Following reactions occur at cathode during the electrolysis of aqueous sodium chloride solution:
Na+(aq) + e− ⟶ Na (s) E0 = 2.71 V
H+(aq) + e− ⟶ `1/2` H2 (g) E0 = 0.00 V
On the basis of their standard reduction electrode potential (E0) values, which reaction is feasible at the cathode and why?
A redox reaction is feasible only if the species with high reduction potential is reduced (accepts an electron) and the species with lower standard reduction potential is oxidised (loses an electron). For a reaction to occur, it is necessary that the species with a greater tendency to lose an electron, loses its electron, which is gained by the species with a greater tendency to gain the electron.
The reactions taking place during the electrolysis of aqueous sodium chloride solution are
Na+ (aq)+e→ Na(s) E° =-2.71 V (I)
H+(aq)+ e → `1/2`H2(g) E° = 0.00 V (II)
One more reaction which takes place during the electrolysis of aqueous sodium chloride solution is
Cl2 +(aq)+2e → Cl (g) E° = 1.36 V (III)
From the above three reactions, we observe that the standard reduction potential sodium (reaction I) is negative, while the standard reduction potential for chlorine (reaction III) is positive. Hence, reaction III is feasible at the cathode.