Working of a Dry Cell:
Anode (Negative Terminal): Zinc
Cathode (Positive Terminal): Carbon coated with MnO2
Electrolyte: Mixture of plaster of Paris, Ammonium Chloride and Zinc Chloride
Dry cells contain a Zinc container which itself acts as a negative electrode. The moist paste is made from a mixture of plaster of Paris, Ammonium Chloride, and Zinc Chloride called sal ammoniac paste. This forms the electrolyte of the cell and takes up the major amount of volume in the battery. Zinc Chloride is hygroscopic in nature and helps to maintain the moistness of the paste. It is wrapped in a canvas sheet.
Anode reaction: The oxidization of Zinc gives two electrons.
Zn(solid) → Zn2 + (aqueous) + 2 (e-)
The carbon rod forms the positive electrode. It is coated with MnO2 and powdered carbon. The powdered carbon reduces the internal resistance of the cell. The top of the cell contains a layer of sawdust. This acts as the base for the top layer of bitumen used for sealing purposes.
2MnO2(solid) + H2(gas)→ Mn2O3(solid) + H2O(liquid)
Electrolyte reaction: Hydrogen from Ammonium chloride
2NH4 + (aqueous ) + 2 (e-) → H2(g) + 2NH3(aqueous)
- Overall reaction in dry cell:
Zn(s) + 2MnO2(s) + 2NH4(+)(aqueous) → Mn2O3(solid) + Zn(NH3)2 (2+)(aqueous) + H2O(liquid)
A vent is provided in this layer to allow the gases formed in the chemical reaction to escape. Irrespective of the size of the dry cell, the EMF is 1.5 V because the zinc and carbon rods used as electrodes specified a chemical equivalent. The chemical equivalent changes from metal to metal and, depending on the type of combination used, the EMF differs.