Types of Redox Reactions

Once a reaction is confirmed to be redox — some element's oxidation number changes — it usually falls into one of a small number of recognizable structural patterns, based on how many reactants and products are involved and how the oxidation-number change is distributed.

Combination reactions join two or more elements or simple compounds into a single product, and are redox whenever at least one participating element starts as a free element — for instance $2\text{Mg} + \text{O}_2 \to 2\text{MgO}$ or $\text{H}_2 + \text{Cl}_2 \to 2\text{HCl}$; both reactants begin at oxidation number $0$ and end up combined.

Decomposition reactions are the reverse pattern — a single compound breaks apart into two or more simpler products — and are redox whenever at least one product is a free element, for example the thermal decomposition $2\text{KClO}_3 \to 2\text{KCl} + 3\text{O}_2$, in which oxygen is released as $\text{O}_2$ (oxidation number $0$) from oxide oxygen (oxidation number $-2$ in the reactant).

Displacement reactions occur when one element in a compound is replaced by another, more reactive element supplied as the free metal (or, less commonly, nonmetal) — the zinc–copper sulfate reaction from earlier in this chapter, $\text{Zn} + \text{CuSO}_4 \to \text{ZnSO}_4 + \text{Cu}$, is the classic example: zinc displaces copper from solution because zinc is the more easily oxidized (more reactive) metal. …