COMEDK 2024 Shift 2 Paper

The correct answer is Option A: It prevents pressure being developed in the cell due to NH3 gas formation.
Here's why:
In a dry cell, the following reactions occur:
At the anode (Zinc casing):
Zn(s)→Zn2+(aq)+2e−

At the cathode (Carbon rod):
2MnO2(s)+2NH4+(aq)+2e−→Mn2O3(s)+2NH3(aq)+H2O(l)
The ammonia (NH3) produced at the cathode is a gas. If it's not controlled, it can build up pressure inside the cell, potentially causing it to leak or even explode.
The role of ZnCl2 is to react with the ammonia to form a complex ion:

Zn2+(aq)+4NH3(aq)⇌[Zn(NH3)4]2+(aq)
This reaction effectively removes the ammonia gas from the system, preventing the buildup of pressure and ensuring the stability of the cell.
Let's look at why the other options are incorrect:
Option B: The cathode is the carbon rod where reduction occurs, and the anode is the zinc casing where oxidation takes place. The ZnCl2 doesn't directly affect this role.
Option C: Zinc acts as the anode, not the cathode. The carbon rod acts as the cathode.
Option D:ZnCl2 doesn't prevent the leakage of the electrolyte. The cell's design and the paste's consistency are more important for preventing leaks.