| Primary Cells |
| We will first consider a modified Ledanche's Cells in which an aqueous solution of the electrolyte is mixed with enough flour or starch to prevent spillage of the electrolyte. It can be represented as |
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Zn
 NH4CI ( 25% ) + ZnCl2 ( 10%) + Mn02 (s) + C (s)  Graphite |
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| Figure 23.1 : Schematic Representation of a Dry Cell (Primary cell)
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1] Outer Cardboard Cover 2]. Zinc Cup anode (Negative Electrode) 3] Plastic or Pitch seal 4] Graphite rod , cathode (positive Electrode) 5. Electrolyte (20 % NH4 Cl + 10% Zn Cl2 + Mn02 (s) + carbon, thickened with starch. |
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The above dry cell is capable of delivering 1.5 V and the cell reactions can be represented as: |
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Anode: Zn - 2e- = Zn2+ (oxidation) |
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Cathode: 2 MnO2 + 2 H2O + 2 e- = 2 MnO ( OH ) + 2 OH- (reduction) |
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Cell reaction: Zn + 2 MnO2 + 2 H2O = Zn2+ + 2 MnO ( OH) + 2 OH- |
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MnO2 is called the 'depolariser', since it prevents the formation of H2 at the cathode by preferentially getting reduced at the cathode. A secondary reaction (local action) results in the consumption of anode material and the electrolyte during the discharging of the cell. Once discharged, the dry cell cannot be charged for reuse. Dry cells deteriorate on storage due to local action and also due to the evaporation of water from the electrolyte. |
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Zn2+ + 2OH- + 2NH4CI
 Zn (NH3 )2Cl2 (s) + 2 H2O |
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