Question

Describe about the working principle of Leclanche cell.

Answer 1

1. Leclanche cell: 

Anode: Zinc container Cathode: Graphite rod in contact with MnO₂ Electrolyte: Ammonium chloride and Zinc chloride in water. 

emf of the cell = 1.5 V

2. Cell reaction: 

Oxidation at anode 

Zn_(s) → Zn²⁺_(aq) + 2e  ……(1)

Reduction at cathode 

2NH₄⁺_(aq) + 2e^- → 2NH_3(aq) + H_2(aq)

3. The hydrogen gas is oxidised to water by 

MnO₂ H_2(g) + 2MnO_2(s) → Mn₂O_3(s) + H₂O_(l) ……(3) 

Adding equations 1,2,3 the overall redox reaction 

Zn_(s) + 2NH⁺ + 2MnO_2(s) → Zn²⁺_(aq) + Mn₂O_3(s) + H₂O_(l) + 2NH₃ …………….(4) 

4. The ammonia produced at the cathode combines with Zn²⁺ to form a complex ion [Zn (NH₃)₄]₂₊ (aq). As the reaction proceeds, the concentration of NH₄⁺ will decrease and the aqueous NH₃ will increase which lead to the decrease in the emf of the cell.