Criteria of product formation during electrolysis

Electrolysis of sodium chloride

i) Molten sodium chloride

The only ions present are Na⁺ and Cl^-

ii) Concentrated aqueous sodium chloride

The products are not the same because water is also involved.

Probable reactions at the cathode

The reduction potential of water is greater so H₂ is evolved.

Probable reactions at the anode

The reduction potentials are nearly the same.

Cl₂ is preferentially evolved because of

iii) Higher concentration of Cl^- ions.

iv) Over voltage of hydrogen. (Extra voltage required for that reaction because it is kinetically slow process)

Actual reactions taking place are:

v) Electrolysis of aqueous CuSO₄ using platinum electrode.

Comparing the reduction potentials, the actual reactions are

vi) Electrolysis of CuSO₄ using copper electrodes

The actual reactions are

Note:

If more than one substance is present in an electrolytic cell the substance liberated at the cathode has highest reduction potential and that at the anode will have the lowest reduction potential.

Therefore,

i) Na⁺, Ca²⁺ and Al³⁺ have lower reduction potential than water so H₂ gas is evolved at the cathode.

ii) Cu²⁺, Ag⁺ have much higher reduction potential than that of water. So these ions are deposited at the cathode.