2002 AIPMT ElectrochemistryProducts of Electrolysis: Overpotential Medium Question 69 of 74 in Electrochemistry

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

The answer is B.

Solution

At the cathode, either $H^+$ (from water) or $Na^+$ can be reduced.
On a platinum cathode, $H^+$ ions are reduced easily, so hydrogen gas is liberated.
On a mercury cathode, the discharge of hydrogen requires a much higher voltage (a large hydrogen overpotential). So it becomes easier to reduce $Na^+$, and the sodium formed dissolves in mercury as sodium amalgam (Castner–Kellner cell).
So the reason is that more voltage is required to reduce $H^+$ at Hg than at Pt.

Concept Hydrogen overvoltage on mercury

Electrochemistry in past papers

74 questions from this chapter have appeared across 27 exam years.