingstructural support.

Every protein is composed of small “buildingblocks” called amino acids. Amino acids are smallermolecules composed of carbon, hydrogen, oxygen,

and nitrogen atoms. Some amino acids also includesulfur atoms. Amino acids chemically bond to form

proteins of many different shapes and sizes, from shortchains of only a few amino acids to large, twisted structuresconsisting of thousands of amino acids. The function of a pro-tein depends on the shape that the bonded amino acids adopt.If even a single amino acid is missing or out of place, the pro-tein may not function correctly or at all. The foods shown inFigure 23 provide amino acids that your body needs to makenew proteins.

Enzymes are proteins that regulate chemical reactions inthe body by acting as catalysts to increase the rate at whichthe reactions occur. Some hormones that help control yourbodily functions are proteins. Insulin, a hormone that helpsregulate the level of sugar in your blood, is one of the small-est proteins, consisting of only 51 amino acids. Oxygen is car-ried by the protein hemoglobin, allowing red blood cells todeliver oxygen throughout your body. There are also large pro-teins that extend through cell membranes and help controlthe transport of materials into and out of cells. Proteins thatprovide structural support often form structures that are easyto see, like those in Figure 24.

Figure 24 Hair and spider webs are made upof proteins that are shaped like long fibers.


All the proteins in your bodyare made from just 20 aminoacids. Nine of these aminoacids are called essentialamino acids because yourbody cannot make them. Youmust get them from the foodyou eat.

Figure 23 Meat, fish, cheese,and beans contain proteins,which are broken down intoamino acids as they are digested.



Nucleic Acids The largest molecules made by living organ-isms are nucleic acids. Nucleic acids are biochemicals that storeinformation and help to build proteins and other nucleic acids.Nucleic acids are sometimes called the “blueprints of life”because they contain all the information needed for the cellto make all of its proteins.

Like proteins, nucleic acids are long chains of smaller mol-ecules joined together. These smaller molecules are composedof carbon, hydrogen, oxygen, nitrogen, and phosphorus atoms.Nucleic acids are much larger than proteins even thoughnucleic acids are composed of only five building blocks.

There are two types of nucleic acids: DNA and RNA. DNA(deoxyribonucleic acid), like that shown in Figure 25, is thegenetic material of the cell. DNA molecules can store an enor-mous amount of information because of their length. If theDNA molecules in a single human cell were placed end toend and stretched out, their overall lengthwould be about 2 m—that’s over 6 ft long!When a cell needs to make a certainprotein, it gets information fromthe DNA in the cell. The impor-tant part of the DNA moleculeis copied. The informationcopied from DNA directs theorder in which amino acids arebonded together to make thatprotein. DNA also containsinformation used to build thesecond type of nucleic acid, RNA(ribonucleic acid). RNA is involvedin the actual building of proteins.

1. What are organic compounds?

2. What are the four categories of biochemicals?

3. What are two functions of proteins?

4. What biochemicals are used to provide energy?

5. Inferring Relationships Sickle-cell anemia is a condi-tion that results from a change of one amino acid in the protein hemoglobin. Why is this condition a genetic disorder?

REVIEW

across the sciencesC O N N E C T I O N

Nucleic acids store information—even about ancient peoples.Read more about these incred-ible biochemicals on page 418.

Figure 25 The DNA from a fruit fly contains all of theinstructions for making proteins,nucleic acids . . . in fact, for

making everything in the organism!


HydrocarbonsOrganic compounds that are composed of only carbon andhydrogen are called hydrocarbons. Hydrocarbons are an impor-tant group of organic compounds. Many fuels, including gaso-line, methane, and propane, are hydrocarbons. Hydrocarbonscan be divided into three categories: saturated, unsaturated,and aromatic.

Saturated Hydrocarbons Propane, like that used in the stovein Figure 26, is an example of a saturated hydrocarbon. Asaturated hydrocarbon is a hydrocarbon in which each carbonatom in the molecule shares a single bond with each of four

other atoms. A single bond is a covalent bond that con-sists of one pair of shared electrons. Hydrocarbons

that contain carbon atoms connected only by sin-gle bonds are called saturated because no otheratoms can be added without replacing an atomthat is part of the molecule. Saturated hydrocar-bons are also called alkanes.

Unsaturated Hydrocarbons Each carbon atom forms fourbonds. However, these bonds do not always have to be sin-gle bonds. An unsaturated hydrocarbon is a hydrocarbon inwhich at least two carbon atoms share a double bond or atriple bond. A double bond is a covalent bond that consistsof two pairs of shared electrons. Compounds that containtwo carbon atoms connected by a double bond are calledalkenes.

A triple bond is a covalent bond that consists of threepairs of shared electrons. Hydrocarbons that contain two car-bon atoms connected by a triple bond are called alkynes.

Hydrocarbons that contain double or triple bonds arecalled unsaturated because the double or triple bond can bebroken to allow more atoms to be added to the molecule.Examples of unsaturated hydrocarbons are shown in Figure 27.

Chapter 16412

CCHHH

H

CH C H

Figure 27 Fruits produce ethene,which helps ripen the fruit. Ethyne,better known as acetylene, isburned in this miner’s lamp and isalso used in welding.

Figure 26 The propane in thiscamping stove is a saturatedhydrocarbon.

CH C HHHH

HCH H


Aromatic Hydrocarbons Mostaromatic compounds are based onbenzene, the compound repre-sented by the model in Figure 28.Look for this structure to helpidentify an aromatic hydrocar-bon. As the name implies, aro-matic hydrocarbons often havestrong odors and are thereforeused in such products as air fresh-eners and moth balls.

Other Organic CompoundsMany other types of organic compounds exist that have atomsof halogens, oxygen, sulfur, and phosphorus in their molecules.A few of these types of compounds and their uses are describedin the chart below.


CC

CC

CH

HH

H

HH

C

Figure 28 Benzene has a ring of sixcarbons with alternating double andsingle bonds. Benzene is the startingmaterial for manufacturing manyproducts, including medicines.

Type of compound Uses Examples

1. What is a hydrocarbon?

2. How many electrons are shared in a double bond? a triplebond?

3. Comparing Concepts Compare saturated and unsatu-rated hydrocarbons.

REVIEW

Types and Uses of Organic Compounds

Alkyl halide starting material for chloromethane (CH3Cl)Teflon bromoethane (C2H5Br)

refrigerant (freon)

Alcohols rubbing alcohol methanol (CH3OH)gasoline additive ethanol (C2H5OH)antifreeze

Organic acids food preservatives ethanoic acid (CH3COOH)flavoring propanoic acid (C2H5COOH)

Esters flavorings methyl ethanoate fragrances (CH3COOCH3)clothing (polyester) ethyl propanoate

(C2H5COOC2H5)


Chapter Highlights

Chapter 16414

SECTION 1 SECTION 2

Vocabularyionic compounds (p. 398)covalent compounds (p. 399)

Section Notes• Ionic compounds contain

ionic bonds and are com-posed of oppositely chargedions arranged in a repeatingpattern called a crystal lattice.

• Ionic compounds tend to bebrittle, have high meltingpoints, and dissolve in waterto form solutions that con-duct an electric current.

• Covalent compounds arecomposed of elements thatare covalently bonded andconsist of independentparticles called molecules.

• Covalent compounds tend tohave low melting points. Mostdo not dissolve well in waterand do not form solutions thatconduct an electric current.

Vocabularyacid (p. 401)base (p. 403)pH (p. 404)salt (p. 406)

Section Notes• An acid is a compound that

increases the number ofhydrogen ions in solution.Acids taste sour, turn bluelitmus paper red, react withmetals to produce hydrogengas, and react with limestoneor baking soda to producecarbon dioxide gas.

• A base is a compound thatincreases the number ofhydroxide ions in solution.Bases taste bitter, feel slip-pery, and turn red litmuspaper blue.

• When dissolved in water,every molecule of a strongacid or base breaks apart toform ions. Few molecules ofweak acids and bases breakapart to form ions.

• When combined, an acid anda base neutralize one anotherto produce water and a salt.

• pH is a measure of hydro-nium ion concentration in asolution. A pH of 7 indicatesa neutral substance. A pH ofless than 7 indicates anacidic substance. A pH ofgreater than 7 indicates abasic substance.

• A salt is an ionic compoundformed from the positive ionof a base and the negativeion of an acid.

LabsCabbage Patch Indicators (p. 582)Making Salt (p. 584)

Skills CheckVisual UnderstandingLITMUS PAPER You can use the ability of acidsand bases to change the color of indicators toidentify a chemical as an acid or base. Litmusis an indicator commonly used in schools.Review Figures 8 and 10, which show how the color of litmus paper is changed by anacid and by a base.

pH SCALE Knowing whether a sub-stance is an acid or a base canhelp explain some of theproperties of the substance.The pH scale shown inFigure 13 illustrates thepH ranges for manycommon substances.


415Chemical Compounds

Vocabularyorganic compounds (p. 407)biochemicals (p. 408)carbohydrates (p. 408)lipids (p. 409)proteins (p. 410)nucleic acids (p. 411)hydrocarbons (p. 412)

Section Notes• Organic compounds are

covalent compounds com-posed of carbon-basedmolecules.

• Each carbon atom forms fourbonds with other carbonatoms or with atoms of otherelements to form straightchains, branched chains, or rings.

• Biochemicals are organiccompounds made by livingthings.

• Carbohydrates are biochemicals that arecomposed of one or more simple sugars bonded together; they areused as a source of energyand for energy storage.

• Lipids are biochemicals thatdo not dissolve in water andhave many functions, includ-ing storing energy and mak-ing up cell membranes.

• Proteins are biochemicalsthat are composed of aminoacids and have many func-tions, including regulatingchemical activities, transport-ing and storing materials,and providing structuralsupport.

• Nucleic acids are biochemi-cals that store informationand help to build proteinsand other nucleic acids.


TOPIC: Ionic Compounds sciLINKS NUMBER: HSTP355TOPIC: Covalent Compounds sciLINKS NUMBER: HSTP360TOPIC: Acids and Bases sciLINKS NUMBER: HSTP365TOPIC: Salts sciLINKS NUMBER: HSTP370TOPIC: Organic Compounds sciLINKS NUMBER: HSTP375


KEYWORD: HSTCMP


SECTION 3

• Hydrocarbons are organiccompounds composed ofonly carbon and hydrogen.

• In a saturated hydrocarbon,each carbon atom in themolecule shares a singlebond with each of four other atoms.

• In an unsaturated hydro-carbon, at least two carbonatoms share a double bondor a triple bond.

• Many aromatic hydrocarbonsare based on the six-carbonring of benzene.

• Other organic compounds,including alkyl halides, alco-hols, organic acids, andesters, are formed by addingatoms of other elements.



To complete the following sentences, choosethe correct term from each pair of terms listedbelow:

1. Compounds that have low melting pointsand do not usually dissolve well in waterare ? . (ionic compounds or covalentcompounds)

2. A(n) ? turns red litmus paper blue.(acid or base)

3. ? are composed of only carbon andhydrogen. (Ionic compounds orHydrocarbons)

4. A biochemical composed of amino acids isa ? . (lipid or protein)

5. A source of energy for living things can befound in ? . (nucleic acids orcarbohydrates)


Multiple Choice

6. Which of the following describes lipids?a. used to store energyb. do not dissolve in waterc. make up most of the cell membraned.all of the above

7. An acid reacts to produce carbon dioxidewhen the acid is added toa. water.b. limestone.c. salt.d. sodium

hydroxide.

8. Which of thefollowing does NOTdescribe ionic compounds?a. high melting pointb. brittlec. do not conduct electric currents in waterd.dissolve easily in water

9. An increase in the amount of hydrogenions in solution ? the pH.a. raisesb. lowersc. does not affectd.doubles

10. Which of the following compounds makesup the majority of cell membranes?a. lipidsb. ionic compoundsc. acidsd.nucleic acids

11. The compounds that store information forbuilding proteins area. lipids.b. hydrocarbons.c. nucleic acids.d.carbohydrates.

Short Answer

12. What type of compound would you useto neutralize a solution of potassiumhydroxide?

13. Explain why the reaction of an acid witha base is called neutralization.

14. What characteristic of carbon atoms helpsto explain the wide variety of organiccompounds?

15. Compare acids and bases based on the ionproduced when each compound is dis-solved in water.

Chapter 16416Copyright © by Holt, Rinehart and Winston. All rights reserved.

Concept Mapping

16. Use the followingterms to create a con-cept map: acid, base,salt, neutral, pH.


17. Fish give off the base ammonia, NH3, aswaste. How does the release of ammoniaaffect the pH of the water in the aquar-ium? What can be done to correct theproblem?

18. Many insects, such as fire ants, injectformic acid, a weak acid, when they biteor sting. Describe the type of compoundthat should be used to treat the bite.

19. Organic compounds are also covalentcompounds. What properties would youexpect organic compounds to have as aresult?

20. Farmers often can taste their soil to deter-mine whether the soil has the correctacidity for their plants. How would tastehelp the farmer determine the acidity ofthe soil?

21. A diet that includes a high level of lipidsis unhealthy. Why is a diet containing nolipids also unhealthy?


Study the structural formulas below, and thenanswer the questions that follow.

22. A saturated hydrocarbon is represented bywhich structural formula(s)?

23. An unsaturated hydrocarbon is repre-sented by which structural formula(s)?

24. An aromatic hydrocarbon is representedby which structural formula(s)?


Take a minute to review your answersto the ScienceLog questions on page397. Have your answers changed? Ifnecessary, revise your answers based onwhat you have learned since you beganthis chapter.

CC

CC

CH

HH

H

H

C

CH

HH CC

CC

C

HH H

H

HH

HH

H

H

H H

C

CHC HHH

CC HH

HHHCH

HH

HC

H

HC

H

HC

H

HC

H

a

c

b

d


timeline unit interactions of - mrs. erin schumacher ...€”combine in different patterns to form...

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