Chapter 7

Chemical Reactions

Tro's "Introductory

Chemistry", Chapter

2

Evidence of Chemical Change

Color Change

Formation of Solid Precipitate Formation of a Gas

Emission of Light Release or Absorption of Heat

3

How Do We Represent A Chemical

Reaction?

• Reactions are represented by Chemical Equations

Chemical Equations

• Short-hand way of describing a reaction.

• Provides information about the reaction. Formulas of reactants and products.

States of reactants and products.

Relative numbers of reactant and product molecules that are required.

Can be used to determine masses of reactants used and products that can be made.

4

Symbols Used in Equations • Symbols used to indicate state after chemical.

(g) = gas; (l) = liquid; (s) = solid.

(aq) = aqueous = dissolved in water.

C(s) + O2(g) CO2(g)

reactants products

• Energy symbols used are:

D = heat.

hn = light.

5

Conservation of Mass

• Matter cannot be created or destroyed.

Therefore, the total mass cannot change.

And the total mass of the reactants will be the

same as the total mass of the products.

• In a chemical reaction, all the atoms present

at the beginning are still present at the end.

If all the atoms are still there, then the mass will

not change.

6

Balancing Equations activity

Rules for balancing equations:

-Balance only one element at a time

-Always balance free elements last

7

Aqueous Solutions

• Many times, the chemicals we are reacting together are dissolved in water.

Mixtures of a chemical dissolved in water are called aqueous solutions.

• Dissolving the chemicals in water helps them to react together faster.

The water separates the chemicals into individual molecules or ions.

The separate, free-floating particles come in contact more frequently so the reaction speeds up.

8

Dissociation • When ionic compounds dissolve

in water, the anions and cations

are separated from each other.

This is called dissociation.

However, not all ionic compounds

are soluble in water!

• When compounds containing

polyatomic ions dissociate, the

polyatomic group stays together

as one ion.

9

Dissociation, Continued • Potassium iodide dissociates in water into

potassium cations and iodide anions.

KI(aq) → K+1(aq) + I-1(aq)

• Copper(II) sulfate dissociates in water into

copper(II) cations and sulfate anions.

CuSO4(aq) → Cu+2(aq) + SO4-2(aq)

K+1 I-1 K I

Cu+2 SO4-2 Cu SO4

10

Dissociation, Continued

• Potassium sulfate dissociates in water into

potassium cations and sulfate anions.

K2SO4(aq) → 2 K+1(aq) + SO4-2(aq)

K+1

SO4-2

K+1

K K SO4

Tro's "Introductory

Chemistry", Chapter

11

Electrolytes

• Electrolytes---

substances whose water

solution is a conductor

of electricity.

• All electrolytes have

ions dissolved in water.

Tro's "Introductory

Chemistry", Chapter

12

Electrolytes, Continued

• In strong electrolytes, all the

electrolyte molecules or formula

units are separated into ions.

• In weak electrolytes, a small

percentage of the molecules are

separated into ions

• In nonelectrolytes, none of the

molecules are separated into

ions.

Tro's "Introductory

Chemistry", Chapter

13

Types of Electrolytes

• Salts = Water soluble ionic compounds.

Mostly strong electrolytes.

• Acids = Form H+ ions and anions in water

solution.

Sour taste.

React and dissolve many metals.

• Bases = Water-soluble metal hydroxides.

Form OH- ions in water solution.

Bitter taste, slippery (soapy) feeling solutions.

14

When Will a Salt Dissolve?

• A compound is soluble in a liquid if it dissolves in that liquid.

NaCl is soluble in water, but AgCl is not.

• A compound is insoluble if a significant amount does not dissolve in that liquid.

AgCl is insoluble in water.

Though there is a very small amount dissolved, but not enough to be significant.

15

Compounds containing the

following ions are generally

soluble

Exceptions

(when combined with ions on the

left the compound is insoluble)

Li+, Na+, K+, NH4+ none

NO3–, C2H3O2

– none

Cl–, Br–, I– Ag+, Hg22+, Pb2+

SO42– Ca2+, Sr2+, Ba2+, Pb2+

Solubility Rules:

Compounds that Are Generally Soluble in Water

16

Compounds containing the

following ions are generally

insoluble

Exceptions

(when combined with ions on the

left the compound is soluble or

slightly soluble)

OH– Li+, Na+, K+, NH4+, H+

Ca2+, Sr2+, Ba2+

S2– Li+, Na+, K+, NH4+, H+

Ca2+, Sr2+, Ba2+

CO32–, PO4

3– Li+, Na+, K+, NH4+, H+

Solubility Rules:

Compounds that Are Generally Insoluble

17

Using the Solubility Rules to Predict an

Ionic Compound’s Solubility in Water

• First check the cation: If it is Li+, Na+, K+, or

NH4+, then the compound will be soluble in water.

Regardless of the anion.

• If the cation is not Li+, Na+, K+, or NH4+, then

follow the rule for the anion.

• If a rule says the compounds are mostly soluble,

then the exceptions are insoluble.

• If a rule says the compounds are mostly insoluble,

then the exceptions are soluble.

Note: slightly soluble insoluble.

18

Examples of Soluble and Insoluble

Compounds

• KOH Soluble, because the cation is K+.

• AgBr Insoluble, even though most compounds

with Br− are soluble, this is an exception.

• CaCl2 Soluble, most compounds with Cl− are

soluble.

• Pb(NO3)2 Soluble, because the anion is NO3−.

• PbSO4 Insoluble, even though most compounds

with SO42− are soluble, this is an exception.

Types of Reactions (Part A)

• Precipitation Reactions

• Acid-Base Reactions

• Gas Evolution Reactions

19

20

Reaction Type 1 A:

Precipitation Reactions

• A reaction that takes place from the exchange of cations and anions

• This results in the formation of a solid known as a precipitate

• Note: The reactants can be in any state other than the solid state

• .

21

Precipitation Reactions, Continued

2 KI(aq) + Pb(NO3)2(aq) 2 KNO3(aq) + PbI2(s)

22

No Precipitate Formation =

No Reaction KI(aq) + NaCl(aq) KCl(aq) + NaI(aq)

All ions still present, no reaction.

23

Process for Predicting the Products of

a Precipitation Reaction 1. Write the formula for the reactants and Determine

what ions each aqueous reactant has.

2. Exchange ions. (+) ion from one reactant with (-) ion from the other.

3. Balance charges of combined ions to get formula of each product.

4. Balance the equation. Count atoms.

5. Determine solubility of each product in water. Use the solubility rules.

If product is insoluble or slightly soluble, it will precipitate.

If neither product will precipitate, no reaction.

24

Example 7.7—Write a balanced chemical equation for

the reaction between an Aqueous Solution of Sodium

Carbonate and an Aqueous Solution of Copper(II)

Chloride 1. Write the form the reactants and Determine

the ions present when each reactant

dissociates.

Na2CO3(aq) + CuCl2(aq)

(Na+ + CO32-) + (Cu+2 + Cl-)

2. Exchange the ions.

(Na+ + CO32-) + (Cu+2 + Cl-) (Na+ + Cl-) + (Cu+2 + CO3

2-)

25

3. Write the formulas of the products.

Cross charges and reduce.

Na2CO3(aq) + CuCl2(aq) NaCl + CuCO3

4. Balance the equation.

Na2CO3(aq) + CuCl2(aq) 2 NaCl + CuCO3

Example 7.7—Write a balanced chemical

equation for the reaction between an Aqueous

Solution of Sodium Carbonate and an

Aqueous Solution of Copper(II) Chloride

26

5. Determine the solubility of each product.

Write an (s) after the insoluble products and a

(aq) after the soluble products

NaCl is soluble.

CuCO3 is insoluble.

Na2CO3(aq) + CuCl2(aq) 2 NaCl(aq) + CuCO3(s)

Example 7.7—Write a balanced chemical

equation for the reaction between an Aqueous

Solution of Sodium Carbonate and an

Aqueous Solution of Copper(II) Chloride

27

Practice–Predict the Products and Balance

the Equation

• KCl(aq) + AgNO3(aq)

• Na2S(aq) + CaCl2(aq)

28

Reaction Type 2 A:

Acid–Base Reactions • Also called neutralization reactions

• Acid contains H+ and base contains OH-

• In all acid-base reactions, the H+ and the OH- combine to

make H2O.

acid + base salt + water

2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l)

29

Predicting the products of an acid-base reactions:

Example 7.11—Write the Molecular Equation for the

Reaction of Aqueous Nitric Acid with Aqueous

Calcium Hydroxide.

1. Write the formulas of the reactants.

HNO3(aq) + Ca(OH)2(aq)

2. Since this is an acid-base reaction, one of the products

must be water! H2O(l).

HNO3(aq) + Ca(OH)2(aq) H2O(l) + other product

30

3. The other product will be formed by combining the ions other than H+ and OH-.

i.e. NO3− and Ca2+

HNO3(aq) + Ca(OH)2(aq) H2O(l) + Ca(NO3)2

4. Balance the equation.

2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2 + 2 H2O(l)

Example 7.11—Write the Molecular Equation for the

Reaction of Aqueous Nitric Acid with Aqueous

Calcium Hydroxide, Continued.

31

5. Determine the solubility of the salt.

Ca(NO3)2 is soluble.

a. Write an (s) after an insoluble salt and an (aq)

after a soluble salt.

2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l)

Example 7.11—Write the Molecular, Ionic, and Net-

Ionic Equation for the Reaction of Aqueous Nitric

Acid with Aqueous Calcium Hydroxide, Continued.

32

Practice—Complete and Balance These

Acid–Base Reactions.

NH4OH(aq) + H2SO4(aq)

Al(OH)3(aq) + H2SO3(aq)

Ba(OH)2(aq) + HCl(aq)

33

Reaction type 3A:

Gas Evolution Reactions • Reactions that produce a gas.

Note: the reactants can be in any state other than a gas.

• To identify a gas evolution reaction one of the reactant is usually an acid or a base but not both.

K2S(aq) + H2SO4(aq) K2SO4(aq) + H2S(g)

• Note that if an acid and a base are present as reactants then the

reaction will be an acid-base reaction.

• Other reactions form a gas by the decomposition of one of their products into a gas and water.

34

Predicting the Products of

Gas Evolving Reactions (Table 7.4) Reactant

type

Reacting

with

Ion

exchange

product

Decom-

pose?

Gas

formed

Example

MetalnS,

metal HS

Acid H2S No H2S K2S(aq) + 2HCl(aq)

2KCl(aq) + H2S(g)

MetalnCO3,

metal HCO3

Acid H2CO3 Yes CO2 K2CO3(aq) + 2HCl(aq)

2KCl(aq) + CO2(g) + H2O(l)

MetalnSO3

metal HSO3

Acid H2SO3 Yes SO2 K2SO3(aq) + 2HCl(aq)

2KCl(aq) + SO2(g) + H2O(l)

(NH4)nanion Base NH4OH Yes NH3 KOH(aq) + NH4Cl(aq)

KCl(aq) + NH3(g) + H2O(l)

Decomposition products

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

H2SO3(aq) SO2(g) + H2O(l)

NH4OH(aq) NH3(g) + H2O(l)

35

Process for Predicting the Products of

a Gas-Evolving Reaction

1. Write the formula of the reactants

2. Use table 7.4 to predict the ion exchange product i.e.: H2S, H2CO3, H2SO3, or NH4OH

3. Form the second product from the remaining cation and anion

4. Check to see if either product decomposes. If so, rewrite as H2O(l) and a gas.

See Table 7.4

5. Balance the equation.

6. Determine solubility of other product in water.

36

Example—Write a molecular equation for the reaction

between an Aqueous Solution of Sodium Sulfite and

an Aqueous Solution of Nitric Acid.

1. Write the formulas of the reactants.

Na2SO3(aq) + HNO3(aq)

2. Use table 7.4 to predict the ion exchange

product i.e.: H2S, H2CO3, H2SO3, or NH4OH

Na2SO3(aq) + HNO3(aq) H2SO3(aq) + other product

37

3. Form the second product from the remaining cation

and anion

Na2SO3(aq) + HNO3(aq) NaNO3 + H2SO3

4. Check to see if either product decomposes. If so, rewrite as H2O(l) and a gas.

See Table 7.4

Na2SO3(aq) + HNO3(aq) NaNO3 + SO2(g) + H2O(l)

Example—Write a molecular equation for the

reaction between an Aqueous Solution of

Sodium Sulfite and an Aqueous Solution of

Nitric Acid.

38

6. Balance the equation.

Na2SO3(aq) + 2 HNO3(aq) 2 NaNO3 + SO2(g) + H2O(l)

7. Determine the solubility of other product.

NaNO3 is soluble.

a. Write an (s) after the insoluble products and an

(aq) after the soluble products.

Na2SO3(aq) + 2 HNO3(aq) 2 NaNO3(aq) + SO2(g) + H2O(l)

Example—When an Aqueous Solution of Sodium

Sulfite Is Added to an Aqueous Solution of Nitric

Acid, a Gas Evolves, Continued.

39

Practice—Complete the Following Reactions.

PbS(s) + H2SO4(aq)

HNO3(aq) + NaHCO3(aq)

40

Ionic Equations

• Equations that describe the chemicals put into the water and

the product molecules are called molecular equations.

2 KOH(aq) + Mg(NO3)2(aq) 2 KNO3(aq) + Mg(OH)2(s)

• Equations that describe the chemicals in terms of ions are

called complete ionic equations.

Aqueous electrolytes are written as ions.

Soluble salts, strong acids, strong bases.

Insoluble substances and nonelectrolytes written in molecule form.

Solids, liquids, and gases are not dissolved, therefore, molecule form.

2K+1(aq) + 2OH-1

(aq) + Mg+2(aq) + 2NO3

-1(aq) 2K+1

(aq) + 2NO3-1

(aq) + Mg(OH)2(s)

Ionic Equations

• Precipitation, acid-base, and gas

evolution reactions can all be written as

ionic equations.

• When Writing Ionic Equations

Only substances in an aqueous state can be

written as ions

41

42

Writing Ionic Equations

• Only aqueous substances can be written as ions

2 KOH(aq) + Mg(NO3)2(aq) 2 KNO3(aq) + Mg(OH)2(s)

2K+1(aq) + 2OH-1

(aq) + Mg+2(aq) + 2NO3

-1(aq) 2K+1

(aq) + 2NO3-1

(aq) + Mg(OH)2(s)

43

Net Ionic Equations

• Ions that are both reactants and products are called spectator ions.

2K+1(aq) + 2OH-1

(aq) + Mg+2(aq) + 2NO3

-1(aq) 2K+1

(aq) + 2NO3-1

(aq) + Mg(OH)2(s)

• An ionic equation in which the spectator ions are

removed is called a net ionic equation.

2OH-1(aq) + Mg+2

(aq) Mg(OH)2(s)

44

**Example**–Write the Ionic and Net Ionic

Equation.

K2SO4(aq) + Ba(NO3)2(aq) 2 KNO3(aq) + BaSO4(s)

45

Other Reaction Types

• Oxidation-reduction reactions

• Combustion reactions

• Decomposition

• Synthesis or combination reactions

• Single Displacement reactions

• Double Displacement reactions

46

Reduction-Oxidation Reactions

• Also known as RedOx

• Involves transferring electrons from one atom to

another

• Can be easily identified as

Reactions in which either one or more of the reactants or

propducts are free elements.

Reactions that involve a change in oxidation state from

reactant to product

47

Oxidation–Reduction Reactions

• Oxidation: An element is oxidized when:

it loses electrons OR

when there is an increase in the oxidation state

• Reduction: An element is reduced when:

it gains electrons OR

when there is an decrease in the oxidation state

• Whenever you think of Oxidation-

Reduction reactions, think of OIL-RIG

• Oxidation

• Is

• Loss

• Reduction

• Is

• Gain

What is “OIL-RIG”?

49

Electron Loss/Gain as Redox

• In the following reaction:

Mg(s) + S(s) MgS(s)

• The magnesium atoms are oxidized.

Mg0 Mg2+ + 2 e

• The oxygen atoms are reduced.

S0 + 2 e S2

50

Practice—Decide Whether Each of the Following

Reactions Is a Redox Reaction, Continued.

2 Al(s) + 3 Br2(l) 2 AlBr3(s)

CaSO3(s) + 2 HCl(aq) CaCl2(aq) + SO2(g) + H2O(l)

Fe2O3(s) + C(s) 2 Fe(s) + 3 CO(g)

SO2(g) + O2(g) + H2O(l) H2SO4(aq)

—Yes, metal + nonmetal.

—No, this is a gas evolving reaction.

—Yes

—Yes, O2 reactant.

51

Combustion Reactions

• Reactions in which O2(g) is a reactant are called combustion reactions.

• Combustion reactions release lots of energy. They are exothermic.

• Combustion reactions are a subclass of oxidation–reduction reactions.

2 C8H18(g) + 25 O2(g) 16 CO2(g) + 18 H2O(g)

52

• To predict the products of a combustion

reaction, combine each element in the other

reactant with oxygen.

Reactant Combustion product

Contains C CO2(g)

Contains H H2O(g)

Contains S SO2(g)

Contains N NO(g) or NO2(g)

Contains metal M2On(s)

Products of Combustion

53

Example—Write the Equation for Each Reaction

• Combustion of the anesthetic cyclopropane,

C3H6(g).

2 C3H6(g) + 9 O2(g) 6 CO2(g) + 6 H2O(g)

• Combustion of the non-toxic antifreeze propylene

glycol, C3H8O2(l).

2 C3H6O2(l) + 7 O2(g) 6 CO2(g) + 6 H2O(g)

54

Classifying Reactions, Continued • Another scheme classifies reactions by what

the atoms do.

Type of reaction General equation

Synthesis A + B AB

Decomposition AB A + B

Single Displacement A + BC AC + B

Double displacement AB + CD AD + CB

55

Combination Reactions • Also known as composition or synthesis

reactions.

• Two (or more) reactants combine together to

make one product.

Simpler substances combining together.

2 CO + O2 2 CO2

2 Mg + O2 2 MgO

HgI2 + 2 KI K2HgI4

56

Decomposition Reactions

• A large molecule is broken apart into smaller molecules or its elements.

Caused by addition of energy into the molecule.

• Have only one reactant, makes 2 or more products.

2h

3

2

223

O 3 O 2

O Hg 2 HgO 2

Cl FeCl 2 FeCl 2

D

)(

)()()(

g(l)(s)

glselec

Tro's "Introductory

Chemistry", Chapter

57

Single Displacement Reactions

• Reactions that involve one atom displacing another and replacing it in a compound.

• In the reaction Zn(s) + 2 HCl(aq) ZnCl2(aq) + H2(g), the atom Zn displaces H from the compound.

• Other examples of displacement reactions are:

Fe2O3(s) + Al(s) Fe(s) + Al2O3(s)

2 Na(s) + 2 H2O(aq) 2 NaOH(aq) + H2(g)

58

Double Displacement Reactions

• Two ionic compounds exchange ions.

• May be followed by decomposition of one

of the products to make a gas.

• X Yq (aq) + A Bq (aq) XB + AY

• Precipitation, acid–base, and gas evolving

reactions are also double displacement

reactions.

59

Examples of Double Displacement

CaCl2(aq) + Na2CO3(aq) CaCO3(s) + 2 NaCl(aq)

Ba(OH)2(s) + 2 HNO3(aq) Ba(NO3)2(aq) + 2 H2O(l)

Li2CO3(aq) + 2 HCl(aq) 2 LiCl(aq) + CO2(g) + H2O(l)

Tro's "Introductory

Chemistry", Chapter

60

Example—Classify the Following Reactions as

Synthesis, Decomposition, Single Displacement, or

Double Displacement, Continued.

3 Mg(s) + 2 FeCl3(aq) 3 MgCl2(aq) + 2 Fe(s)

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

3 KOH(aq) + H3PO4(aq) K3PO4(aq) + 3 H2O(l)

)(CO )CaO( )(CaCO 23 gssheat

Single displacement.

Double displacement.

Decomposition.

Synthesis.

61

Recommended Study Problems Chapter 7

NB: Study problems are used to check the student’s understanding

of the lecture material. Students are EXPECTED TO BE ABLE

TO SOLVE ALL THE SUGGESTED STUDY PROBLEMS.

If you encounter any problems, please talk to your professor or seek

help at the HACC-Gettysburg learning center.

Questions from text book Chapter 7, p 232

5, 7, 29, 37, 53, 55, 61, 65, 67, 75, 83, 85, 87, 89, 93, 115

ANSWERS

-The answers to the odd-numbered study problems are found at

the back of your textbook