Q. 34

Value of standard electrode potential for the oxidation of Cl ions is more positive than that of water, even then in the electrolysis of aqueous sodium chloride, why is Cloxidised at anode instead of water?

Answer :

A positive value for the oxidation of Cl- ions means the reduced form Cl- is more stable than Cl2 but still, oxidation of Cl- takes place.

Cl (aq) 1/2 Cl2 (g) + e Ecell = 1.36 V ---(1)


2H2O (l) O2 (g) + 4H+ (aq) + 4e Ecell = 1.23 V ---(2)


Reaction (2) should take place at anode instead of Reaction (1) due to lower reduction potential.


In reaction (2), the oxidation of water to oxygen is kinetically unfavourable and requires excess potential called over-potential. Over-potential is the excess potential required to drive a reaction at a particular rate.


Therefore, oxidation of water does not take place at anode.


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