Question

Explain the criteria for product formation during electrolysis taking the example of aqueous sodium chloride.

Answer 1

When an electric current is passed through a concentrated aqueous solution of sodium chloride, hydrogen gas is evolved at the cathode and chlorine gas at the anode. The reactions taking place at the two electrodes and the overall reaction are as follows.

At anode: \[\ce{2Cl^{-}_{ (aq)} -> Cl2_{(g)} + 2e-}\] (Oxidation)

At cathode: \[\ce{2H2O_{(l)} + 2e- -> H2_{(g)} + 2OH^{-}_{ (aq)}}\] (Reduction) 
                                                                                                                               
Overall reaction: \[\ce{2H2O_{(l)} + 2e- + 2Cl^{-}_{ (aq)} -> Cl2_{(g)} + H2_{(g)} + 2OH^{-}_{ (aq)}}\]
Although Na⁺ ions are present in the solution, they do not become reduced at the cathode. Now the question arises: why do water molecules get reduced at the cathode in preference to Na⁺ ions which are also present in the solution?