Combination reactions

A combination reaction has the general form:

For this to be a redox reaction, at least one of the reactants (A or B, or both) must be in its elemental form (oxidation number zero). When elements combine, their oxidation numbers change — one element gets oxidised (its oxidation number increases) and another gets reduced (its oxidation number decreases).

All combustion reactions that use elemental dioxygen ($O_2$) are redox combination reactions. But the category is broader — any reaction where an element combines with another substance qualifies.

Examples with oxidation numbers shown:

Carbon goes from 0 to +4 (oxidation), oxygen goes from 0 to –2 (reduction).

Magnesium is oxidised (0 → +2), nitrogen is reduced (0 → –3).

$\overset{-4}{C}\overset{+1}{H_4}(g) + 2\overset{0}{O_2}(g) \xrightarrow{\Delta} \overset{+4}{C}\overset{-2}{O_2}(g) + 2\overset{+1}{H_2}\overset{-2}{O}(l)$ …