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Uses of Electrolysis

There are several uses for electrolysis, these include:

Purification of Copper

Purification of copper is carried out using CuSO₄ solution as electrolyte, the impure copper as the anode and the cathode a strip of pure copper. During the process, the anode (impure copper) decomposes and goes into solution as Cu²⁺,

i.e. Anode reaction: Cu(s) → Cu²⁺(aq) + 2e^-(aq) - oxidation reaction.

While at the cathode, pure copper is deposited i.e. Cathode reaction:

Cu²⁺(aq) + 2e^-(aq) → Cu(s) - reduction reaction

The impurities, which include silver (Ag) and gold (Au) are not discharged in the process, but collect in a sludge at the bottom of the cell and are recovered from it.

Production of Aluminium

In the production of aluminium, pure aluminium oxide, Al₂O₃ (alumina) is obtained from the chief raw material – bauxite, Al₂O₃.2H₂O. The purified Al₂O₃ is then electrolyzed in solution in molten cryolite (Na₃AlF₆ – sodium hexaflouroaluminate (III)) to obtain pure aluminium.

Cryolite is unchanged in the process and can therefore be used indefinitely.

Cathodic reaction: 2Al³⁺(aq) + 6e^-(aq) → 2Al(s)

Anodic reaction: 3O^2-(aq) → 3/2 O₂(g) + 6e^-(aq)

Note: The carbon anodes are constantly attacked by the oxygen produced at the anode. Hence, the anodes are usually changed.

The cryolite serves to dissolve the Al₂O₃, thereby enabling it to conduct electricity and decompose. Alumina, Al₂O₃ is not dissolved in water because it reacts vigorously with water.

Production of Na and Cl₂

We have already discussed the electrolysis of fused NaCl (without water). Sodium and chlorine are produced commercially by the electrolysis of NaCl in the molten condition (Down’s process). Due to high melting point of sodium chloride (about 800^oC), calcium chloride is added to it to lower it to about 600^oC.

Cathodic reaction: 2Na⁺(aq) + 2e^-(aq) → 2Na(s) – reduction.

Anodic reaction: 2Cl^-(aq) → Cl₂(g) + 2e^-(aq) – oxidation

Chlorine can also be produced through the electrolysis of conc. sodium chloride solution (Brine).

Production of Oxygen

The electrolysis of dilute H₂SO₄ and CuSO₄ solutions produce oxygen at the anode. These have earlier been treated here.

Production of NaOH

The electrolysis of conc. sodium chloride solution using carbon electrodes results in the solution being highly concentrated of sodium hydroxide. Sodium hydroxide may be prepared commercially by this method. This process has earlier been treated here.

Also, using mercury cathode, Na⁺ is discharged, and then reduces to the metal. i.e. Na⁺(aq) + e^-(aq) → Na(s)

Then Na(s) combines with the mercury to for a mixture - sodium amalgam Na/Hg.

The sodium amalgam is run into water, resulting in the reaction of sodium with water to form NaOH solution, while the mercury is dissociated from it.