Key terms in reactivity

Edexcel wants you to define oxidation and reduction in two ways: an oxygen definition (the older, historical one) and an electron definition (the more powerful, general one).

Process	Oxygen definition	Electron definition
Oxidation	gain of oxygen	loss of electrons
Reduction	loss of oxygen	gain of electrons

The mnemonic that examiners expect you to know is OIL RIG:

• OIL — Oxidation Is Loss (of electrons).
• RIG — Reduction Is Gain (of electrons).

Why two definitions?

• The oxygen definition is historical — early chemists studied metals burning in air, so 'gaining oxygen' became the original meaning of oxidation.
• The electron definition is more general — it works even when no oxygen is involved (e.g. Mg + Cl₂ → MgCl₂ is redox, but there is no oxygen).

Worked tip. Always check which clue the question gives you. If a species gains an oxygen (e.g. Mg → MgO), use the oxygen definition. If you can see electrons being lost or gained (e.g. Mg → Mg²⁺ + 2e⁻), use the electron definition. Either earns the mark — but match it to the evidence.

A redox reaction is one in which oxidation and reduction happen at the same time. You cannot have one without the other: the electrons lost by the species being oxidised are exactly the electrons gained by the species being reduced.

Worked example — magnesium burning.

$2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}$

• Oxygen view: Mg gains oxygen → Mg is oxidised.
• Electron view: Mg → Mg²⁺ + 2e⁻ (loses electrons → oxidised); O₂ + 4e⁻ → 2O²⁻ (gains electrons → reduced).

Both views agree: Mg is oxidised, oxygen is reduced — so it is a redox reaction.

Worked example — no oxygen present.

$\text{Mg(s)} + \text{Cl}_2\text{(g)} \rightarrow \text{MgCl}_2\text{(s)}$

The oxygen definition cannot help here (no oxygen). The electron definition still works:

• Mg → Mg²⁺ + 2e⁻ → Mg oxidised.
• Cl₂ + 2e⁻ → 2Cl⁻ → chlorine reduced.

This is why the electron definition is so useful — it identifies redox even with no oxygen in sight.

To answer 'which species is oxidised and which is reduced?', take each species in turn and ask what happened to it.

Method.

1. Write down what each species starts as and ends as.
2. Decide if it gained or lost oxygen, OR gained or lost electrons.
3. Label using OIL RIG: lost electrons / gained oxygen → oxidised; gained electrons / lost oxygen → reduced.

Worked example — displacement.

$\text{Fe(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Fe}^{2+}\text{(aq)} + \text{Cu(s)}$

• Iron: Fe → Fe²⁺ + 2e⁻ → loses electrons → oxidised.
• Copper: Cu²⁺ + 2e⁻ → Cu → gains electrons → reduced.

Worked example — extracting a metal with carbon.

$2\text{CuO(s)} + \text{C(s)} \rightarrow 2\text{Cu(s)} + \text{CO}_2\text{(g)}$

• Copper oxide loses oxygen → copper is reduced.
• Carbon gains oxygen → carbon is oxidised.

Exam discipline. Name the species, not just 'the metal'. Write "Fe is oxidised" and "Cu²⁺ is reduced" — being precise about the ion vs the atom (Cu²⁺ is reduced, not 'copper') keeps the mark.

An agent is the species that brings about a change in something else.

• An oxidising agent oxidises another species. It does this by taking (accepting) electrons from it. So the oxidising agent is itself reduced.
• A reducing agent reduces another species. It does this by giving (donating) electrons to it. So the reducing agent is itself oxidised.

Worked example. In $\text{Mg(s)} + \text{Cl}_2\text{(g)} \rightarrow \text{MgCl}_2\text{(s)}$:

• Cl₂ takes electrons from Mg → Cl₂ is the oxidising agent (and Cl₂ is itself reduced).
• Mg gives electrons to Cl₂ → Mg is the reducing agent (and Mg is itself oxidised).

The mark-saving trick. The species that is oxidised is the reducing agent; the species that is reduced is the oxidising agent. So once you have labelled oxidised/reduced, you already know the agents — just flip the word.

Common agents you should recognise.

• Oxidising agents: O₂, Cl₂, Br₂ — they readily accept electrons.
• Reducing agents: reactive metals (Mg, Zn, Fe), carbon (C), hydrogen (H₂) — they readily donate electrons.