In a concentration cell, the electrodes are the same material and the half-cells differ only in concentration.

A concentration cell generates voltage by transferring the electrons from a half-cell with lower concentration of electrolyte to the cell with the higher concentration. The concentration cell, a type of voltaic cell, has the same electrodes but different concentrations of electrolytes on either side. The electrolytes on either side of the cell joined by a salt bridge. This difference in concentration is enough for a voltage to be generated as the cells reach equilibrium.

Take an example of a concentration cell of 2 Zn electrodes with ZnSO4 electrolytes (0.1M and 1M concentrations). The less concentrated cell would need to increase its Zn2+ ion concentration, and this can be achieved by oxidation of Zn electrode (anode):

Zn(s) → Zn2+(aq) + 2e–

In the more concentrated cell, the amount of Zn2+ ions need to decrease from the electrolyte solution. This can be achieved if the Zn2+ ions gain electrons and form solid Zn (deposited on the electrode). The electrode in the more concentrated solution becomes the cathode, where reduction happens.

Zn2+(aq) + 2e–  → Zn(s)

The two compartments of a cell must be separated, but they should be linked to allow ion transfer. If there is no salt bridge, the cell on the cathode side (more concentrated) will accumulate negative charge, where the anode side (less concentrated) will accumulate positive ions (Zn2+) in this case. This quickly prevents further reaction, and hence production of electricity.

Key Points

• A  concentration cell is a voltaic cell where the electrodes are the same material and the half-cells differ only in concentration.

• A concentration cell generates voltage by transferring the electrons from a half-cell with lower concentration of electrolytes to the cell with the higher concentration. This difference in concentration is enough for a voltage to be generated as the cells reach equilibrium.

• The electrolytes on either side of the cell are joined by a salt bridge.

Key Terms

Half-cell: Either of the two parts of an electrochemical cell containing an electrode and an electrolyte.

Electrode: The terminal through which electric current passes between metallic and nonmetallic parts of an electric circuit. In electrolysis, the electrodes are placed in the solution separately.

Voltaic cell: A cell, such as in a battery, in which an irreversible chemical reaction generates electricity; a cell that cannot be recharged.

Electrolyte: a substance containing free ions that carry electric current.

Anode: The electrode of an electrochemical cell at which oxidation (loss of electrons) occurs.

Cathode: The electrode of an electrochemical cell at which reduction (gain of electrons) occurs.