Electrochemistry

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

1. Hg is more inert than Pt
2. more voltage is required to reduce H+ at Hg than at Pt
3. Na is dissolved in Hg while it does not dissolve in Pt
4. concentration of H+ ions is larger when Pt electrode is taken

Answer

(2) more voltage is required to reduce H+ at Hg than at Pt

Solution

At the cathode, Na+ and H+ (from water) compete for reduction.

At a Pt cathode, H+ (water) is reduced more easily ⇒ H2 is liberated.

As per NCERT, the discharge of hydrogen at a mercury cathode needs a high overpotential, so at a Hg cathode Na+ is reduced instead and the sodium dissolves in mercury to form sodium amalgam.

Na+ + e- + Hg Na-amalgam

∴ More voltage is required to reduce H+ at Hg than at Pt.

Answer: (2)