1. Chemical Reactions and Equations
Chapter 5:
Chemical Reactions and Equations
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Questions for Consideration
What happens in a chemical reaction?
How do we know whether a chemical reaction takes place?
How do we represent a chemical reaction with a chemical equation?
How are chemical reactions classified? How can the products of different classes of chemical reactions be predicted?
How do we represent chemical reactions in aqueous solution?

3. Chapter 5 Topics: What is a Chemical Reaction?
How Do We Know a Chemical Reaction Occurs?
Writing Chemical Equations
Predicting Chemical Reactions
Representing Reactions in Aqueous Solution

4. Introduction Chemical reactions occur all around us.
How do we make sense of these changes? What patterns can we find?
Figure 5.1
Figure 5.29
Figure 5.8F

5. 5.1 What is a Chemical Reaction?
A chemical reaction is a chemical change.
A chemical reaction occurs when one or more substances is converted into one or more new substances.
Reactant – a substance that we start with that undergoes a change
Product – a new substance that forms during the reaction
Products differ from reactants only in the arrangement of their component atoms.

6. 5.2 How do We Know a Chemical Reaction Occurs?
What observations might indicate that a chemical reaction has taken place?
Figure 5.7

7. What clues tell you a reaction is likely occurring?
Figure 5.8

8. Physical Clues of a Chemical Reaction
Common observations that may accompany a chemical reaction are:
change in color
production of light
formation of a solid (such as a precipitate in solution, or smoke in air, or a metal coating)
formation of a gas (bubbles in solution or fumes in the gaseous state)
absorption or release of heat (sometimes appearing as a flame)

9. 5.3 Writing Chemical Equations
A chemical equation is a symbolic representation of a chemical reaction.
A chemical equation shows:
the formulas for reactants and products
the physical states of each substance (s, l, g, aq)
relative numbers of reactants that combine and products that form
special conditions required for the reaction, such as constant heating.

10. Chemical Reactions
When hydrogen gas is ignited in the presence of oxygen, an explosive reaction occurs producing gaseous water molecules.
Is mass conserved?
Figure 5.5
Figure 5.6

11. Writing Chemical Equations
Balanced equation
The number of atoms of each element is the same in the products as in the reactants.
Conservation of mass is always maintained.

12. Writing Chemical Equations
When a powdered mixture of aluminum metal and iron(III) oxide is heated, it reacts to form liquid iron metal and aluminum(III) oxide.
Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l)
Figure 5.9

13. Chemical Equations Must be Balanced
Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l)
Is this equation balanced?
What has to change?
Figure from p. 171

14. Balance Equations with Coefficients
Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l)
We balance the atoms in equations with coefficients.
The aluminum atoms are not balanced so we place a coefficient of 2 in front of the Al reactant:
2Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l)
The iron atoms are not balanced so we place a coefficient of 2 in front of the Fe product:
2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(l)
Figure
from
p. 172

15. Writing Chemical Equations
Methane + oxygen  carbon dioxide + water
CH4(g) + O2(g)  CO2(g) + H2O(g)
The number of atoms of each element in the unbalanced equation is shown below. These numbers were obtained by multiplying the subscript to the right of the element’s symbol by the stoichiometric coefficient.
# of atoms (reactants)
# of atoms (products)
1 C
4 H
2 H
2 O
3 O

16. Writing Chemical Equations
CH4(g) + O2(g)  CO2(g) + H2O(g)
First, we look at the carbon atoms. Since the number of carbon atoms on the reactant side is already equal to the number of carbon atoms on the product side, we don’t need to add coefficients.
Next, we look at the hydrogen atoms. Currently, there are four hydrogen atoms on the reactant side and 2 hydrogen atoms on the product side. Thus, we need to add a coefficient of 2 in front of water to make the hydrogen atoms equal.
CH4(g) + O2(g)  CO2(g) + 2H2O(g)

17. Writing Chemical Equations
CH4(g) + O2(g)  CO2(g) + 2H2O(g)
Finally, we look at the oxygen atoms. Currently, there are 2 oxygen atoms on the reactant side and 4 oxygen atoms (combined from carbon dioxide and water) on the product side. Thus, we add a coefficient of 2 in front of the oxygen gas.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
Now the equation is balanced!

18. A General Approach to Balancing Equations
Identify the reactants and products and write their correct formulas.
Write a skeletal equation including physical states.
Change coefficients one at a time until the atoms of each element are balanced. (Start with the elements that occur least often in the equation.)
Make a final check by counting the atoms of each element on both sides of the equation.

19. Activity: Balancing Equations
Balance the following equations:
Potassium chlorate  potassium chloride + oxygen
Aluminum acetate reacts with potassium sulfate to form potassium acetate and aluminum sulfate
Hexane (C6H14) reacts with oxygen gas to form carbon dioxide and water
Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas

20. Activity Solutions: Balancing Equations
Potassium chlorate  potassium chloride + oxygen
First, translate the chemical names into chemical formulas:
Potassium chlorate  potassium chloride + oxygen
K+ ClO3− K+ Cl− O (diatomic)
KClO3(s)  KCl(aq) O2(g)
Next, balance the chemical equation:
2KClO3(s)  2KCl(aq) + 3O2(g)

21. Activity Solutions: Balancing Equations
Aluminum acetate reacts with potassium sulfate to form potassium acetate and aluminum sulfate
Aluminum acetate + potassium sulfate 
Al(C2H3O2) K2SO4 
potassium acetate + aluminum sulfate
KC2H3O Al2(SO4)3
Al(C2H3O2)3(aq) + K2SO4(aq) 
KC2H3O2(aq) + Al2(SO4)3(s)
Balanced:
2Al(C2H3O2)3(aq) + 3K2SO4(aq) 
6KC2H3O2(aq) + Al2(SO4)3(s)

22. Activity Solutions: Balancing Equations
Hexane (C6H14) reacts with oxygen gas to form carbon dioxide and water
Hexane was given. Oxygen gas is diatomic. Carbon dioxide has 1 carbon atom and 2 oxygen atoms (using the Greek prefixes) and water has 2 hydrogen atoms and 1 oxygen atom.
C6H14(l) + O2(g)  CO2(g) + H2O(g)
Balanced:
C6H14(l) + 9.5O2(g)  6CO2(g) + 7H2O(g)
Fractional coefficients are not acceptable, so we multiply all coefficients by 2:
2C6H14(l) + 19O2(g)  12CO2(g) + 14H2O(g)

23. Activity Solutions: Balancing Equations
Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas
Zinc + Hydrochloric acid  Zinc chloride + hydrogen
Zn(s) + HCl(aq)  ZnCl2(aq) + H2(g)
Balanced:
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)

24. 5.4 Predicting Chemical Reactions
Since chemical reactions are rearrangements of atoms, let’s look at the possible rearrangements for the following two elements and two compounds.
A and B are elements, either monatomic or diatomic.
C, D, E, and F might be atoms, monatomic ions, or polyatomic ions
When making a list of the possible reactions, do not react more than two elements or compounds.
Figure from p. 178

25. Each compound could react to form its elements:
Figure from p. 179
Compound AB could react similarly.

26. The elements could combine to form a compound:
Figure from p. 179
Similarly, an element could combine with a compound to form a more complex compound:
Figure from p. 179

27. An element could react with a compound to form a new element and a new compound:
Figure from p. 179
In the example, why did zinc displace copper
instead of chlorine?

28. Two compounds can combine to form one new compound:
Figure from p. 179

29. Two compounds can react to form two new compounds:
Figure from p. 179
In the example, why did silver ion combine with chloride ion instead of potassium ion?

30. Summary of Reactions
All of the combinations we looked at fall into one of these four categories:
Table from p. 180

31. Summary of Reactions
These four categories have names:

32. Activity: Classes of Reactions
Classify each of the following as a decomposition, combination, single-displacement, or double-displacement reaction.
NH3(g) + HCl(g)  NH4Cl(s)
CuCl2(aq) + Na2S(aq)  CuS(s) + 2 NaCl(aq)
NiSO3(s)  NiO(s) + SO2(g)
Ca(s) + PbCl2(aq)  CaCl2(aq) + Pb(s)

33. Activity Solutions: Classes of Reactions
Classify each of the following as a decomposition, combination, single-displacement, or double-displacement reaction.
NH3(g) + HCl(g)  NH4Cl(s) Combination
CuCl2(aq) + Na2S(aq)  CuS(s) + 2 NaCl(aq) Double-displacement
NiSO3(s)  NiO(s) + SO2(g) Decomposition
Ca(s) + PbCl2(aq)  CaCl2(aq) + Pb(s)
Single-displacement

34. Activity: Classify each of the following reactions:
Decomposition
Combination
Single-displacement
Double-displacement
Figures from p. 179

35. Activity: Can you classify any of these reactions?
Figure 5.8

36. Activity: Classify this reactionFe Cu
Single Displacement
Figure 5.2

37. Activity: Classify these reactions:
Combination
Decomposition

38. Activity: Classify these reactions:
Single
Displacement
Double
Displacement

39. Activity: Classify this reaction
Figure from p. 199
Combination

40. Activity: Classify this reactionB
Figure from p. 201
Decomposition

41. Activity: Classify this reaction
Figure from p. 202
Single Displacement

42. Activity: Classify this reactionD.
Decomposition
Figure from p. 202

43. Activity: Classify the following reactions, based on the changes happening at an atomic/molecular level.
1. AlF3(aq) + 3H2O(l)  Al(OH)3(s) + 3HF(aq)
2. BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)
3. Ca(OH)2(s)  CaO(s) + H2O(g)
4. Ca(s) + 2H2O(l)  Ca(OH)2(aq) + H2(g)
5. CaO(s) + CO2(g)  CaCO3(s)
6. Cl2(aq) + 2NaI(aq)  2NaCl(aq) + I2(aq)
7. Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
8. Fe(s) + 2HCl(aq)  FeCl2(aq) + H2(g)
9. H2SO3(aq)  H2O(l) + SO2(g)
10. 2HgO(s)  2Hg(l) + O2(g)
11. KOH(aq) + HNO3(aq)  KNO3(aq) + H2O(l)
12. 4Li(s) + O2(g)  2Li2O(s)
13. Na2S(aq) + 2HCl(aq)  2NaCl(aq) + H2S(g)
14. NH3(g) + HCl(g)  NH4Cl(s)
15. NiCO3(s)  NiO(s) + CO2(g)
16. P4(s) + 10F2(g)  4PF5(g)
double displacement
decomposition
single displacement
combination

44. Decomposition Reactions (CD  C + D)
In a decomposition reaction, a single compound breaks down into elements and/or simpler compounds.
2HgO(s)  2Hg(l) + O2(g)
Figure 5.13 Before
Figure 5.13 After
Figure 5.14

46. Activity: Predicting Products
Predict the products and balance the equation for the following decomposition reaction.
Platinum(IV) chloride decomposes to its elements:
PtCl4(s) →
heat
PtCl4(s) → Pt(s) + 2Cl2(g)
heat

47. Combination Reactions (A + B  AB)
During a combination reaction, two substances combine to form a single compound.
Most metals react with most nonmetals to form ionic compounds.
A nonmetal may react with a more reactive nonmetal to form a molecular compound.
A compound and an element may combine to form another compound if one exists with a higher atom: atom ratio.
2CO(g) + O2(g)  2CO2(g)
Figure 5.19
Figure 5.18

48. Combination Reactions

49. Activity: Predicting Products
We can often predict the products of combination reactions involving metal and nonmetal element reactants.
Predict the product and balance the following equation:
Al(s) + Br2(l) 
Figure 5.16
2Al(s) + 3Br2(l)  2AlBr3(s)

50. Single-Displacement Reactions (A + CD  AD + C)
In a single-displacement, a free element displaces another element from a compound to produce a different compound and another free element.
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
Figure 5.22
Figure 5.23

51. Single-Displacement Reactions
2Al(s) + Fe2O3(s)  Al2O3(s) + 2Fe(l)
Fe(s) + CuO(s)  FeO(s) + Cu(s)
What happens to the charge on the metal doing the displacing?
Figure 5.9
Figure 5.24

52. Predicting Single-Displacement Reactions
Does copper displace silver or does silver displace copper? Which is more active?
Figure 5.25

53. Relative Activities (based on reactions we have examined)
Cu > Ag
Fe > Cu
Al > Fe
How can we make predictions without doing experiments?
We can use an activity series, which is a list of elements in order of decreasing reactivity.

54. The Activity Series Will magnesium displace H+? Mg(s) + 2HCl(aq) 
Will aluminum displace H+?
Will iron displace zinc?
Mg(s) + 2HCl(aq) 
MgCl2(aq) +H2(g)
Yes No
Figure 5.21

55. The Activity Series
Write a balanced equation for the reaction of calcium metal with water.
Ca(s) + 2H2O(l)  Ca(OH)2(aq) +H2(g)
Figure 5.20
Figure 5.21

56. Double-Displacement Reactions (CD + EF  CF + ED)
In a double-displacement reaction, two compounds exchange ions to form two new compounds.
There are three types:
Precipitation Reactions
Gas Formation Reactions
Acid-Base Neutralization Reactions

57. Double-Displacement Reactions Precipitation Reactions
In a precipitation reaction, an insoluble solid called a precipitate is formed.
BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)
What is the white solid in the beaker?
Figure 5.26

58. Double-Displacement Reactions Precipitation Reactions
BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)
Which ions undergo a chemical change?
Figure 5.27
Figure 5.26

59. Predicting Precipitation Reactions
How do we know if a compound is insoluble?
See solubility table (Table 5.4).

60. Activity: Predicting Precipitation Reactions
Predict the products of the reaction, including their physical states, and then balance the equation:
Pb(NO3)2(aq) + KI(aq) 
Pb(NO3)2(aq) + 2KI(aq) 
PbI2(s) + 2KNO3(aq)
Figure 5.4

61. Activity: Precipitation Reactions
Jennifer mixes solutions of cadmium(II) nitrate, Cd(NO3)2, and sodium sulfide, Na2S. She obtains an orange precipitate that is the pigment known as cadmium orange. Identify the precipitate, and write a balanced chemical equation for the reaction she carried out.
Cd(NO3)2(aq) + Na2S(aq) 

62. Activity Solution: Precipitation Reactions
The starting solutions contain Cd2+(aq), NO3−(aq), Na+(aq), and S2−(aq).
The solubility rules say that most compounds of sodium are soluble and all nitrates are soluble, so the precipitate must be formed between cadmium(II) ions and sulfide ions.
By matching the positive and negative charges to get electrical neutrality, we can determine the formula of the precipitate to be CdS(s).

63. Activity Solution: Precipitation Reactions
The solution contains Na+(aq) and NO3−(aq) ions in a 1:1 ratio, so we write NaNO3(aq).
We can write the skeletal equation for this reaction as:
Cd(NO3)2(aq) + Na2S(aq)  CdS(s) + NaNO3(aq)
Balanced, the equation is:
Cd(NO3)2(aq) + Na2S(aq)  CdS(s) + 2NaNO3(aq)

64. Extra Activity: Precipitation Reactions
Given that a precipitate forms in this reaction, predict the products and their physical states, and balance the equation.
BaCl2(aq) + Na2CrO4(aq)  ?
BaCl2(aq) + Na2CrO4(aq) 
BaCrO4(s) + 2NaCl(aq)
Figure from p. 190

65. Double-Displacement Reactions Gas-Formation Reactions
In some double-displacement reactions, a gas is formed, which helps drive the reaction to completion.
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2CO3(aq)
CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g)
Metal carbonates react with acids to form carbonic acid, which decomposes to carbon dioxide gas and water.
Figure 5.28

66. Acid Rain deteriorated this sculpture
Acid reacts with the calcium carbonate in the sculpture.
Figure 5.29
1908
1969

67. Double-Displacement Reactions Gas-Formation Reactions
A similar reaction occurs when a metal sulfite reacts with an acid:
MgSO3(s) + 2HCl(aq)  MgCl2(aq) + H2SO3(aq)
MgSO3(s) + 2HCl(aq)  MgCl2(aq) + H2O(l) + SO2(g)
Metal sulfites react with acids to form sulfurous acid, which decomposes to sulfur dioxide gas and water.

68. Double-Displacement Reactions Acid-Base Neutralization Reactions
In an acid-base neutralization reaction, the driving force is the formation of H2O, a molecular compound.
An acid reacts with a base to form an ionic compound (a salt) and water.
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
Acid Base Salt Water

69. Activity: Acid Base Neutralizations
Calcium oxide is the white powder, lime. When added to water, it makes slaked lime, which is a solution of the base calcium hydroxide. If sulfuric acid is added to slaked lime, what products form? Write the balanced chemical equation for this reaction.

70. Activity Solution: Acid-Base Neutralizations
The starting solutions contain H+(aq), SO42−(aq), Ca2+(aq), and OH−(aq).
The solubility rules say that most sulfate compounds are soluble, unless sulfate is combined with calcium. Thus, calcium sulfate is a precipitate. Hydrogen ions and hydroxide ions form water, H2O(l).
By matching the positive and negative charges to get electrical neutrality, we can determine the formula of the precipitate to be CaSO4(s).

71. Activity Solution: Acid-Base Neutralizations
The solution contains no spectator ions. We can write the skeletal equation for this reaction as:
H2SO4(aq) + Ca(OH)2(aq)  CaSO4(s) + H2O(l)
Balanced, the equation is:
H2SO4(aq) + Ca(OH)2(aq)  CaSO4(s) + 2H2O(l)

72. Activity: Double-Displacement Reactions Acid-Base Neutralization Reactions
Complete and balance the following equation:
HCl(aq) + Mg(OH)2(s)  ?
2HCl(aq) + Mg(OH)2(s) 
MgCl2(aq) + 2H2O(l)
Figure 11.6

73. Combustion Reactions
A combustion reaction is a 5th type of reaction that does not always fall into one of the earlier categories.
In a combustion reaction, a substance reacts with oxygen in a reaction that burns to produce a flame.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
Figure 5.8F
Figure 5.31

74. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
Figure 5.30
Figure 5.8F
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

75. Combustion of Hydrocarbons
The most common combustion reactions involve the burning of a hydrocarbon, or a hydrocarbon containing oxygen. The products are always carbon dioxide and water.
2C8H18(g) O2(g)  16CO2(g) H2O(g)

76. 5.5 Representing Reactions in Aqueous Solution
When reactions occur in water, soluble ionic compounds exist as separate ions, not grouped together with each other as might seem so from the equation, sometimes called a molecular equation:
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
NaCl(aq) exists in solution as separate ions

77. Representing Reactions in Aqueous Solution
An ionic equation shows soluble ionic compounds as separate ions:
Ag+(aq) + NO3(aq) + Na+(aq) + Cl(aq) 
AgCl(s) + Na+(aq) + NO3(aq)
Insoluble compounds do not exist as ions.
Spectator ions are ions that occur on both sides of the equation.

78. Ionic Equations and Spectator Ions
What are the spectator ions in the ionic equation?
Ag+(aq) + NO3(aq) + Na+(aq) + Cl(aq) 
AgCl(s) + Na+(aq) + NO3(aq)
Since the spectator ions do not change, we can leave them out of the net ionic equation:
Ag+(aq) + Cl(aq)  AgCl(s)
NO3(aq) + Na+(aq)
Na+(aq) + NO3(aq)

79. Activity: Representing Reactions in Aqueous Solution
Barium sulfate, used in the white pigment lithopone, can be prepared by mixing solutions of barium chloride and sodium sulfate. Write balanced molecular, ionic, and net ionic equations for this reaction.
Figure 5.26

80. Activity Solution: Representing Reactions in Aqueous Solution
Both reactants are strong electrolytes, so they dissociate into the ions Ba2+(aq), Cl-(aq), Na+(aq) and SO42-(aq) in solution.
Possible formulas for the precipitate are BaCl2, Na2SO4, NaCl, or BaSO4.
The first two choices can be eliminated because they are starting reactants.
The solubility rules (Table 5.4) indicate that NaCl is soluble and BaSO4 is insoluble.
Thus, we can write the molecular equation:
BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)

81. Activity Solution: Representing Reactions in Aqueous Solution
BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)
Since BaCl2, Na2SO4, and NaCl are all strong electrolytes in solution, we can write their formulas as separate ionic formulas, giving the following ionic equation:
Ba2+(aq) + 2Cl−(aq) + 2Na+(aq) + SO42−(aq) 
BaSO4(s) + 2Na+(aq) + 2Cl−(aq)

82. Activity Solution: Representing Reactions in Aqueous Solution
Ba2+(aq) + 2Cl−(aq) + 2Na+(aq) + SO42−(aq) 
BaSO4(s) + 2Na+(aq) + 2Cl−(aq)
Finally, we note that Na+ and Cl− occur on both sides of the equation, so they are spectator ions. They can be eliminated, giving us the following net ionic equation:
Ba2+(aq) + SO42−(aq)  BaSO4(s)

83. Activity: Spectator Ions and Net Ionic Equations
Identify the spectator ions and write the net ionic equations from the following molecular equations:
1) Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)
2) 2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g)
Cu(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag(s)
2Na(s) + 2H2O(l)  2Na+(aq) + 2OH−(aq) + H2(g)
Figure from p. 201

84. Activity: Net Ionic Equations
What is the net ionic equation for the reaction of KCl(aq) and NaNO3(aq)?
Ionic “equation”
K+(aq) + Cl−(aq) + Na+(aq) + NO3−(aq)  K+(aq) + NO3−(aq) + Na+(aq) + Cl−(aq)
No reaction occurs!
Figure 5.7