Oxidation and reduction

The idea of oxidation and reduction grew over time.

The classical view: oxidation is the addition of oxygen (or another electronegative element) to a substance, or the removal of hydrogen (or an electropositive element). Reduction is the opposite: removing oxygen, or adding hydrogen. When magnesium burns, 2Mg + O₂ → 2MgO, magnesium is oxidised.

The electron-transfer view is more general: oxidation is loss of electrons, and reduction is gain of electrons. In 2Na + Cl₂ → 2Na⁺Cl⁻, sodium loses an electron (oxidised) and chlorine gains one (reduced), even though no oxygen is involved.

A memory trick: OIL RIG, Oxidation Is Loss, Reduction Is Gain. (Some teachers say GIR LEO: Gain Is Reduction, Loss of Electrons is Oxidation.)

Electrons lost by one substance must be gained by another, so oxidation and reduction always happen together. Such a reaction is a redox reaction. Two terms that confuse many students:

• The substance that is reduced is the oxidising agent (oxidant): it takes electrons and so causes the oxidation of the other.
• The substance that is oxidised is the reducing agent (reductant): it gives electrons and so causes the reduction of the other.

Competitive electron transfer: dip a zinc rod in copper sulphate solution. Zinc gives up electrons more readily than copper, so zinc goes into solution as Zn²⁺, and Cu²⁺ ions take the electrons and come out as copper metal on the rod: Zn + Cu²⁺ → Zn²⁺ + Cu. But put a copper rod in a solution of Zn²⁺, and nothing happens. In this way metals can be ranked by how easily they give up electrons.