Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
208 Chemistry: Matter and Change
Solving Problems: A Chemistry Handbook
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 20
Balance electrons in the half-reactions by multiplying the reduction
half-reaction by 3 and the oxidation half-reaction by 2. Then add the
half-reactions to obtain the overall cell reaction.
3Cr 2ϩ (aq) ϩ 6e Ϫ 0 3Cr(s)
2Al(s) 0 2Al 3ϩ (aq) ϩ 6e Ϫ
3Cr 2ϩ (aq) ϩ 2Al(s) 0 3Cr(s) ϩ 2Al 3ϩ (aq)
Calculate the standard potential of the voltaic cell.
E 0
cell ϭ E 0
reduction Ϫ E 0
oxidation
E 0
cell ϭ E 0
Cr 2ϩ ΈCr Ϫ E 0
Al 3ϩ ΈAl
ϭ Ϫ0.913 V Ϫ (Ϫ1.662 V)
ϭ ϩ0.749 V
Notice that the standard reduction potentials of the half-reactions are
not multiplied by their coefficients in the balanced overall equation.
Recall your study of properties of matter from Chapter 3. Standard
potential is an intensive property that does not depend on the amount
of material used.
The overall reaction can be expressed in cell notation, with oxidation on the left and reduction on the right.
AlΈAl 3ϩ ΈΈCr 2ϩ ΈCr
Practice Problems
1. For each pair of half-reactions, write the balanced equation for
the overall cell reaction, calculate the standard cell potential,
and express the reaction using cell notation. Use E 0 values from
Table 21-1 in your textbook.
a. Mg 2ϩ (aq) ϩ 2e Ϫ 0 Mg(s)
Pd 2ϩ (aq) ϩ 2e Ϫ 0 Pd(s)
b. Cu ϩ (aq) ϩ e Ϫ 0 Cu(s)
Cd 2ϩ (aq) ϩ 2e Ϫ 0 Cd(s)
c. Ce 3ϩ (aq) ϩ 3e Ϫ 0 Ce(s)
2H ϩ (aq) ϩ 2e Ϫ 0 H 2 (g)