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Electrochemistry
Products of Electrolysis: Overpotential
1 question from 1 paper, each with its answer.
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Concepts tested
- Hydrogen overvoltage on mercury
All Questions
2002 AIPMT 1 question
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In electrolysis of NaCl when Pt electrode is taken then H2 is liberated at cathode while with Hg cathode it forms sodium amalgam : -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.