Dry Cell (or) Leclanche's Cell

Dry Cell (or) Leclanche's Cell - PRIMARY BATTERIES

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

Description

Anode : Zinc

Cathode : Carbon rod (graphite)

Electrolyte : ( NH₄Cl + ZnCl₂ + MnO₂ + Starch + H₂O )

Emf : 1.5 V

Construction

A dry cell consists of a zinc cylinder, which acts as anode. This zinc cylinder is filled with an electrolyte consisting of NH₄Cl, 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.

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 NH₄⁺ ions from the electrolyte.

Cell reactions:

At anode: Zn →  Zn²⁺ + 2e^‒

At cathode: NH₄⁺(aq) + MnO_2(s) + 2e^‒ → MnO(OH) ̄ + NH₃

Overall Reaction:

 Zn + NH₄⁺_(aq) + MnO_2(s) → Zn²⁺ + MnO(OH) ̄ + NH₃  

In cathode reaction, Mn is reduced from +4 oxidation +3 oxidation state. The liberation of NH₃ gas, which disrupts the current flow, is prevented by a reaction of NH_3(g) with Zn²⁺ (from ZnCl₂).

 ZnCl₂ + 2NH₃ →  [ Zn(NH₃)₂ ] Cl₂(s)

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

Advantages and Disadvantages of Leclanche's cell

Advantages

1. The cost of dry cell is cheap and easily available.

2. The compact size of it makes it suitable for powering small electronic devices.

3. The electrolyte used in dry cell is not harmful and no leakage.

4. It is easy and safe for transportation.

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.

3. This cell can not be recharged.

4. Voltage decreases with continuous usage.

Uses:

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