Question

How does overvoltage affect the electrolysis of an aqueous solution of sodium chloride?

Answer 1

In the electrolysis of aqueous sodium chloride (NaCl), overvoltage plays an important role in determining which gas is evolved at the electrodes, especially at the anode.

At the anode, both Cl⁻ ions and water molecules can be oxidised:

Oxidation of Cl⁻: \[\ce{2Cl- -> Cl2_{(g)} + 2e-}\] E° = +1.36 V,

Oxidation of H₂O: \[\ce{2H2O -> O2_{(g)} + 4 H+ + 4e-}\] E° = +1.23 V,

Although the standard potential for water oxidation is lower (i.e., it should occur more readily), the evolution of oxygen involves a high overvoltage, which raises the actual required potential.

As a result, Cl₂ is liberated at the anode, not O₂.