Question

In electrolysis of NaCl when Pt electrode is taken then H₂ is liberated at cathode while with Hg cathode it forms sodium amalgam

(a) Hg is more inert than Pt

(b) More voltage is required to reduce H⁺ at Hg than at Pt

(c) Na is dissolved in Hg while it does not dissolve in Pt

(d) Conc. of H⁺ ions is larger when Pt electrode is taken.

Answer 1

Correct option (b) More voltage is required to reduce H⁺ at Hg than at Pt

Explanation :

When sodium chloride is dissolved in water, it ionises as

Water also dissociates as

During passing of electric current through this solution using platinum electrode, Na⁺ and H⁺ ions move towards cathode. However, only H⁺ ions are discharged more readily than Na⁺ ion because of their low discharge potential (In the electromotive series hydrogen is lower than sodium). These H⁺ ions gain electrons and change into neutral atoms.

At cathode H⁺ + e → H, H + H → H₂ 

Cl^- and OH^- ions move towards anode. Cl^- ions lose electrons and change into neutral atom.

At anode, Cl^- - e → Cl, Cl + Cl → Cl₂ 

If mercury is used as cathode H⁺ ions are not discharged at mercury cathode because mercury has a high hydrogen over-voltage. Na⁺ ions are discharged at the cathode in preference to H⁺ ions, yielding sodium, which dissolves in mercury to form sodium amalgam. At cathode : Na⁺ + e = Na