Important Primary Battery: Dry Cell (or) Leclanche's Cell

IMPORTANT PRIMARY BATTERY

1. Dry Cell (or) Leclanche's Cell

It is a primary cell, which works without fluid component.

Description

A dry cell consists of a zinc cylinder, which acts as anode. This zinc cylinder is filled with an electrolyte consisting of NH₄C1, ZnCl₂ and MnO₂ in the form of paste using starch and water. A carbon rod (graphite), acts as cathode, is immersed in the electrolyte in the centre of the cell. The zinc cylinder has an outer insulation of cardboard case. During use, the zinc cylinder gets consumed and at the end, it will develop holes which are responsible for leakages.

Fig. 8.1 Leclanche's Cell

Working

When the cell is working, zinc loses electrons and Zn²⁺ ions gets dissolved in the electrolyte. The electrons pass through the circuit and are consumed at cathode. This causes discharge of NH⁺₄ ions from the electrolyte.

Cell reactions:

At anode:

Zn → Zn²⁺ + 2e^-

At cathode:

NH⁺_{4 (aq)} + MnO_{2 (s)}+ 2e^- → MnO(OH) ^- + NH₃

Overall Reaction:

Zn + NH⁺_{4 (aq)} + MnO_{2 (s)} → Zn²⁺ + MnO(OH) ^- + NH₃

In cathode reaction, Mn is reduced from +4 oxidation state to +3 oxidation state. The liberation of NH₃ gas, which disrupts the current flow, is prevented by a reaction of NH₃ (g) with Zn²⁺ (from ZnCl₂).

ZnCl₂ + 2 NH3 → [Zn(NH₃)₂] C1₂(s)

The voltage of Leclanche's cell is about 1.5 V.

Disadvantages

1. This dry cell does not have an indefinite life, because NH₄Cl being acidic corrodes the zinc container, even if it is not used.

2. When current is drawn rapidly from it, products build up on the electrodes, so voltage drop occurs.

Uses:

It is used in transistor radios, calculators, Flash lights, torches etc.,