Redox
Oxidation and reduction in terms of oxygen and electrons, oxidising and reducing agents, and oxidation numbers.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 6.4.1Define oxidation and reduction in terms of oxygen loss/gain and electron loss/gain.
- 6.4.2Identify oxidising and reducing agents in a redox reaction and use oxidation numbers.Supplement
7 minute read
Redox reactions
Redox is short for "reduction–oxidation" — reactions in which oxidation and reduction happen simultaneously, one substance being oxidised while another is reduced.
Two ways to define oxidation and reduction
In terms of oxygen: oxidation is the gain of oxygen; reduction is the loss of oxygen. This is the simpler, older definition, still useful for reactions like the blast furnace, where iron(III) oxide is reduced to iron.
In terms of electrons (the more general, more useful definition): oxidation is the loss of electrons; reduction is the gain of electrons. Remember it as OIL RIG: Oxidation Is Loss, Reduction Is Gain.
Both definitions describe the same underlying process viewed two different ways, and the electron definition works for reactions — like metal displacement — where oxygen is not involved at all.
Oxidising and reducing agents
An oxidising agent oxidises another substance by taking electrons from it — and in doing so, the oxidising agent itself is reduced.
A reducing agent reduces another substance by giving it electrons — and the reducing agent itself is oxidised.
Oxidation numbers
An oxidation number tracks how oxidised or reduced an atom is. Oxidation numbers increase during oxidation and decrease during reduction, giving a quick way to spot which species is which in a more complex equation.
Think of it like this
A redox reaction is like a game of "electron tag" — one species chases down and steals electrons from another (getting reduced as it gains them), while the other species loses electrons and gets oxidised. Both things always happen together; you cannot have oxidation without reduction happening to something else at the same time.
Worked examples
Method, step by step
In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, state which species is the oxidising agent and which is the reducing agent.
- 1Zn → Zn²⁺ + 2e⁻ (zinc loses electrons: oxidised, so zinc is the reducing agent)
- 2Cu²⁺ + 2e⁻ → Cu (copper ions gain electrons: reduced, so Cu²⁺ is the oxidising agent)
Reducing agent: zinc. Oxidising agent: Cu²⁺ (copper(II) ions).
Common misconceptions
- Thinking a reaction can involve oxidation without any reduction taking place elsewhere. Every redox reaction has both happening together — one substance's gain of electrons is exactly another substance's loss.
- Mixing up which species is the oxidising agent and which is oxidised. The oxidising agent is the one that IS reduced (it does the oxidising to something else); the substance that loses electrons is the one that gets oxidised.
- Assuming oxidation always literally involves oxygen. The electron definition is more general — displacement reactions between metals are redox reactions with no oxygen involved at all.
In the exam
- When identifying oxidising and reducing agents, write out the half-equation for each species first (showing electrons lost or gained) — this makes it far easier to assign the correct label to each.
- OIL RIG is worth memorising exactly as written — many students mix up which letter pair refers to loss and which to gain under exam pressure.