Reduction

Reduction is the gain of electrons by a species. It can also be described as the loss of oxygen or the gain of hydrogen, and it always makes the oxidation number of that species decrease. Malay: penurunan · Chinese: 还原.

Learn the electron definition with its direction fixed, because that is the wording the examiner rewards: in reduction, electrons arrive at the species. The memory aid OIL RIG, Reduction Is Gain, states it directly. As the species takes in negative electrons it becomes more negative, so its oxidation number falls. The oxygen and hydrogen definitions still appear: a metal oxide is reduced when carbon removes its oxygen, and unsaturated compounds are reduced when they gain hydrogen.

Example. When copper(II) oxide is heated with carbon, each Cu²⁺ ion gains two electrons to become copper metal; its oxidation number falls from +2 to 0, so the copper(II) ion is reduced. In a voltaic cell, Cu²⁺ ions gain electrons at the copper electrode (Cu₂⁺ plus 2e- to Cu), so those ions are reduced.

Confusion to avoid. Do not confuse reduction with oxidation, its paired opposite, and do not assume that removing oxygen is the only way a species can be reduced. The safe test is the electron count or the oxidation number: a fall in oxidation number always signals reduction, whether or not oxygen is involved. Remember too that the reducing agent is oxidised, not reduced.

Reduction never occurs alone; every time one species is reduced another is oxidised, so the two make up a single redox reaction. In a reduction half-equation the electrons always appear on the left-hand side, being added to the species, and the species that supplied them is the reducing agent. Identifying which species is reduced, tracking its fall in oxidation number, and writing its gain-of-electrons half-equation, is a common structured-question step in SPM Chemistry.