RedoxOxford AQA IGCSE Chemistry: Revision notes

The original, simplest definitions of oxidation and reduction are based on oxygen:

• Oxidation = gain of oxygen
• Reduction = loss of oxygen

Example: when iron rusts, iron gains oxygen to form iron oxide — this is oxidation. Example: in the blast furnace, iron oxide loses oxygen to form iron — this is reduction.

A more general definition of oxidation and reduction is based on electron transfer, and applies even when oxygen is not involved (e.g. displacement reactions, electrolysis):

• Oxidation = loss of electrons
• Reduction = gain of electrons

A simple way to remember this is the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

A redox reaction is one in which oxidation and reduction happen at the same time — one substance is oxidised while another is reduced.

Example: in the reaction between magnesium and copper sulfate solution:

Mg + CuSO₄ → MgSO₄ + Cu

• Magnesium atoms lose electrons to form Mg²⁺ ions — oxidation
• Copper ions gain electrons to form copper atoms — reduction

This is a redox reaction because both processes occur together.

Redox ideas apply across the specification:

• Electrolysis: at the cathode, positive ions gain electrons (reduction); at the anode, negative ions lose electrons (oxidation)
• Metal displacement: a more reactive metal loses electrons to form ions (oxidised) while ions of a less reactive metal gain electrons to form atoms (reduced)
• Half-equations can be written to show the electron transfer for each species separately, e.g. Cu²⁺ + 2e⁻ → Cu (reduction)