Question

Explain the construction and working of a dry cell (or Leclanche’s cell). 

OR

Write a note on dry cell.

Answer 1

(A) Principle :

• Leclanche’s cell is a primary voltaic cell. 
• It doesn’t contain mobile liquid electrolyte but contains moist viscose aqeuous paste of the electrolytes. 
• It is an irreversible voltaic cell which can’t be recharged.

 (B) Construction :

(i) It consists of a small zinc vessel which serves as an anode (negative electrode).

(ii) The zinc vessel contains a porous paper bag containing an inert graphite (C) electrode which serves as cathode, immersed in a paste of MnO₂ and carbon black. This paper bag divides the dry cell into two compartments, namely anode and cathode compartments.

(iii) The rest of the cell is filled with a moist paste of NH₄Cl and ZnCl₂ which acts as an electrolyte for zinc anode.

(iv) The graphite rod is fitted with a metal cap and the cell is sealed to prevent the drying of moist paste by evaporation.

(C) The dry cell can be represented as,

^-Zn|ZnCl_2(aq), NH₂Cl_(aq) , MnO_2(s) |C⁺.

(D) Reactions in the dry cell :

(i) Oxidation at zinc anode :

Zn_(s) → \(Zn^{2+}_{(aq)}\) + 2e^- (oxidation half reaction)

(ii) Reduction at graphite (C) cathode : 

The electrons released in the oxidation reaction at anode, flow to cathode through external circuit.

Hydrogen in NH₄ ion is reduced to molecular hydrogen which reduces MnO₂ to Mn₂O₃.

(iii) Zn²⁺ react with NH₃ and form a complex.

\(Zn^{2+}_{(aq)}\) + 4NH_3(aq) ⟶ [Zn(NH₃)₄]²⁺_(aq)

Since Zn²⁺ ions are removed, the overall cell reaction can’t be reversed.

(E) Uses of dry cell :

• Dry cell is used as a source of electric power in radios, flashlights, torches, clocks, etc
• Since they are available in small size and portable, they can be used conveniently.