This chapter discusses chemical reactions and their principles. It introduces chemical equations and how they must be balanced to satisfy the law of mass conservation. Reactions can be exothermic or endothermic depending on whether energy is released or absorbed. Reaction rates depend on factors like concentration, temperature, and catalysts. Acids donate protons while bases accept them. pH is a measure of acidity. Acid rain forms from air pollution while oceans are basic. Oxidation involves losing electrons while reduction involves gaining them. Batteries and fuel cells use redox reactions to generate electricity through oxidation and reduction at electrodes. Electrolysis and corrosion are types of redox reactions.

1. Chapter 13:
Chemical Reactions
Principles of Science II

2. This lecture will help you understand:
• Chemical Equations
• Energy and Chemical Reactions
• Reaction Rates
• Catalysts
• Acids Donate Protons, Bases Accept Them
• Relative Strengths of Acids and Bases
• Acid, Basic, and Neutral Solutions
• Acidic Rain and Basic Oceans
• Losing and Gaining Electrons
• Harnessing the Energy of Flowing Electrons
• Electrolysis
• Corrosion and Combustion

3. Reactants Products
Chemical Equations
• During a chemical reaction, one or more new
compounds are formed as a result of the
rearrangement of atoms.

4. Chemical Equations
• Law of mass conservation: No atoms are gained or lost
during any reaction.
• The number of times atoms appear before the arrow
must be equal to the number of times atoms appear after
the arrow.
• One of the most important principles of chemistry is the
law of mass conservation, which states that matter is
neither created nor destroyed during a chemical
reaction.
– Instead, atoms simply change partners to form new
materials.

5. Chemical Equations
• The number and type of atoms before a reaction,
therefore, are always the same as the number and type
of atoms after the reaction.
• To reflect this fact, the chemical equation must be
BALANCED.
Balancing guidelines
1. Balance one element at a time.
2. If you incidentally unbalance an element, leave it alone.
3. Make successive passes.

6. Al2O3 C CO2+ Al +
(not balanced)
Chemical Equations

7. 4 32 3
Al Al
OOO C
Al
C OO
Al
C OO
C OOAl Al
OOO
C
C
Al
Al
Al2O3 C CO2+ Al +
Chemical Equations
Balanced

8. Energy and Chemical Reactions
• Bond energy is the amount of energy required to
pull two bonded atoms apart, which is the same
as the amount of energy released when they are
brought together.
• In an EXOTHERMIC reaction there is a net
release of energy.

9. Energy and Chemical Reactions

10. 2 H2 O2 2 H2O+
Energy and Chemical Reactions
The reaction is EXOTHERMIC.
Energy absorbed:
Energy released:
–486 kJ/molNet energy of
reaction:
–1856 kJ/mol
+1370 kJ/mol

11. Energy and Chemical Reactions
Reaction progress
Energy
This reaction is exothermic.

12. Energy and Chemical Reactions
• In an ENDOTHERMIC reaction there is a net
absorption of energy.

13. Energy and Chemical Reactions

14. +1444 kJ/mol
–1262 kJ/mol
Energy absorbed:
Energy released:
Net energy of
reaction:
N2 O2
2 NO+
Energy and Chemical Reactions
+182 kJ/mol
This reaction is ENDOTHERMIC.

15. Energy and Chemical Reactions
Reaction progress
Energy
This reaction is endothermic.

16. Reaction Rates
• The reaction rate is how quickly the
concentration of reactants decreases and how
quickly the concentration of products increases.
• The rate of a reaction is dependent on the
collisions among molecules.
• Increasing the concentration increases the
number of collisions per second and therefore
increases the rate of the reaction.

17. Reaction Rates

18. Reaction Rates
• Not all collisions lead to products. For example,
at ambient temperatures, N2 and O2 molecules
do not have sufficient energy to form nitrogen
monoxide.
• So, where might this reaction occur?

19. Reaction Rates
• Activation energy, Ea, is the minimum energy
required to overcome the initial breaking of
bonds in reactants.

20. Reaction progress
Energy
Reaction Rates

21. Reaction Rates
• Reactants must be moving fast enough (have
sufficient kinetic energy) to overcome the energy
of activation.

22. Reaction Rates
• Reaction rates are affected by:
– Concentration
– Temperature
– Catalysts
Premise:
Reactant molecules have to make physical
contact with each other in order to transform into
products.

23. Reaction Rates
• The more concentrated a sample of nitrogen
and oxygen, the greater the likelihood that N2
and O2 molecules will collide and form nitrogen
monoxide.

24. Reaction Rates
• Slow-moving molecules may collide without
enough force to break the bonds. In this case,
they cannot react to form product molecules.

25. Catalysts
• A catalyst is a substance that lowers the
activation energy of a chemical reaction, which
allows for the reaction to proceed at a faster
rate.

26. Reaction progress
Energy
Ozone
O3 Oxygen
O2
2 O3 3 O2
Cl
Reaction progress
Energy
Ozone
O3
Oxygen
O2
2 O3 3 O2
Catalysts

27. CCl3F CCl2F2
Catalysts
• Chlorofluorocarbons were once commonly used
as refrigerants, until it was recognized that these
compounds are a major source of chlorine atoms
in the stratosphere.

28. Catalysts

29. Catalysts
Concentration of
chlorinemonoxide, ClO
Concentration of ozone, O3

30. Catalysts
• Chemists have been able to harness the power
of catalysts for numerous beneficial purposes.

31. Acids Donate Protons, Bases Accept Them
• An acid is a chemical that donates a hydrogen
ion, H+
.
• A base is a chemical that accepts a hydrogen
ion, H+
.

32. H3O+
Cl–
H2OHCl + +
Acids Donate Protons, Bases Accept Them
(acid) (base)
d
o
n
o
r
a
c
c
e
p
t
o
r

33. NH4
+
OH–
+H2O NH3
d
o
n
o
r
a
c
c
e
p
t
o
r
+
Acids Donate Protons, Bases Accept Them
(acid) (base)

34. NH4
+
OH–
+H2O NH3+
Acids Donate Protons, Bases Accept Them
(acid) (base) (acid) (base)

35. A salt is an

36. Acids Donate Protons, Bases Accept Them

37. Relative Strengths of Acids and Bases
• Strong acids and bases
ionize completely in
water.

38. Relative Strengths of Acids and Bases
• Weak acids and bases
do not ionize completely
in water.

39. Acidic, Basic, and Neutral Solutions
• Water can behave as an acid or a base.

40. Acidic, Basic, and Neutral Solutions
• In pure water, for every one hydronium ion, H3O+
,
formed, there is one hydroxide ion, OH–
, formed.
• So, in pure water, [H3O+
] = [OH–
] = 0.0000001 M
= 10–7
M.

41. Acidic, Basic, and Neutral Solutions
• Add hydronium ions, H3O+
, and the solution is
"acidic."
• Add hydroxide ions, OH–
, and the solution is
"basic."

42. © 2013 Pearson Education, Inc.
Acidic, Basic, and Neutral Solutions

43. Acidic, Basic, and Neutral Solutions
• pH is a measure of the concentration of
hydronium ions, H3O+
:
pH = –log [H3O+
]
• For pure water:
pH = –log (10–7
)
pH = –(– 7)
pH = 7

44. Acidic, Basic, and Neutral Solutions
• The "log" of a number is simply the power to
which 10 is raised. The log of 103
, for example, is
3.
Quiz Time
What is the log of 102
?
Log 102
= 2
(the power to which 10 is raised)

45. Acidic, Basic, and Neutral Solutions
103
= 1000
102
= 100
101
= 10
100
= 1
10–1
= 0.1
10–2
= 0.01
10–3
= 0.001

46. Acidic, Basic, and Neutral Solutions
pH = –log [H3O+
]
For acidic water pH < 7; for example:
pH = –log (10–5
)
pH = –(–5)
pH = 5

47. Acidic, Basic, and Neutral Solutions
pH = –log [H3O+
]
For basic water pH > 7; for example:
pH = –log (10–9
)
pH = –(–9)
pH = 9

48. Acidic, Basic,
and Neutral
Solutions

49. Acidic Rain and Basic Oceans
• Acid rain has a pH lower than 5.
2 SO2(g) + O2(g)  SO3(g)
SO3(g) + H2O(l)  H2SO4(aq)
• SO2 is released from burning coal and oil.

50. Acidic Rain and Basic Oceans
• CO2 levels in the atmosphere are rising.
CO2(g) + H2O(l)  H2CO3(aq)

51. Acidic Rain and Basic Oceans

52. Acidic Rain and Basic Oceans

53. Losing and Gaining Electrons
• Acid–base reactions involve the transfer of a
proton.
• Oxidation–reduction reactions involve the
transfer of an electron.

54. Losing and Gaining Electrons
• Oxidation is the loss of an electron.
2 Na 2 Na+
+ 2 e–
• Reduction is the gain of an electron.
Cl2 + 2 e–
2 Cl–

55. Losing and Gaining Electrons
2 Na + Cl2 2 Na+
+ 2 Cl–

56. Losing and Gaining Electrons

57. Losing and Gaining Electrons

58. Harnessing the Energy of Flowing Electrons
• Electric currents are generated by
oxidation– reduction reactions.
• Oxidation–reduction reactions are used in
batteries and fuel cells.

59. Harnessing the Energy of Flowing Electrons
• Ions must be able to flow in order to generate a
current.

60. Harnessing the Energy of Flowing Electrons
• A salt bridge allows this to happen.

61. © 2013 Pearson Education, Inc.
Harnessing the Energy of Flowing Electrons
• A battery is a self-contained voltaic cell.

62. Harnessing the Energy of Flowing Electrons
• The positive electrode is the cathode, where
reduction occurs.
• The negative electrode is the anode, where
oxidation occurs.

63. Harnessing the Energy of Flowing Electrons
• Several types of batteries:
– Dry cell
– Alkaline
– Rechargeable
• NiMH
• Lithium

64. Harnessing the Energy of Flowing Electrons
• Fuel cells convert the chemical energy of a fuel
into electric energy.

65. © 2013 Pearson Education, Inc.
Harnessing the Energy of Flowing Electrons

66. Electrolysis
• Electrolysis uses electric energy to produce
chemical change.
• Examples:
– Recharging your car battery
– Purifying metal ores

67. Electrolysis

68. Corrosion and Combustion
• Corrosion is the process in which a metal
deteriorates through oxidation–reduction
reactions.
• Corrosion can be prevented by coating the metal
with zinc, which oxidizes first.

69. Corrosion and Combustion
• Combustion is an oxidation–reduction reaction
between a nonmetallic material, such as wood,
and oxygen.