Q.In Mercury cell which of the following acts as a Cathode?

This is a matching problem linking battery types to their real-world applications. The correct pairing is: Lead storage cell → Inverter (d), Mercury cell → Wrist watch (c), Dry cell → Wall clock (a), Fuel cell → Apollo Space Programme (b). So the answer is option (B).

The mercury cell has a zinc-mercury amalgam anode and an HgO/carbon cathode paste in an alkaline (KOH/ZnO) electrolyte; because the electrolyte composition never changes as the cell discharges, it gives a remarkably steady voltage throughout its life.

At the anode (oxidation): Zn(Hg) + 2OH- → ZnO(s) + H2O + 2e-

At the cathode (reduction): HgO(s) + H2O + 2e- → Hg(l) + 2OH-

Adding the two half-reactions (the H2O and OH- cancel):

Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)

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The mercury cell's anode is a zinc-mercury amalgam; the cathode is a paste of HgO and carbon.

The mercury cell (used in hearing aids, watches) is a primary cell with:

• Anode: Zinc mercury amalgam (Zn–Hg), i.e. zinc metal amalgamated with mercury
• Cathode: a paste of HgO and carbon
• Electrolyte: a paste of KOH and ZnO

Electrode reactions:

Anode: $Zn(Hg) + 2OH^- \rightarrow ZnO(s) + H_2O + 2e^-$

Cathode: $HgO + H_2O + 2e^- \rightarrow Hg(l) + 2OH^-$

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