Q40 of 136 Page 38

Unlike dry cell, the mercury cell has a constant cell potential throughout its useful life. Why?

The following diagram represents a mercury cell.



The reactions take place inside a mercury cell are as follows:


Anode: Zn(Hg) + 2OH–→ ZnO(s) + H2O + 2e–


Cathode: HgO + H2O + 2e–→ Hg(l) + 2OH–


Galvanic cell equation: Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)


The above overall reaction contains no ions whose concentration can change over time. So, the cell potential remains constant throughout the mercury cell life.


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