Classification of Chemical Reactions

Three underlying ways for reactions to happen

A chemical reaction changes reactants into products by rearranging atoms and their electrons. Although reactions have many familiar names—such as neutralisation, displacement and combustion—AQA identifies three fundamental ways in which chemical reactions take place: proton transfer, electron transfer and electron sharing.

These describe what happens to particles during the reaction. They are not always mutually exclusive labels: the same reaction may be classified in other ways according to its energy change, observable pattern or overall use. For example, a combustion reaction can also be exothermic and an electron-transfer reaction.

Proton transfer: moving $H^+$

A proton is the positively charged nucleus of a hydrogen atom. In acid–base chemistry it is represented as $H^+$. A proton-transfer reaction occurs when one particle donates $H^+$ and another accepts it.

An acid is the proton donor and a base is the proton acceptor. In neutralisation by hydroxide ions, the essential change is

$H^+ + OH^- \rightarrow H_2O$

The hydroxide ion accepts the proton and becomes a water molecule. Other ions may be present, but if they remain unchanged they do not explain the central reaction. Looking at the ionic equation therefore makes the proton transfer clearer than looking only at a word equation.

Electron transfer: oxidation and reduction together

In an electron-transfer reaction, electrons move from one reacting particle to another. The electron donor is oxidised because it loses electrons; the electron acceptor is reduced because it gains electrons. Since an electron cannot be lost without being accepted somewhere else, oxidation and reduction always occur together. Such a reaction is called a redox reaction.

For the displacement of copper by iron, the overall ionic equation is

$Fe + Cu^{2+} \rightarrow Fe^{2+} + Cu$

It can be separated into two half-equations:

$Fe \rightarrow Fe^{2+} + 2e^-$

$Cu^{2+} + 2e^- \rightarrow Cu$

Iron loses two electrons, so iron is oxidised. Each copper ion gains two electrons, so copper ions are reduced. The two half-equations show that the same number of electrons is released and accepted.

Electron sharing: making covalent bonds

Atoms of non-metallic elements can be held together when they share pairs of electrons. The attraction between the shared negative electrons and both positive nuclei forms a covalent bond.

In an electron-sharing reaction, shared electron pairs are formed or rearranged as covalent bonds change. For example, when hydrogen and oxygen react to form water, bonds within the reactant molecules are broken and new O–H covalent bonds form. Each O–H bond contains a shared pair of electrons.

Electron sharing does not mean that an electron permanently leaves one atom and becomes the property of another. That would be electron transfer. Instead, the shared electrons are attracted to both bonded nuclei.

How to classify a reaction

Begin by identifying the particle-level change rather than relying only on the names of the substances:

1. If $H^+$ moves from a donor to an acceptor, classify the change as proton transfer.
2. If electrons are lost by one particle and gained by another, classify it as electron transfer or .

Equations provide useful evidence. An ionic equation containing $H^+$ moving to a base indicates proton transfer. Balanced half-equations containing $e^-$ demonstrate electron transfer. Displayed formulae or dot-and-cross diagrams can show electron sharing because they represent the shared pairs in covalent bonds.

These classifications focus on the mechanism of chemical change. Labels such as combustion, displacement, neutralisation, exothermic and endothermic describe other features and do not replace the particle-level explanation.