Question

What is the significance of electrode potentials in predicting the products of electrolysis of an aqueous solution of an electrolyte? Explain with a suitable example.

Answer 1

The electrode potentials play an important role in predicting the products of electrolysis, especially when more than one species is capable of being oxidised or reduced.

At the cathode (reduction occurs), the species with a higher (more positive) reduction potential is preferred and gets reduced.

At the anode (oxidation occurs), the species with lower reduction potential (i.e., more easily oxidised) is discharged.

Thus, the standard reduction potentials (E° values) help in identifying which ion will be discharged during electrolysis.

In aqueous NaCl solution, the following reactions are possible:

At the cathode:

\[\ce{Na+ + e− -> Na}\], E° = −2.71 V

\[\ce{2H2O + 2e− -> H2 + 2OH−}\], E° = −0.83 V

Water is reduced (not Na⁺) because it has a higher reduction potential. Hence, hydrogen gas is evolved at the cathode.

At the anode:

\[\ce{2Cl- -> Cl2 + 2e-}\] = +1.36 V

\[\ce{2H2O -> O2 + 4H+ + 4e−}\], reverse of reduction potential E° = +1.23 V

Though the discharge potential of water is lower, Cl⁻ ions are discharged due to overvoltage for oxygen and a higher concentration of Cl⁻. Hence, chlorine gas is evolved at the anode.