CHAPTER 21 - Chapter 21: Electrochemistry 1...

1
Chapter 21:
Electrochemistry

2
Oxidation-Reduction Reactions
Oxidation-Reduction Reactions

Zn added to HCl yields the spontaneous reaction
Zn( s ) + 2H
+
( aq ) → Zn
2+
( aq ) + H
2
( g ).

The oxidation number of Zn has increased from 0 to
2+.

The oxidation number of H has reduced from 1+ to 0.

Therefore, Zn is oxidized to Zn
2+
while H
+
is reduced
to H
2
.

H
+
causes Zn to be oxidized and is the oxidizing
agent.

Zn causes H
+
to be reduced and is the reducing
agent.

Note that the reducing agent is oxidized and the
oxidizing agent is reduced.

3
Balancing Oxidation-Reduction
Balancing Oxidation-Reduction
Equations
Equations

Law of conservation of mass : the amount of
each element present at the beginning of the
reaction must be present at the end.

Conservation of charge : electrons are not lost
in a chemical reaction.

In complicated redox reactions, we need to
look at the transfer of electrons carefully.
Half-Reactions
Half-Reactions

Half-reactions are a convenient way of
separating oxidation and reduction reactions.

4
Balancing Oxidation-Reduction
Balancing Oxidation-Reduction
Equations
Equations
Half-Reactions
Half-Reactions

The half-reactions for
Sn
2+
( aq ) + 2Fe
3+
( aq ) → Sn
4+
( aq ) + 2Fe
2+
( aq )
are
Sn
2+
( aq ) → Sn
4+
( aq ) +2e
-
2Fe
3+
( aq ) + 2e
-
→ 2Fe
2+
( aq )

Oxidation: electrons are products.

Reduction: electrons are reagents.

5
Balancing Equations by the Method of
Balancing Equations by the Method of
Half-Reactions
Half-Reactions

Consider the titration of an acidic solution of
Na
2
C
2
O
4
(sodium oxalate, colorless) with
KMnO
4
(deep purple).

MnO
4
-
is reduced to Mn
2+
(pale pink) while the
C
2
O
4
2-
is oxidized to CO
2
.

The equivalence point is given by the
presence of a pale pink color.

If more KMnO
4
is added, the solution turns
purple due to the excess KMnO
4
.

6
Balancing Equations by the Method of
Balancing Equations by the Method of
Half-Reactions
Half-Reactions

7
Balancing Equations by the Method of
Balancing Equations by the Method of
Half-Reactions
Half-Reactions

What is the balanced chemical equation?
1. Write down the two half reactions.
2. Balance each half reaction:
a. First with elements other than H and O.
b. Then balance O by adding water.
c. Then balance H by adding H
+
.
d. Finish by balancing charge by adding electrons.

8
Balancing Equations by the Method of
Balancing Equations by the Method of
Half-Reactions
Half-Reactions
3. Multiply each half reaction to make the
number of electrons equal.
4. Add the reactions and simplify.
5. Check!
For KMnO
4
+ Na
2
C
2
O
4
:

9
Balancing Equations by the Method of
Balancing Equations by the Method of
Half-Reactions
Half-Reactions
1. The two incomplete half reactions are
MnO
4
-
( aq ) → Mn
2+
( aq )
C
2
O
4
2-
( aq ) → 2CO
2
( g )
2. Adding water and H
+
yields
8H
+
+ MnO
4
-
( aq ) → Mn
2+
( aq ) + 4H
2
O
•
There is a charge of 7+ on the left and 2+ on
the right. Therefore, 5 electrons need to be
added to the left:
5e
-
+ 8H
+
+ MnO
4
-
( aq ) → Mn
2+
( aq ) + 4H
2
O

10
Balancing Equations by the Method of
Balancing Equations by the Method of
Half-Reactions
Half-Reactions
