Concentration Cells

Concentration cells are those cells whose is zero.

Concentration cells are of two types,

1. Electrode Concentration Cells 
2. Electrolyte Concentration Cells

Electrode Concentration Cells

In these cells, two similar electrodes at different concentrations are dipping in the same solution. Two hydrogen electrodes at unequal gas pressures immersed in the same solution of hydrogen ions constitute an electrode concentration cell. This may be represented as follows

Pt ⏐H₂(p1) | solution of H+ ions |H₂(p₂)⏐ Pt

The reactions occurring are

At cathode: 2H⁺ + 2e^–  →  H₂(p₂)(Reduction)

At anode:H₂(p₁) → 2H⁺ + 2e–(Oxidation)

Net reaction:H₂(p₁) → H₂(p₂)

This reaction is evidently independent of the concentration of the electrolyte.

At moderate pressures, H₂ can be considered to be an ideal gas so that the ratio of the fugacities can be considered to be equal to the ratio of the gas pressures. Hence, the Nernst equation may be written as

At 25 °C

Since, by definition, = 0, we have

Ecell = –0.0295 log = 0.0295 log

When p₂ <p₁, the EMF is positive so that the whole process is spontaneous.

Another example of the electrode concentration cell is that of an amalgam with two different concentrations of the same metal

Hg–Pb(c₁)⏐PbSO₄ (soln.)⏐Hg–Pb(c₂)

The electrode reactions are

At cathode: Pb²⁺ + 2e– → Pb(c2)(Reduction)

At anode:Pb(c1) → Pb²⁺ + 2e–(Oxidation)

Net reaction:Pb(c1) →Pb(c₂)

The EMF of the cell is given by

Here, too, if c₂ < c₁, the EMF is positive so that the whole process is spontaneous, i.e., lead will go spontaneously from the high concentration to the low concentration amalgam.

Electrolyte Concentration Cells

In these cells, the two electrodes of the same metal are dipping in solutions of metal ions of different concentrations. One such cell is represented as

Zn⏐ Zn²⁺ (c₁)|| Zn²⁺(c₂)⏐Zn

In this case, both the electrodes are of the same metal (Zn) and these are in contact with solutions of the their metal ions ( Zn²⁺). The concentrations and hence activities of the ions are however, different. Let (c1) and (c2) be the concentrations of zinc ions in the two electrolyte (ZnSO₄) solutions which are separated from each other by a salt bridge.

The electrode reactions are

At cathode: Zn²⁺ (c2) + 2e− → Zn(s) (Reduction)

At anode:Zn(s) →  Zn²⁺(c1) + 2e– (Oxidation)

Net reaction: Zn²⁺(c₂) →  Zn²⁺(c1)

According to Nernst equation, the reduction potentials of cathode and anode are given by

For the process to be feasible (spontanoues), EMF should be positive. Hence, c2 > c1.