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

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Solution

Generally, in a dry cell, cell potential is maintained by a continuous movement of ions which get completely used up after a period of time, and hence, cell potential is not maintained and the dry cell stops working. But, this does not happen in case of a mercury cell because it does not involve the movement of ions.

The reactions inside a mercury cell are as follows:

Anode:
$Z n (H g) + 2 O H^{-} (a q) \to Z n O (s) + H_{2} O (l) + 2 e^{-}$

Cathode:
$H g O (s) + H_{2} O (l) + 2 e^{-} \to H g (l) + 2 O H^{-} (a q)$

Overall reaction for the cell:
$Z n (H g) + H g O (s) \to Z n O (s) + H g (l)$

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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Z n (H g) + H g O_{(s)} \to Z n O_{(s)} + H g_{(l)}

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{N a}^{+} (a q) + e^{-} ⟶ N a (s) E^{0} = - 2.71 V

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