Discussion
Consider the following electrochemical cell, connected to the potential source (Potentiostat) with Hg as a working electrode :
Hg/Cd2+(0.01M), Zn2+ (0.01M), HCl(1.0M) || HCl(1.0M)| H2(g)|Pt
when the potential is change towards (i) more negative direction and (ii) change towards the more positive direction from the equilibrium potential value, the following observations are expected :
Given :
E°(Cd2+/Cd°) = − 0.351 V,
E°(Zn2+/Zn°) = − 0.762 V, E°(Hg2+/Hg°) = + 0.855 V
Answer: B
The expected observations are (i) Cd deposits first, followed by Zn; (ii) Hg starts oxidising.
On the negative sweep, the ion with the less negative reduction potential is reduced first: Cd2+ (E° = −0.351 V) before Zn2+ (−0.762 V), both depositing into the mercury as amalgams. Hydrogen evolution, which would otherwise come first in 1 M HCl, is suppressed by the very high hydrogen overvoltage on mercury, which is why mercury is used as the working electrode in polarography.
On the positive sweep, the electrode itself is oxidised long before water: mercury dissolves (in chloride medium as Hg2Cl2), which limits the anodic range of a mercury electrode, so O2 evolution (A, C) is not reached. Option D leaves out the later deposition of zinc.
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