Chemistry Matter and Change

2. Redox Reactions
Section 19.1 Oxidation and
Reduction
Section 19.2 Balancing Redox
Equations
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3. Section 19.1 Oxidation and Reduction
• Describe the processes
of oxidation and
reduction.
spectator ion: an ion
that does not participate
in a reaction and is not
usually shown in an
ionic equation• Identify oxidizing and
reducing agents.
• Determine the oxidation
number of an element in
a compound.
• Interpret redox
reactions in terms of
change in oxidation
state.

4. Section 19.1 Oxidation and Reduction (cont.)
oxidation-reduction reaction
redox reaction
oxidation
Oxidation and reduction are
complementary—as an atom is
oxidized, another atom is reduced.
reduction
oxidizing agent
reducing agent

5. Electron Transfer and Redox Reactions
• An oxidation-reduction reaction, or
redox reaction involves the transfer of
electrons from one atom to another.

6. Electron Transfer and Redox Reactions (cont.)

7. Electron Transfer and Redox Reactions (cont.)
• Oxidation is defined as the loss of
electrons from atoms of a substance.
Na → Na+
+ e–
• Reduction is defined as the gain of electrons
by the atoms of a substance.
Cl2
+ 2e–
→ 2Cl–

8. Electron Transfer and Redox Reactions (cont.)
• The oxidation number of an atom in an
ionic compound is the number of electrons
lost or gained by the atom when it forms an
ion.
• When an atom or ion is reduced, the
numerical value of its oxidation number
decreases.
• When an atom or ion is oxidized, its oxidation
number increases.

9. Electron Transfer and Redox Reactions (cont.)
• Oxidation numbers are tools that scientists
use to keep track of the movement of
electrons in a redox reaction.

10. Oxidizing and Reducing Agents
• The substance that oxidizes another
substance by accepting its electrons is
called an oxidizing agent.
• The oxidizing agent is the substance that is
reduced in a redox reaction.

11. Oxidizing and Reducing Agents (cont.)
• The substance that reduces another
substance by losing its electrons is the
reducing agent.
• The reducing agent is
the substance that is
oxidized in a redox
reaction.

12. Redox and Electronegativity
• Redox reactions are not limited to atoms of
an element changing to ions.
• Some redox reactions involve changes in
molecular substances or polyatomic ions.
N2
(g) +3H2
(g) → NH3
(g)
• N is reduced and H is oxidized.

13. Redox and Electronegativity (cont.)
• To determine which was oxidized and
which was reduced, you must know which
atom is more electronegative.
• Elements with high electronegativity are
strong oxidizing agents.

14. Redox and Electronegativity (cont.)

15. Determining Oxidation Numbers
• To understand all types of redox reactions,
the oxidation number of the atoms involved
in the reaction must be determined.

16. Determining Oxidation Numbers (cont.)

17. Oxidation Numbers in Redox Reactions
• Oxidation-reduction reactions are changes
in oxidation number.
• Atoms that are reduced have their oxidation
number decreased.
• Atoms that are oxidized have their oxidation
number increased.

18. Oxidation Numbers in Redox Reactions (cont.)

19. A. A
B. B
C. C
D. D
A
B
C
D
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Section 19.1 Assessment
In a redox reaction, the reducing agent is:
A. the substance that is reduced
B. the substance that is oxidized
C. the substance that gains
electrons
D. none of the above

20. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
Section 19.1 Assessment
In redox reactions, more electronegative
elements tend to:
A. be reduced
B. be reducing agents
C. lose electrons
D. not change

22. Section 19.2 Balancing Redox Reactions
• Relate changes in
oxidation number to the
transfer of electrons.
net ionic equation: an
ionic equation that
includes only the
particles that participate
in the reaction• Use changes in
oxidation number to
balance redox equations.
• Balance net ionic redox
equations using the
oxidation-number
method.

23. Section 19.2 Balancing Redox Reactions (cont.)
oxidation-number method
species
half-reaction
Redox equations are balanced when
the total increase in oxidation numbers
equals the total decrease in oxidation
numbers of the atoms involved in the
reaction.

24. The Oxidation-Number Method
• Chemical equations must be balanced to
show the correct quantities of reactants
and products.
• The number of electrons transferred from
atoms must equal the number of electrons
accepted by other atoms.

25. The Oxidation-Number Method (cont.)
• The total increase in oxidation numbers
must equal the total decrease in oxidation
numbers in the reaction.
• This method is called the oxidation number
method.

26. Balancing Net Ionic Redox Equations
• Sometimes it is preferred to express redox
reactions in the simplest possible terms,
showing only the oxidation and reduction
processes.
• When balancing equations in acidic solution,
hydrogen ions (H+
) or water molecules can be
added to either side of the equation.
• When balancing equations in basic solution,
hydroxide ions (OH–
) or water molecules can
be added to either side of the equation.

27. Balancing Redox Equations Using
Half-Reactions
• In chemistry, a species is any kind of
chemical unit involved in a process.
• Oxidation-reduction reactions occur whenever
a species that can give up electrons comes in
contact with another species that can accept
them.

28. Balancing Redox Equations Using
Half-Reactions (cont.)
• A half-reaction is one of the two parts of a
redox reaction—the oxidation half of the
reduction half.

29. Balancing Redox Equations Using
Half-Reactions (cont.)

30. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
Section 19.2 Assessment
A redox reaction split into two parts is
called ____.
A. net reaction
B. oxidation-reaction
C. half-reaction
D. reduction-reaction

31. A. A
B. B
C. C
D. D
Section 19.2 Assessment
A
B
C
D
0% 0%0%0%
In acid solution, what would you use to
balance the charge in a redox-reaction?
A. electrons
B. hydrogen ions and water
C. hydroxide ions and water
D. hydrogen ions and
hydroxide ions

33. Chemistry Online
Study Guide
Chapter Assessment
Standardized Test Practice
Image Bank
Concepts in Motion

34. Section 19.1 Oxidation and Reduction
Key Concepts
• Oxidation-reduction reactions involve the transfer of
electrons from one atom to another.
• When an atom or ion is reduced, its oxidation number
is lowered. When an atom or ion is oxidized, its
oxidation number is raised.
• In oxidation-reduction reactions involving molecular
compounds (and polyatomic ions with covalent bonds),
the more-electronegative atoms are treated as if they
are reduced. The less-electronegative atoms are
treated as if they are oxidized.

35. Section 19.1 Oxidation and Reduction
(cont.)
Key Concepts

36. Section 19.2 Balancing Redox
Equations
Key Concepts
• Redox equations in which the same element appears in
several reactants and products can be difficult to
balance using the conventional method.
• The oxidation-number method is based on the number
of electrons transferred from atoms equaling the
number of electrons accepted by other atoms.
• To balance equations for reactions in an acid solution,
add enough hydrogen ions and water molecules to
balance the equation.

37. Section 19.2 Balancing Redox
Equations (cont.)
Key Concepts
• To balance equations for reactions in a basic solution,
add enough hydroxide ions and water molecules to
balance the equation.
• A half-reaction is one of the two parts of a redox
reaction.

38. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
What type of reaction involves the transfer
of electrons from one atom to another?
A. synthesis
B. decomposition
C. double replacement
D. redox

39. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
Less electronegative atoms in redox
reactions are most often ____.
A. oxidized
B. reduced
C. oxidizing agents
D. neutral

40. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
Any chemical unit involved in a process is
called a(n) ____.
A. atom
B. type
C. species
D. ion

41. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
Determine the oxidation number of
oxygen in HNO3
.
A. +3
B. –3
C. +2
D. –2

42. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
Determine the oxidation number of
nitrogen in HNO3
.
A. +1
B. +4
C. +5
D. –5

43. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
In the reaction MgI2
+ Br2
→ MgBr2
+ I2
,
what is the oxidation number of I2
?
A. –1
B. +1
C. 0
D. +½

44. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
Which is NOT an oxidizing agent in a
redox reaction?
A. substance reduced
B. electron acceptor
C. oxidizer of another substance
D. electron donor

45. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
How does the oxidation number change in
sodium in the following equation?
2NaI(aq) + Cl2
(aq) → 2NaCl(aq) + I2
(aq)
A. It changes from 0 to –1.
B. It changes from –1 to 0.
C. It changes from 2 to –2.
D. no change

46. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
If the temperature in a system is held
constant, the pressure of a gas must do
what when the volume is increased?
A. no change
B. increase
C. decrease
D. unable to determine

47. A. A
B. B
C. C
D. D
A
B
C
D
0% 0%0%0%
What is the oxidizing agent in the
following equation?
Na2
SO4
+ 4C → Na2
S + 4CO
A. C
B. S
C. O
D. Na

48. Click on an image to enlarge.

58. 9.2 Redox Reaction
.1 Summary of Redox Reactions

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