Ncert Exercises & Solutions

Brine is electrolysed by using inert electrodes. The reaction at the anode is:

1. Cl^-(aq) → \(\frac{1}{2}\)Cl₂(g) + e^-; \(E_{cell}^{o}\) = 1.36 V

2. 2H₂O(l) → O₂(g) + 4H⁺ + 4 e^-; \(E_{cell}^{o}\) = 1.23 V

3. Na⁺(aq) + e^- → Na(s); \(E_{cell}^{o}\) = 2.71 V

4. H⁺(aq) + e^- → \(\frac{1}{2}\)H₂(g); \(E_{cell}^{o}\) = 0.00 V

HINT: Cl₂ produces at anode.

Explanation:

STEP 1:

Brine is electrolysed by using inert electrodes. The possible reactions occurring at anode are :

{Cl}^{-} (aq) \to \frac{1}{2} {Cl}_{2} (g) + e^{-}

;

E_{Cell}^{⊝}

=1.36V

2 H_{2} O (l) \to O_{2} (g) + 4 H^{+} + 4 e^{-}

;

E_{Cell}^{⊝}

= 1.23 V

STEP 2:

The reaction at anode with lower value of E° is preferred and therefore water should get oxidised in preference to

{Cl}^{-}

(aq). However,

{Cl}_{2}

is produced instead of

O_{2}

This unexpected result is explained on the basis of the fact that water needs greater voltage for oxidation to

O_{2}

(as it is kinetically slow process) than that needed for oxidation of

{Cl}^{-}

ions to

{Cl}_{2}