Quantitative Relations In Electrolysis
313
For aluminum,
- T
•
.. / IT r> 8 \ (1 rnoleX
i-Tequivwt = I 27.0
, I I- -p—I
M
\
mole/ \3 F /
g
o
= 9.0 |r = 9.0
M
-
F
' mole of electrons
Some substances have more than one electron-transfer equivalent weight.
For example, consider iron in the following reactions:
2Fe
3+ + Zn -> 2Fe
2+ + Zn
2+
Fe + 2H
+ -> Fe
2+ + H 2 f
2Fe + 3C1 2 -» 2FeCl 3
One mole of Fe is involved in the transfer of 1 mole of electrons in the
first reaction, 2 moles of electrons in the second reaction, and 3 moles in
the third reaction. This, in turn, means that the equivalent weight of Fe in
the first reaction is 55.85 g/F, in the second is 27.92 g/F, and in the third is
18.62 g/F.
§
Electrical Equivalents (Faradays)
The quantity of electricity involved in an electrolysis reaction is determined
by the current (amperes) and the length of time the current is passed. By
definition,
coulomb
ampere =
sec
or,
coulombs = (amperes)(secs)
A number of practical problems can be solved by applying these simple
principles. The following problems are typical.
PROBLEM:
Calculate the weight of copper and the volume of O 2 (at 25.0°C and 730 torr,
dry) that would be produced by passing a current of 0.500 amp through a CuSO 4
solution between Ft electrodes for a period of 2.00 hr.
SOLUTION:
The number of faradays of electricity passed is