Question

Mention the criteria to predict the formation of products during electrolysis of an aqueous solution of an electrolyte. Apply these criteria to the electrolysis of an aqueous solution of copper sulphate using inert electrodes.

Answer 1

During electrolysis of an aqueous salt solution, the ions discharged at their respective electrodes depend upon their standard electrode potentials (discharge tendencies).

Ions with lower discharge potential (i.e., higher reduction potential) are discharged preferentially, leaving the other ions in solution.

For example, in the electrolysis of aqueous copper sulphate solution using inert electrodes (like platinum or graphite), the following reactions take place:

Reaction at cathode: 

\[\ce{Cu{^{2+}_{(aq)}} + 2e- -> Cu_{(s)}}\]

Reaction at the anode:

\[\ce{2H2O_{(l)} -> O2_{(g)} + 4H{^+_{(aq)}} + 4e-}\]

At the cathode, copper ions are discharged in preference to hydrogen ions, so copper gets deposited.

At the anode, oxygen is liberated in preference to sulphate ions because \[\ce{SO^{2-}_4}\] is a stable oxoanion and is not oxidised.

A powdery, uneven layer of copper metal is deposited over the inert cathode. The solution becomes acidic due to the formation of sulphuric acid from H⁺ and \[\ce{SO^{2-}_4}\] ions remaining in solution.