Write short note on dry cell.

Explanation

Dry cell is the most familiar commercial cell. It consists of a cylinderical vessel made up of zinc. It is filled with a paste of `NH_(4)Cl` along the small amount of zinc chloride. Zinc cylinder acts as anode. A carbon rod is placed at the centre. A carbon rod is placed at the centre. It acts as cathode. This cathode is surrounded by a black paste of `MnO_(2)`. This arrangement is shown in the given figure :
When the cell is working, Zn loses electrons to form `Zn^(2+)` ions. These electrons pass through the external circuit and are taken at the cathode. This causes the discharge of `NH_(4)^(+)` ions.
Following reactions take place at the respective electrodes :
At anode : `Zn(s)rarrZn^(2+)(aq)+2e^(-)`
At cathode : `2NH_(4)^(+)(aq)+2MnO_(2)(s)+2e^(-)rarrMn_(2)O_(3)(s)+2NH_(3)(aq)+H_(2)O(l)`
Ammonia combines with `Zn^(2+)` ions to form a complex ion, `[Zn(NH_(3))_(2)]^(2+)`.