The reaction at cathode in the cells commonly used in clocks involves.

Solution

Chemistry ma’am · 4 steps

A clock cell is a dry cell, and its cathode turns MnO₂ into MnO(OH), so Mn drops from +4 to +3.

1. Which cell. Clocks, torches and remotes use the dry cell (Leclanché cell). Its anode is a zinc container, and its cathode is a carbon rod packed in a paste of MnO₂ and carbon.
2. Cathode reaction. The cathode always gains electrons, so the reaction there is a reduction. In the dry cell, manganese dioxide is the species that gets reduced.

   MnO₂ + NH₄⁺ + e⁻ → MnO(OH) + NH₃

3. Oxidation states. In MnO₂, Mn is +4. In MnO(OH) the oxygens give -2 - 1 = -3, so Mn is +3. One electron is gained per Mn.
4. So. The cathode reaction is the reduction of Mn from +4 to +3, which is option (b).

The idea to keep. Cathode means reduction, every time. For the dry cell, remember the pair MnO₂ → MnO(OH): Mn goes from +4 to +3.

Answer from the NTA final answer key. Solution by PrepWiser.