Full chapter
Redox Chemistry
Track electrons through reactions, electrolysis and electricity-producing cells.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Recognise oxidation and reduction
Use the model that makes the electron change easiest to see.
| Model | Oxidation | Reduction |
|---|---|---|
| Oxygen transfer | Gain of oxygen | Loss of oxygen |
| Hydrogen transfer | Loss of hydrogen | Gain of hydrogen |
| Electron transfer | Loss of electrons | Gain of electrons |
| Oxidation state | Increase | Decrease |
Oxidation and reduction happen together: electrons lost by one species are gained by another. In CuO(s) + H2(g) -> Cu(s) + H2O(g), copper(II) oxide loses oxygen and is reduced; hydrogen gains oxygen and is oxidised. During addition of hydrogen to an alkene, the organic compound gains hydrogen and is reduced.
Oxidation state is an accounting number. An uncombined element has state 0. A monatomic ion has its ion charge. In the common compounds here, oxygen is usually -2 and hydrogen +1; exceptions must be supplied or recognised from context. The states sum to zero in a neutral compound or to the overall charge in an ion.
Worked example
Follow electrons and oxidation states
Explain Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s).
- The reacting ions give Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s).
- Zn -> Zn2+ + 2e-: oxidation state rises 0 to +2, so zinc is oxidised.
- Cu2+ + 2e- -> Cu: state falls +2 to 0, so copper ions are reduced.
Zinc transfers electrons to copper(II) ions. Sulfate is a spectator.
Check your understandingIs H+ + OH- -> H2O a redox reaction?Think it through, then reveal the answer
Identify the agent from what it does
The oxidising agent is itself reduced.
An oxidising agent accepts electrons and causes another species to be oxidised. It is reduced. A reducing agent supplies electrons and is oxidised. In the zinc/copper-ion reaction, Cu2+ is the oxidising agent and Zn is the reducing agent; copper metal is a product, not the agent added at the start.
| Reagent | Observation | Inference |
|---|---|---|
| Aqueous potassium iodide | Initially colourless solution becomes brown as iodine forms | Unknown oxidises I- to I2; it acts as an oxidising agent |
| Acidified potassium manganate(VII) | Purple solution becomes colourless | Unknown reduces manganate(VII); it acts as a reducing agent |
For the iodide test, 2I-(aq) -> I2(aq) + 2e- shows iodide losing electrons. For manganate(VII), use the acidified reagent specified: conditions affect its products and colours. A colour change is an observation; the assignment of oxidising or reducing behaviour is the inference.
Check your understandingAn unknown decolourises acidified potassium manganate(VII). Is the unknown oxidised or reduced?Think it through, then reveal the answer
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Meaning |
|---|---|
| Oxidation | Electron loss; oxidation state rises |
| Reduction | Electron gain; oxidation state falls |
| Oxidising agent | Accepts electrons and is reduced |
| Reducing agent | Supplies electrons and is oxidised |
Scope and references
Learning outcomes and sources
7. Redox Chemistry (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
7(a) Define oxygen/hydrogen redox
- Oxygen gain/loss
- Hydrogen loss/gain
7(b) Define electron/oxidation-state redox
- Electron loss/gain
- Oxidation-state increase/decrease
7(c) Test redox agents
- KI to brown iodine
- Acidified KMnO4 purple to colourless
- 2026 Pure Chemistry 6092
Official topic 7, pages 16-17. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Pure Chemistry K324
Official topic 7, pages 16-17. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2026 Combined Chemistry 5086 / 5088
Official topic 7, pages 32. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Combined Chemistry K326 / K328
Official topic 7, pages 32. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- Grail: 6092 Chemistry Complete Notes, Version 1
Background consultation: Chapter 13, electrolysis and selective discharge, pp. 55-56. Teaching additions and examples are original; syllabus scope and chemistry independently checked.