Redox reaction

A redox reaction is a reaction in which oxidation and reduction occur together, because the electrons lost by one species are exactly the electrons gained by another. The word itself is a blend of reduction and oxidation. Malay: tindak balas redoks · Chinese: 氧化还原反应.

The two halves are inseparable: you cannot have oxidation without reduction, since released electrons must go somewhere. This is why redox is treated as a single event with two linked changes. To show it, chemists split the reaction into two half-equations, one for the loss of electrons and one for the gain, and the number of electrons in each must balance when they are combined. Many familiar reactions are redox: combustion, displacement, corrosion, the reactions in cells, and metal extraction.

Example. When zinc reacts with copper(II) sulfate, zinc atoms lose electrons (Zn to Zn²⁺ plus 2e-, oxidation) and copper(II) ions gain them (Cu₂⁺ plus 2e- to Cu, reduction). The two electrons lost by zinc are the two gained by each copper(II) ion, so the overall change Zn plus Cu²⁺ to Zn²⁺ plus Cu is a redox reaction.

Confusion to avoid. Do not label a reaction redox just because it looks vigorous. A reaction is redox only if oxidation numbers change; neutralisation and precipitation usually involve no change in oxidation number and are not redox. Check each element before and after to be sure electrons have actually transferred.

A useful habit is to assign oxidation numbers to every element before and after the reaction; if any element’s number changes, the reaction is redox, and the size of the change tells you how many electrons moved. Recognising a redox reaction, and naming which species is oxidised and which is reduced, threads through displacement, cells, rusting and extraction in this chapter, and is asked directly in SPM Chemistry.