Question

What type of battery is Mercury cell? Why it is more advantageous than dry cell? Write overall reaction taking place in Mercury cell.

Hint:

Mercury cells are ideal for small, low-current devices like digital watches and hearing aids where a stable voltage is essential for accuracy.

Answer 1

Step 1: Understanding the Concept:
Batteries convert chemical energy into electrical energy. Primary cells are designed for single use. Constant voltage is a desirable trait for sensitive electronics.
Step 2: Detailed Explanation:
1. Type: The mercury cell is a primary battery. This means the cell reaction is irreversible, and the battery must be discarded once the active chemicals are exhausted.
2. Advantages: In a standard dry cell (Leclanché cell), the voltage gradually decreases from \(1.5\) V as chemicals are consumed and reaction products accumulate.
In contrast, the mercury cell provides a constant potential of approximately \(1.35\) V throughout its entire service life.
This is because the overall reaction only involves solids and liquids, and no ions in the electrolyte are consumed or produced in a way that changes their molar concentration.
3. Overall Reaction: The anode is a zinc-mercury amalgam, and the cathode is a paste of mercury(II) oxide mixed with carbon.
The half-reactions are:
Anode: \(Zn(Hg) + 2OH^- \rightarrow ZnO(s) + H_2O + 2e^-\)
Cathode: \(HgO(s) + H_2O + 2e^- \rightarrow Hg(l) + 2OH^-\)
Summing these gives the overall cell reaction:
\[ Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l) \]
Step 3: Final Answer:
The mercury cell is a primary cell. It is better than a dry cell because it maintains a steady voltage. The reaction is \(Zn(Hg) + HgO(s) \rightarrow ZnO(s) + Hg(l)\).