Anode: Zn(s) →→ Zn2+ + 2e-
Cathode: MnO2 + NH4+ + e-→→ MnO(OH) + NH3
In the reaction at cathode, manganese is reduced from the + 4 oxidation state to the +3 state. Ammonia produced in the reaction forms a complex with Zn2+ to give [Zn (NH3) 4] 2+.The cell has a potential of nearly 1.5 V.
The overall reaction of mercury cell is represented by
Zn(Hg) + HgO(s) →ZnO(s) + Hg(l)
The cell potential is approximately 1.35 V and remains constant during its life as the overall reaction does not involve any ion in solution whose concentration can change during its life time. Unlike mercury cell, Dry cell has shorter life (or) Cell potential in mercury cell remains constant but not in dry cell.
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