Example of Dry Cell - Leclanche Cell:
The most common example of dry cell is Leclanche cell invented by French scientist Georges Leclanche. We use to call it a battery. It has a cylindrical Zn container which works as the anode. At the center of the Zn container a carbon or graphite rod is placed vertically in a mixture of MnO2 and graphite powder. The Zn container is then filled up with a paste of ZnCl2 and NH4Cl. The above described Leclanche cell may be represented as follows,
The chemical changes that takes place in the Lecanche cell is complicated, but the main reactions may be regarded as –
Hydrogen gas is oxidized by MnO2 and water is formed which intern reacts with ammonia and ammonium hydroxide is produced.
Therefore, the cathodes reduction may be represented as,
Therefore, the total cell reaction is,
The surface area of carbon rod is increased by using the mixture of carbon and MnO2 powder. The polarization effect of hydrogen gas produced in the reduction reaction at the cathode is removed by the absorption of H2 gas by MnO2. Similarly the polarization effect of ammonia gas is also removed by the dissolution of ammonia in water. In dry cell the zinc container takes part into oxidation reaction. As a result on prolong use it is being perforated, paste leaks out from the cell.
Leclanche cell is irreversible cell. That means, it converts chemical energy into electrical energy by an irreversible process. Therefore the original change can not be restored by applying electrical energy from the external sources.