Electrolysis of Aqueous Ionic Solutions and the
Phenomenon of Overvoltage
An example of overvoltage is the electrolysis of aqueous NaCl.
Water is easier to reduce, so H2 forms at the electrode even with an
overvoltage of 0.6 V.
Na+(aq) + e- Na(s) Eo = -2.71 V
2 H2O(l) H2 (g) + 2 OH-(aq) E = - 0.42 V (~ 1 V with
overvoltage) [reduction]
But Cl2 forms at the anode, even though the electrode potentials suggest
O2 should form:
2 H2O(l) O2 (g) + 4 H+(aq) + 4 e- -E = -0.82 V (~ -1.4 V
with overvoltage) [reduction]
2 Cl-(aq) Cl2 (g) + 2 e- -Eo = -1.36 V [oxidation]
Keeping the chloride ion concentration high favors the production of Cl2,
even though O2 formation is still slightly favored with the overvoltage.
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