Oxidation and reduction

This page covers one Form 5 content standard from Redox Equilibrium: oxidation and reduction. It is the foundation the whole chapter is built on, so make the definitions second nature before you move on to oxidation numbers, displacement, rusting, electrolysis and cells. Almost every marks-losing mistake later in the chapter comes back to a shaky start here.

Three ways to define oxidation and reduction

The syllabus wants you to describe oxidation and reduction in three complementary ways. A reaction can be a redox reaction under one description even when another description does not obviously apply, so knowing all three lets you recognise redox in any question.

1. In terms of oxygen. Oxidation is the gain of oxygen; reduction is the loss of oxygen. When magnesium burns, 2Mg + O₂ → 2MgO, the magnesium gains oxygen and is oxidised. When copper(II) oxide is heated with hydrogen, CuO + H₂ → Cu + H₂O, the copper(II) oxide loses oxygen and is reduced.

2. In terms of hydrogen. Oxidation is the loss of hydrogen; reduction is the gain of hydrogen. When hydrogen sulfide reacts with chlorine, H₂S + Cl₂ → 2HCl + S, the H₂S loses hydrogen and is oxidised to sulfur, while the chlorine gains hydrogen and is reduced.

3. In terms of electron transfer. This is the most powerful definition and the one examiners expect at Form 5. Oxidation is the loss of electrons; reduction is the gain of electrons. The memory aid is OIL RIG, Oxidation Is Loss, Reduction Is Gain (of electrons).

Because the electrons lost by one species are exactly the electrons gained by another, oxidation and reduction always occur together. A reaction in which both happen is called a redox reaction.

Oxidising agents and reducing agents

Two more terms are examined constantly, and students mix them up under pressure.

• An oxidising agent is the substance that oxidises another species. To do that it must accept the electrons, so the oxidising agent is itself reduced.
• A reducing agent is the substance that reduces another species. To do that it must donate electrons, so the reducing agent is itself oxidised.

Say it as a chain and it never gets tangled: the reducing agent gives electrons away, so it is oxidised; the oxidising agent takes electrons, so it is reduced. The agent is named for what it does to the other reactant, not for what happens to itself.

Half-equations

An electron-transfer reaction can be split into two half-equations: one showing the loss of electrons (oxidation) and one showing the gain of electrons (reduction). Electrons are written as e-. For example, when zinc reacts with copper(II) ions:

• Oxidation half-equation: Zn → Zn²⁺ + 2e-
• Reduction half-equation: Cu²⁺ + 2e- → Cu

The electrons cancel when the two halves are added, giving the overall ionic equation Zn + Cu²⁺ → Zn²⁺ + Cu. Being fluent with half-equations here makes the later standards on displacement, electrolysis and cells far easier.

Worked example

Question. When chlorine gas is bubbled into colourless potassium iodide solution, the solution turns brown as iodine forms: Cl₂ + 2KI → 2KCl + I₂. Using the electron-transfer definition, state which species is oxidised, which is reduced, and name the oxidising agent and the reducing agent. Support your answer with half-equations.

Step 1, write the ionic species. Potassium is a spectator ion, so ignore K⁺. The reacting species are Cl₂ and I⁻, forming Cl⁻ and I₂.

Step 2, write the two half-equations.

• Iodide loses electrons: 2I⁻ → I₂ + 2e- (loss of electrons = oxidation).
• Chlorine gains electrons: Cl₂ + 2e- → 2Cl⁻ (gain of electrons = reduction).

Step 3, identify what is oxidised and reduced. The iodide ion, I⁻, is oxidised to iodine (it loses electrons). Chlorine, Cl₂, is reduced to chloride (it gains electrons).

Step 4, name the agents. Chlorine takes the electrons, so chlorine is the oxidising agent (it oxidises iodide while being reduced itself). The iodide ion donates the electrons, so iodide is the reducing agent (it reduces chlorine while being oxidised itself).

Answer. I⁻ is oxidised; Cl₂ is reduced. Chlorine is the oxidising agent and iodide is the reducing agent. The brown colour is the iodine, I₂, produced by the oxidation of iodide.

Practice question

Ammonia reacts with copper(II) oxide when passed over the heated oxide: 2NH₃ + 3CuO → N₂ + 3Cu + 3H₂O. Using the oxygen and hydrogen definitions, state which reactant is oxidised and which is reduced, and name the reducing agent.

Answer. Ammonia, NH₃, loses hydrogen to form N₂, so ammonia is oxidised. Copper(II) oxide, CuO, loses oxygen to form copper, so CuO is reduced. Because ammonia is oxidised, it is the species that reduces the copper(II) oxide, so ammonia is the reducing agent (and copper(II) oxide is the oxidising agent).

Exam tip

When a question asks you to identify oxidation and reduction, always state which definition you are using and back it with evidence from the equation, “sodium is oxidised because it loses electrons: Na → Na⁺ + e-”. A bare word like “oxidised” without the reason often earns nothing. Also keep the agents straight with one sentence you can recall under exam pressure: the oxidising agent is itself reduced; the reducing agent is itself oxidised. Writing correct half-equations, with the electrons balanced, is the single habit that protects your marks across the whole of SPM Chemistry Redox Equilibrium.

Where this fits in the chapter

This standard is the entry point to Redox Equilibrium. The next standard, oxidation number and its changes, gives you a precise, number-based tool for spotting oxidation and reduction even when there is no obvious oxygen, hydrogen or simple electron transfer to point at. After that, displacement reactions, rusting, electrolysis and electrochemical cells are all just redox in different settings.

• Up to the chapter hub: Redox Equilibrium.
• Sideways for a full recap: the chapter’s revision notes and step-by-step worked examples.

Our online 1-to-1 SPM Chemistry teachers drill this until identifying the oxidising and reducing agent, and writing balanced half-equations, becomes automatic, because those two skills reappear in every remaining standard of this chapter and in Paper 2 essay answers. Lessons are conducted in English, from RM50 per hour, and a one-to-one teacher can check that every “oxidised/reduced” statement in your answers is paired with the reason the marking scheme rewards.