Electrolysis of Aqueous Ionic solutions and the
Phenomenon of Overvoltage
When two half-reactions are possible at an electrode, the one with the
more positive (or less negative) electrode potential occurs.
For example, the electrolysis of KI: Cathode reaction:
K+(aq) + e- K(s) Eo = -2.93 V
2 H2O(l) + 2 e- H2 (g) + 2 OH-(aq) Eo = -0.42 V [reduction]
The less negative potential for water means that H2 forms at the cathode.
Anode reaction:
2 I -(aq) I2 (s) + 2 e- -Eo = -0.53 V [oxidation]
2 H2O(l) O2 (g) + 4 H+(aq) + 4 e- -Eo = -0.82 V
The less negative potential for I - means I2 forms at the anode.
For gases such as H2 (g) and O2 (g) to be formed at the metal electrodes an
additional voltage is required; this is called the overvoltage!
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