Page 216 - Modern physical chemistry

9.3 Electrolyte Transference 209

In a conductance experiment, ~lectrons are supplied to the cathode by an external
source of potential. These may react in the following ways:
1. A cation discharges; for example

Ag+ +e~Ag, [9.14]

Cu++ +2e~ Cu. [9.15]
2. A cation is reduced to another cation; thus
[9.16]

Sn 4+ + 2e ~ Sn ++ . [9.17]

3. An anion is produced. Thus if one bubbles chlorine over a platinum electrode, the
following reaction occurs:
Cl 2 +2e~2Cr. [9.18]
4. The hydrogen ion from water is discharged. This reaction occurs when no other
reduction can readily occur:
[9.19]

Example9.1
If 0.500 A current was passed for 55 min through a CuS0 4 solution using copper elec-
trodes, how much copper plated out on the cathode and how much left the anode?
From equation (9.3), the number of coulombs passing any cross section in the circuit is

1
Q = It =(0.500 AX55 minX60 s min- ) = 1650 C.
But following formula (9.1), the number of equivalents reduced at the cathode and oxi-
dized at the anode is
1650 C .
n = = 0.01710 eqmv.
96,485.3 equiv- 1
Since the reaction at the cathode is

Cu++ +2e~ Cu,
the equivalent mass of copper is its atomic mass divided by 2. So the weight of copper
plating out is
w =(0.01710 equivX31.773 g equiv- ) = 0.543 g.
1
At the anode, the reaction is
Cu ~ Cu++ + 2e.
and the weight of copper leaving is
w =0.543 g.

9.3 Electrolyte Transference

When electric current is driven through a solution by an external source of poten-
tial, cations move toward the cathode and anions move toward the anode. Furthermore,