Chemistry Unit 2

Wear your safety goggles!

Structure of an Atom

Protons and an Atoms

identity

Valence Electrons

and Reactivity

The Periodic

Table

Chemical Formulas

Chemical Reactions

Chemical Equations

Law of Conservation of Mass

C h e m i s t r y U n i t 1

C h e m i s t r y U n i t 2

Teks for Unit 1 and 2• (5) Matter and energy. The student knows that matter is composed of atoms and has chemical

and physical properties. The student is expected to:

• Unit 1• (A) describe the structure of atoms, including the masses, electrical charges, and locations, of

protons and neutrons in the nucleus and electrons in the electron cloud;• (B) identify that protons determine an element's identity and valence electrons determine its

chemical properties, including reactivity; • (C) interpret the arrangement of the Periodic Table, including groups and periods, to explain how

properties are used to classify elements;

• Unit 2• (D) recognize that chemical formulas are used to identify substances and determine the

number of atoms of each element in chemical formulas containing subscripts;• (E) investigate how evidence of chemical reactions indicate that new substances with different

properties are formed; and• (F) recognize whether a chemical equation containing coefficients is balanced or not and how

that relates to the law of conservation of mass.

8.5 D Questions

• The chemical formula for water is H2O. What do the H and O represent? What does the subscript 2 mean?

• The chemical formula for hydrogen peroxide is H2O2. What does the chemical formula tell you about the hydrogen peroxide molecule?

• Is CO the same as CO2? Explain. • One calcium (Ca) atom combines with two chlorine (Cl)

atoms to make calcium chloride. What is the chemical formula of calcium chloride?

• Suppose that all you know about a molecule is its chemical formula. What can the chemical formula tell you?

8.5 D Key Concepts

• An element is made up of only one type of atom. Hydrogen, helium, and oxygen are examples of elements. Elements are pure substances. A pure substance is matter that has the same chemical composition throughout and cannot be separated into its parts by physical means.

Elements form compounds. A compound is a pure substance that forms when two or more elements join chemically in a fixed proportion.

• Just as an element is made up of one kind of atom, a compound is made up of one kind of molecule. A molecule is a group of two or more atoms held together by very strong chemical bonds. These bonds form between atoms that share or transfer electrons. A molecule is the smallest unit of a compound that has all the properties of that compound.

• A molecule can be made up of more than one atom of the same element. For example, two atoms of oxygen join to form a molecule of oxygen gas.

• A molecule can also be made up of two or more different elements. A water molecule is made up of two hydrogen atoms and one oxygen atom. The illustration below is a model of a water molecule.

• Molecules are identified by chemical formulas. A chemical formula is a group of chemical symbols and numbers that shows the kinds and numbers of atoms in a molecule.

• The formula for a water molecule is H2 O. H is the chemical symbol for hydrogen. O is the chemical symbol for oxygen. The number 2 in the formula is called a subscript. It shows that the molecule contains two atoms of hydrogen. The O has no subscript. That means that the molecule contains only one atom of oxygen.

Chemistry Vocabulary, Unit 2, part 1

• pure substance: matter that has the same chemical composition throughout • compound: a substance that forms when two or more elements join chemically • molecule: a group of two or more atoms held together by chemical bonds • chemical formula: a group of chemical symbols and numbers that shows the

kinds and numbers of atoms in a molecule

How many chemical formulas can you name?

Matter

Compound

Smallest unit is a molecule

H2O

Pure Substance

Smallest unit is an atom

H

Chemical FormulasSubscripts

And Lettering

carbon dioxide CO2

Chemical Formulas of Household

Items

Common Name Chemical Name Chemical Formula

aluminum foil aluminum Alvitamin C ascorbic acid C6 H8 O6

vinegar acetic acid (diluted) CH3 COOH

table salt sodium chloride NaCl

road salt calcium chloride CaCl2

table sugar sucrose C12 H22 O11

baking soda sodium bicarbonate NaHCO3

wood alcohol (solvent) methanol CH3 OH

hydrogen peroxide hydrogen peroxide (diluted) H2 O2

dry ice carbon dioxide CO2

nail polish remover acetone CH3 COCH3

chalk, some antacidscalcium carbonate CaCO3

lighter fluid butane C4 H10

tincture of iodine iodine I2

soda water carbonic acid (diluted) H2 CO3

Chemical Formula

CaC03

NaCl

C8H10N4O2 Mg3Al2Si3O12

O3 C6 H12 O6

Common Name

calcite

Atoms Present

1 Calcium1 Carbon3 Oxygen

Chemical Formula

CaC03

NaCl

C8H10N4O2 Mg3Al2Si3O12

O3 C6 H12 O6

Common Name

calcite

Atoms Present

1 Calcium1 Carbon3 Oxygen

Chemical Formula

CaC03

NaCl

C8H10N4O2 Mg3Al2Si3O12

O3 C6 H12 O6

Common Name

calcite

Atoms Present

1 Calcium1 Carbon3 Oxygen

Chemistry Vocabulary, Unit 2pure substance: matter that has the same chemical composition throughout

compound: a substance that forms when two or more elements join chemically

molecule: a group of two or more atoms held together by chemical bonds

chemical formula: a group of chemical symbols and numbers that shows the kinds and numbers of atoms in a molecule

physical change: a change in the form or appearance of a substance without a change in the identity of the substance\chemical reaction: a process in which chemicals react, or change, to form new types of matter

precipitate: a solid that forms during a chemical reaction in a solution

reactant: a substance that enters into a chemical reaction

product: a substance that forms during a chemical reaction

combustion reaction: a chemical reaction that occurs when oxygen combines with certain other substances to release heat

rusting: a slow chemical reaction between oxygen and a metalchemical reaction: a process in which chemicals react, or change, to form new types of matter

reactants: the chemicals that enter into a chemical reaction; also called reagents

products: the chemicals that form during a chemical reaction

chemical equation: a statement that shows the reactants and products of a chemical reaction

law of conservation of mass: the principle that states that in any chemical reaction, the amount of matter in the reactants must equal the amount of matter in the products

coefficient: in a chemical equation, a number placed in front of a reactant or product that tells how many molecules of that substance participate in the reaction

8.5 E Questions

• What is a chemical reaction? • What is the difference between a physical

change and a chemical reaction? • How can you tell that a chemical reaction has

taken place? • Are the products of a chemical reaction the

same as the reactants? Explain why or why not. • What types of chemical reactions are there?

8.5 E Key Concepts• A physical change alters the form or appearance of a substance without changing the identity of the substance.

During a physical change, no changes occur in the elements or compounds that make up the matter. For example, when water boils, water vapor is released. The water changes from a liquid to a gas, but it is still water.

A chemical reaction is a process in which elements and compounds combine in new ways to form new substances

Usually a physical reaction can be easily reversed. For example, to change water vapor back into a liquid, you simply let it cool down. A chemical reaction, on the other hand, is not easily reversed.

Some signs that indicate a chemical reaction has occurred include a change in color and the release of heat or light. Sometimes a precipitate forms. A precipitate is a solid that forms during a chemical reaction that takes place in a solution.

• Some chemical reactions cause release of a gas. If you drop an antacid tablet into water, a chemical reaction occurs. The bubbles that you see in the water are carbon dioxide given off by a reaction between a compound in the tablet and the water. But release of gas does not always mean a chemical reaction has taken place. For example, when water boils, this change from liquid water to water vapor is a physical change, not a chemical change.

• The reactants are the chemicals that enter into a chemical reaction. The products are the chemicals that form during a chemical reaction. For example, when acetic acid (vinegar) combines with sodium bicarbonate (baking soda), acetic acid and sodium bicarbonate are the reactants. The products are a salt, water, and a gas—carbon dioxide.

• A combustion reaction is a chemical reaction that occurs when oxygen combines with certain other substances to release heat. A combustion reaction takes place when something burns. When wood burns, water vapor and carbon dioxide are given off. The ash that remains is very different from the wood that burned. It is a new substance with different properties.

Rusting is another example of a chemical reaction. Rusting is a slow reaction between oxygen and a metal. A nail contains iron. Over time, the iron combines with oxygen in the air to form rust. The surface of the nail changes from smooth and shiny to rough and reddish brown. The rust is a different substance from iron.

Reaction Types

• Physical Reaction

• Chemical Reaction

Types of Reactions Types of Reactions Types of Reactions

Types of ReactionsTypes of ReactionsTypes of Reactions

C25H52 + 38 O2 → 25 CO2 + 26 H2O1. What are the reactants in the chemical reaction that occurs when a candle burns?

2. The products of the reaction are carbon dioxide and water vapor. What two types of energy are also released?

3. What happened when your teacher placed a glass jar over the burning candle? Why do you think this happened?

4. Explain How do you know this is a combustion reaction?

5. Apply How do you know this is a chemical reaction and not just a physical change?

Combustion Reaction C25H52 + 38 O2 → 25 CO2 + 26 H2O

1. What are the reactants in the chemical reaction that occurs when a candle burns?

2. The products of the reaction are carbon dioxide and water vapor. What two types of energy are also released?

3. What happened when your teacher placed a glass jar over the burning candle? Why do you think this happened?

4. Explain How do you know this is a combustion reaction?

5. Apply How do you know this is a chemical reaction and not just a physical change?

Combustion Reaction

Evidence of a Chemical Reaction

Precipitation Gas Bubbles Change in color or

temperature

N2 + H2 → NH3

Types of Chemical Reactions

OxidizingCombustion

8.5 F Questions

• Why are coefficients included in a chemical equation?• What is the law of conservation of mass? How does it

apply to a chemical reaction?• If 12 oxygen atoms enter a chemical reaction, must the

same 12 oxygen atoms be included in the products of the reaction? How do you know?

• How can you tell if a chemical equation is balanced or unbalanced?

• When balancing a chemical equation, which element should you balance first?

8.5 F Key Concepts• A chemical equation shows the reactants and products of a chemical reaction. The reactants are shown on the left side of the equation, and the products are shown on

the right side.

Here is an example of a chemical equation: CH4 + O2 CO2 + H2 O

• This equation shows that methane (CH4) and oxygen (O2) react to form carbon dioxide (CO2) and water (H2 O).

The equation above is an unbalanced equation, which means that the numbers of atoms on the left and right sides of the arrow do not match. The equation shows two oxygen atoms in the reactants (on the left side), and three oxygen atoms in the products (on the right side). Also, there are four hydrogen atoms in the reactants and only two hydrogen atoms in the products.

The balanced form of the chemical equation shown above is as follows:

CH4 + 2 O2 → CO2 + 2 H2 O

Placing a coefficient of 2 in front of H2 O makes four hydrogen atoms on both sides of the equation. Then, placing a coefficient of 2 in front of O2 makes four oxygen atoms on both sides of the equation.

• A balanced chemical equation shows how a reaction agrees with the law of conservation of mass. Every atom has a specific mass, and that mass cannot be increased or decreased by a chemical reaction. So, if four oxygen atoms enter a chemical reaction, then four oxygen atoms must leave the reaction as well.

• In balanced chemical equations, the coefficients show the ratios in which chemicals combine and form. In the reaction between methane and oxygen described above, the ratio of the reactants is 1:2—one methane molecule to two oxygen molecules. This shows that 10 methane molecules will combine with 20 oxygen molecules, or that 1,000 methane molecules will combine with 2,000 oxygen molecules.

• When you balance an equation, remember the following: – If no coefficient is visible in front of a reactant or product, the coefficient of that reactant or product is 1. – You might not need to change every coefficient when balancing a chemical equation. – A coefficient in front of a reactant or product affects each element inside that reactant or product. For example, putting a coefficient of 2 in front of H2 O means

that there are four hydrogen atoms and two oxygen atoms. – Begin by balancing the element that appears in the fewest places in the equation.

When you balance an equation, remember the following:

• If no coefficient is visible in front of a reactant or product, the coefficient of that reactant or product is 1.

• You might not need to change every coefficient when balancing a chemical equation.

• A coefficient in front of a reactant or product affects each element inside that reactant or product. For example, putting a coefficient of 2 in front of H 2 O means that there are four hydrogen atoms and two oxygen atoms.

• Begin by balancing the element that appears in the fewest places in the equation.

Balancing Chemical Reactions

• Unbalanced Reaction– CH4 + O2 CO2 + H2 O

• Balanced Reaction– CH4 + 2 O2 CO2 + 2 H2 O

Evidence of a Chemical Reaction

• Color Change• Temperature Change• Release of Gas Bubbles• Precipatate• Light• Odor