malone and dolter - basic concepts of chemistry 9e1 chapter 6 chemical reactions

Malone and Dolter - Basic Concepts of Chemistry 9e

1

Chapter 6Chapter 6

Chemical ReactionsChemical Reactions


2

Setting the Stage – Global Warming

Accepted as a fact after some controversy At least partly due to increase of CO2 in

atmosphere CO2 (along with other gases) traps heat in

the atmosphere by preventing heat loss to space


3

Burning Fossil Fuels – Source of Most CO2

Natural gas, coal and oiland all carboncompounds produce CO2 on combustion.Other gases (variouspollutants) are potent“greenhouse gases”


4

Carbon Cycle

Photosynthesis converts CO2 to carbohydrates (CH2O)n

Carbohydrates undergo reaction with oxygen (combustion) to yield H2O and CO2

In the past 100-200 years, more combustion occurs than photosynthesis, resulting in an increase in CO2 relative to (CH2O)n


5

Reactions

Combustion (reaction with oxygen) is one of a number of possible reactions that chemicals can undergo

Many reactions occur in water, especially since water can hold ionic and polar compounds in solution


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Setting a Goal – Part AThe Representation of Chemical Changes and Three Types of Changes

You will learn to use the symbolic language of chemistry by writing balanced chemical reactions for several identifiable reaction types


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Objectives for Section 6-1 Write a chemical equation from a

word description of the reaction Balance a simple chemical equation

by inspection


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Professor’s Little Joke

Old chemistry professors never die, they just fail to react


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6-1 Chemical Equations

A chemical equation is the representation of a chemical reaction using the symbols of the elements and the formulas of the compounds

The reacting species (starting materials or reactants) are listed on the left, and the products of the reaction are listed on the right


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Example Equation

N2(g) + 3 H2(g) 2 NH3(g)

reactants products

phases


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Chemical Equations

Chemical equations are the representation of a chemical reaction using the symbols of the elements and the formulas of the compounds

The arrow means “produces” or “yields” Reactants are shown to the left of the arrow, and

products are shown to the right of the arrow A plus sign is used to indicate that substances

combine or react with each other


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Chemical Equations

Symbols commonly used in chemical equations are listed in Table 6-1 (memorize this)

We balance chemical equations because of the Law of the Conservation of Mass

We use coefficients in front of the element or compound formula, which are whole numbers that adjust the amounts of the species

Note that we cannot balance an equation by changing the subscripts of the compound


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Balancing Coefficients

The subscripts of the compound are fixed: they cannot be changed in an equation

The coefficients used should be the smallest whole numbers possible

The coefficient multiplies every number in the formula (e.g. 2 MgCl2 means that there are 2 Mg atoms and 4 Cl atoms)

We will balance equations by inspection


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Balancing Strategies

Consider PbO2 + HCl → PbCl2 + Cl2 + H2O Balance Pb first, then O, H and finally Cl

Cl last because it is in its free state, Cl2

Pb first since it is not H or O O because it is bonded to Pb (in PbO2)

H because it is bonded to O (in H2O)

PbO2 + 4HCl PbCl2 + Cl2 + 2H2O


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Combustion, Combination and Decomposition Reactions

All chemical reactions can be represented by balanced chemical equations

These reactions can also be classified into one of five groups (although other groupings are possible)

Objective for next section is to classify certain chemical reactions as being combustion, combination or decomposition


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Objective for Section 6-2

Classify certain chemical reactions as being combustion, combination, or decomposition reactions


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6-2 Combustion, Combination, and Decomposition Reactions

Combustion Reactions Involve the combination of an element or compound

with oxygen (as O2) Usually liberates a lot of heat, and may be

accompanied by a flame Combustion reactions may be incomplete,

especially if there is insufficient oxygen.

E.g. 2C8H18(l) + 17O2(g) 16CO(g) + 18H2O(l)


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Combustion Reactions

Balancing combustion reactions involving hydrocarbons is best done by starting with C, then H, followed by O

Consider the complete combustion of ethane. The products are CO2 and H2O

The initial balanced equation is C2H6 (g) + 7/2O2 (g) 2CO2(g) + 3H2O(l)

Converting to whole numbers gives:

2C2H6 (g) + 7O2 (g) 4CO2(g) + 6H2O(l)


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Combustion Reactions - Example

Strip of magnesium combusting


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Combination Reactions

Often called synthesis reactions Involves two or more substances

combining to form a single substance Consider:

2Mg(s) + O2 (g) 2MgO(s)

2Na(s) + Cl2(g) 2NaCl(s)

NH3(g) + HCl(g) NH4Cl(s)


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Decomposition Reactions

The reverse of a combination (or synthesis) reaction

One substance breaking apart into two or more substances

Consider:H2CO3(aq) H2O(l) + CO2(g)

N2O4(g) 2NO2(g)


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Setting a Goal - Part BIons In Water And How They React

You will learn how ions are formed in aqueous solution and how these ions interact in two important types of chemical reactions


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Objective for Section 6-3 Write the ions formed when ionic

compounds or acids dissolve in water


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6-3 The Formation of Ions in Water

When we consider a substance dissolving in another substance, we define the solute as the substance in lesser amount, and the solvent as the substance in greater amount

Solubility is the relative degree to which a substance dissolves in a solvent

Soluble substances dissolve to a considerable degree

Insoluble substances do not dissolve at all or only to a small degree


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Dissolution of Ionic Compounds When ionic compounds dissolve in water, they dissociate into separate

anions and cations:

NaCl(s) Na+(aq) + Cl-(aq) See Figure 6-4 The ions are often highly solvated. Ion solvation is an energetically

favorable process; the energy lost more than compensates for the energy needed to break down the crystal lattice

H2O


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Dissolution of Ionic Compounds - Example

Sodium chloride crystallattice (NaCl(s))Na+ Cl-

Water molecules

Na+

Cl-

Cl- Na+

Cl-

Na+

Solvated ions


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Dissolution of Polyatomic Salts

It is important to be able to recognize polyatomic ions, since they do not dissociate into simple ions H2O

Ca(ClO4)2(s) → Ca2+(aq) + 2 ClO4-(aq)

The salts do dissociate into anions and cations, but the polyatomic ions remain intact


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Dissolution of Acids

Acids are so named because when they dissolve, they ionize to yield H+

and the associated anionHCl(aq) H+(aq) + Cl-(aq)

or HCl(g) + aq H+(aq) + Cl-(aq)


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Acids Strong acids completely dissociate into

ions upon dissolution in water Examples are HX (X = Cl-, Br-, I-), H2SO4,

HNO3, HClO4

Weak acids dissolve, but do not completely dissociate into H+ and anions

Examples are HF, HCN, HOCl, HCO2H and CH3CO2H (+ most organic carboxylic acids)


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Acids – Indicator Test

The indicatorphenolphthalein is pink in alkaline solution and colorless in acidsolution


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Objective for Section 6-4 Given the activity series, complete

several single-replacement reactions as balanced molecular, total ionic, and net ionic equations


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6-4 Single-Replacement Reactions

An element replaces another in a compoundZn(s) + CuCl2(aq) ZnCl2(aq) + Cu(s)

This particular form of the equation is termed the molecular equation, where all reactants and products are shown as neutral compounds


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Reaction of Zn with Cu2+

Immersing a strip of Zn in a solution containing Cu2+ causes Cu metal to form a coat on the Zn strip

See Figure 6-5.


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Ionic Equations

The total ionic equation separates all of the anions and cations Zn(s) + Cu2+(aq) + 2Cl-(aq)

Zn2+ (aq) + 2Cl-(aq) + Cu(s) Spectator ions (such as the Cl- ions)

are those that are in the same state on both sides of the equation


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Net Ionic Equation

The net ionic equation leaves out the spectator ionsThe complete ionic equation:Zn(s) + Cu2+(aq) + 2Cl-(aq)

Zn2+(aq) + 2Cl-(aq) + Cu(s)The net ionic equation:

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)


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Activity Series

Table 6-2 indicates the common metals in decreasing order of their ability to replace metal ions in aqueous solution

When comparing two metals, the one higher in the table replaces the one lower in the table (e.g. Zn can replace Cu but not Al)


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Metal Corresponding Ion

K (most active) Na Ca

MgAlZnCrFeNiSnPbH2

CuAg

Au (least active)

K+

Na+

Ca2+

Mg2+

Al3+

Zn2+

Cr3+

Fe2+

Ni2+

Sn2+

Pb2+

H+

Cu2+

Ag+

Au3+

The Activity Series (Table 6-2)


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Activity Series (Examples)

Feasible (spontaneous)Ni(s) + 2 Ag+(aq) → Ni2+(aq) + 2 Ag(s)

Mg(s) + Zn2+(aq) → Mg2+(aq) + Zn(s)

Not Feasible (no spontaneous reaction)3Zn(s) + 2Al3+(aq) → 3Zn2+(aq) + 2Al(s)

2Ag(s) + Ni2+(aq) → 2 Ag+(aq) + Ni(s)


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Objectives for Section 6-5 Given a table of solubility rules,

determine whether a specific ionic compound is soluble or insoluble in water

Write balanced molecular, total ionic, and net ionic equations for precipitation and neutralization reactions


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6-5 Double Replacement Reactions – Precipitation

Where the anions and cations in an ionic equation exchange counterionsFeCl3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaCl(aq)

Precipitates are indicated by (s) or


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What Causes Double Replacements?

There has to be some process to cause the reaction to take place Precipitation of a solid Evolution of a gas Neutralization reaction between an acid

and a base If one of these processes does not occur,

there is no double replacement reaction


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Examining Reactions

Molecular equation Pb(NO3)2(aq) + 2 NaI(aq) → PbI2(s) + 2 NaNO3(aq)

Total ionic equationPb2+(aq) + 2 NO3

-(aq) + 2 Na+(aq) + 2I-(aq) → PbI2(s) + 2 Na+(aq) + 2 NO3

-(aq) Net ionic equation

Pb2+(aq) + 2I-(aq) → PbI2(s)


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Types of Double Replacement Reactions

PrecipitationAgNO3(aq) + NaCl(aq) AgCl(s)

+ NaNO3(aq) Gas evolution

2HCl(aq) + Na2CO3(aq) 2NaCl(aq) + CO2(g)

Formation of a molecular compoundHCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)


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Precipitation Reactions

Formation of a precipitate occurs when solutions of two soluble ionic compounds are mixed to produce an insoluble salt

The two cations exchange anions in such reactions


45

Precipitation

Use the solubility rules (Table 6-3) to predict whether two ions combine to form an insoluble salt

Pb(NO3)2(aq) + 2 NaI(aq) → PbI2(s) + 2 NaNO3(aq)

NH4NO3 (aq) + NaI(aq) → no precipitate


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Anion Solubility Rule

Mostly soluble

Cl-, Br-, I- All cations form soluble compounds, except Ag+, Hg22+,

and Pb2+. (PbCl2 and PbBr2 are slightly soluble)

NO3-, ClO4

-, CH3CO2-

(acetate)All cations form soluble compounds. (KClO4 and

CH3CO2Ag are slightly soluble)

SO42- All cations form soluble compounds, except Pb2+, Ba2+

and Sr2+. (Ca2+ and Ag+ form slightly soluble compounds)

Mostly insoluble

CO32-, PO4

3- All cations form insoluble compounds, except Group IA metals and NH4

+

S2- All cations form insoluble compounds, except Group IA and Group IIA metals and NH4

+

OH- All cations form insoluble compounds, except Group IA metals, Ba2+, Sr2+, and NH4

+. [Ca(OH)2 is slightly soluble]

Solubility Rules for Some Ionic Compounds (Table 6-3)


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What Happens if all Salts are Soluble?

Mixing aqueous solutions of CuCl2 and KNO3 results in a solution containing all four ions in solution


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Analyzing a Possible Precipitation Reaction

When aqueous solutions of two salts are mixed, consider the solubility of the salts that would result if the anions and cations exchanged partners

For example, if a solution of Na2CO3 is mixed with a solution of CaCl2, you need to consider the solubility of CaCO3 and NaCl

If one of the new salts is insoluble, then a precipitate forms


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Objective for Section 6-6

Write balanced molecular, total ionic, and net ionic equations for neutralization reactions


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6-6 Double-Replacement Reactions - Neutralization

Neutralization In water, an acid is a proton donor In water, a base is a hydroxide donor Proton plus hydroxide yields water

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)


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Strong Acids and Bases

Both chemical types are hazardous Strong acids completely ionize into

H+(aq) and an anion Strong bases dissolve in water to

produce the hydroxide ion, OH-

The strong bases are the hydroxides of the group IA and IIA metal ions (except Be)


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Reaction of Strong Acids and Bases

When solutions of strong acids and bases are mixed, H+ ion and the OH- ion react to form the molecular compound H2O

The other product is a salt This double replacement reaction of

acids and bases is termed a neutralization reaction


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Examples

HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq) The net ionic equation for all strong acid-

strong base neutralization reactions is the sameH+(aq) + OH-(aq) → H2O(l)

The key to balancing neutralization reactions is to ensure that there is one H+ for each OH-

malone and dolter - basic concepts of chemistry 9e1 chapter 6 chemical reactions

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