Summary

Redox reactions form an important class of reactions in which oxidation and

reduction occur simultaneously. The chapter builds the idea three ways — the

classical view (addition/removal of oxygen or hydrogen and of electronegative or

electropositive elements), the electronic view (electron transfer), and the

oxidation-number view — and defines oxidation, reduction, oxidising agent (oxidant) and reducing agent (reductant) under each. The mnemonic OIL RIG

(Oxidation Is Loss, Reduction Is Gain — of electrons) captures the electronic view.

Oxidation numbers are assigned in accordance with a consistent set of rules: free

elements are 0; a monatomic ion equals its charge; oxygen is $-2$ (peroxides $-1$,

superoxides $-\frac{1}{2}$, positive when bonded to fluorine); hydrogen is $+1$ (metal

hydrides $-1$); fluorine is always $-1$; and the algebraic sum equals zero for a

compound, or the charge for a polyatomic ion. Oxidation is an increase in an element's

oxidation number; reduction is a decrease.

Redox reactions are classified into four categories: combination, decomposition,

displacement and disproportionation. In a disproportionation reaction the same

element, in an intermediate oxidation state, is simultaneously oxidised and reduced —

e.g. $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$

(oxygen: $-1 \rightarrow -2$ and $0$), or chlorine in alkali,

$\text{Cl}_2 + 2\text{OH}^- \rightarrow \text{ClO}^- + \text{Cl}^- + \text{H}_2\text{O}$.

Redox equations are balanced by two systematic methods: the oxidation number method

(equalise the total increase and decrease in oxidation number, then balance charge with

H⁺/OH⁻ and hydrogen with H₂O) and the ion-electron (half reaction) method (balance

each half reaction for atoms and charge, equalise the electrons, and add). In basic

medium, balance as if acidic, then neutralise every H⁺ with OH⁻ on both sides.

Redox reactions are the basis of redox titrations: the end point is signalled by a

self-indicator (the lasting pink of MnO₄⁻), by an added redox indicator (diphenylamine

turning intense blue with the first excess of Cr₂O₇²⁻), or by the starch–iodine colour

in iodine/thiosulphate titrations.

A redox couple is the oxidised and the reduced form of a substance taking part in an

oxidation or reduction half reaction, written oxidised form/reduced form (e.g. Zn²⁺/Zn,

Cu²⁺/Cu). When the two half reactions of a spontaneous redox reaction are physically

separated and connected through a wire and a salt bridge — the Daniell cell — the …