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.