Revision Notes

Electrolysis

GCSE Chemistry

This is the (chemical) process by which a compound in its molten state is decomposed to form its elements. Some of its uses are as follows:

  • Extraction of reactive metals from their ore. (e.g. – aluminium from bauxite)
  • Electroplating
  • Chloralkali Industry – electrolysis of brine to produce chlorine, hydrogen, and the alkali, sodium hydroxide.
  • Purification of metals (e.g. – copper)
  • Anodising

Metals

Metals conduct electricity in their solid state or when they are melted but they are NOT decomposed in the process (i.e. a chemical reaction does not occur). Metals can conduct electricity because in their structure, each metal atom gives up its outer shell electrons to form a sea of delocalised electrons. This means that the electrons are free to move through the metal’s structure and therefore conduct electricity.

Electrolytes

Non-metals, apart from graphite, do not possess freely moving electrons and so, they are insulators.

A compound that is in its molten or aqueous state conducts electricity and is decomposed by it. The current is carried by ions which are free to move through the liquid to the oppositely charged electrodes where they are discharged (acids / alkalis / salts are all examples). Ions are not mobile when a compound like this is in its solid state.

Electrodes

Cathode (-) electrode – Positive ions are attracted to it. Positive ions are called Cations (e.g. Na+)

Anode (+) electrode – Negative ions are attracted to it. Negative ions are called Anions (e.g. Cl–)

An Example of Electrolysis

Lead Bromide (PbBr2) can undergo electrolysis to form bromine gas and lead. The lead bromide is melted in a heat resistant vessel. The electrodes are put into the liquid and they are connected to an electrical circuit.

After a short time, a layer of metal (lead) is deposited around the cathode (negative). Also, clouds of orange gas (bromine) form around the anode (positive).

The following equations show the reactions taking place:

  • (At the cathode) Pb2+ + 2e– —-> Pb (neutral atom)
  • (At the anode) 2Br– – 2e– —-> Br2 (bromine molecule)