Redox (II): Storage cells: the Leclanche cell.

Edexcel A level Chemistry (2017)

Topic 14: Redox (II): Storage cells: the Leclanche cell.

14/15. To be able to understand the application of electrode potentials to storage cells.

The Leclanche cell

This is the original dry cell, the versatile and reliable if not very long–lasting carbon-zinc cell, invented in 1866 and in use until fairly recently when alkaline dry cells superceded it.

It uses a moist paste rather than a liquid as an electrolyte.

You can see how the Leclanche cell is constructed in the diagram below:

The Leclanche cell has a zinc cup for an cathode, a carbon rod for an anode, and a wet paste made of powdered carbon, ammonium chloride (NH₄Cl), zinc chloride (ZnCl₂), and manganese (IV) oxide (MnO₂) for an electrolyte.

Powdered carbon and manganese(IV)oxide fill the space close to the carbon rod whereas zinc chloride and ammonium chloride are close to the zinc electrode. 

Several reactions take place in the cell.

At the zinc electrode zinc atoms are oxidized and release electrons into the cell.

Zn  ⟶  Zn²⁺   +   2e–

The cell works until there is little zinc to be oxidized.

Ammonium ions pick up these electrons and form ammonia and hydrogen gas.

2NH₄⁺    +   2e^–      ⟶     2NH₃     +   H₂

The ammonia is very soluble in the paste so presents little problems initially until the cell has almost run down.  The problem lies with the hydrogen because it is not soluble in the electrolyte.  However, two further reactions take place in the cell to absorb and remove the gases formed and stop the possible build up of gas and the possible explosion of the cell.

First zinc chloride reacts with the ammonia to form zinc ammonium chloride

ZnCl₂(aq)   +    2NH₃(g)    →      Zn(NH₃)₂Cl₂(s)

The hydrogen reduces the manganese(IV)oxide to form water and manganese(III) oxide (Mn₂O₃)

2MnO₂(s)    +    H₂(g)    →     Mn₂O₃(s)     +     H₂O(l)

In this way the threat of the cell exploding is eliminated and the cell gradually runs down until the zinc electrode is worn out and the cell starts to leak.

Overall we can write this

Zn + 2MnO₂ + 2NH₄Cl →  ZnCl₂ + Mn₂O₃ + 2NH₃ + H₂O

.

A cell diagram looks something like this:

Zn(s) | Zn²⁺ (aq) || [2NH₄⁺(aq) , 2NH₃(g) + H₂(g)] |C(s)

The EMF of the cell is initially  +1.5V.

If the half cell redox potential of the zinc half cell is –0.76v then the redox potential of the other half cell must be

E _cell =  E_rhs  – E_lhs

So +1.5v   =   E_rhs  –  –0.76v

E_rhs  =  +1.5v – 0.76v   =  +0.74v

It runs down fairly quickly especially in cold weather.  So storage of charge is not well accomplished and so this cell is now surpassed by the use of alkaline nickel cadmium cells and lithium cells.

I will post on these alkaline cells next