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K326 / K328 / 2027
Redox Chemistry

Full chapter

Redox Chemistry

Track electrons through reactions, electrolysis and electricity-producing cells.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

01

Recognise oxidation and reduction

Use the model that makes the electron change easiest to see.

Equivalent redox descriptions
ModelOxidationReduction
Oxygen transferGain of oxygenLoss of oxygen
Hydrogen transferLoss of hydrogenGain of hydrogen
Electron transferLoss of electronsGain of electrons
Oxidation stateIncreaseDecrease

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 exampleFollow electrons and oxidation states

Explain Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s).

Try it first. Then show the worked stepsHide the worked steps
  1. The reacting ions give Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s).
  2. Zn -> Zn2+ + 2e-: oxidation state rises 0 to +2, so zinc is oxidised.
  3. Cu2+ + 2e- -> Cu: state falls +2 to 0, so copper ions are reduced.
Answer

Zinc transfers electrons to copper(II) ions. Sulfate is a spectator.

Check your understanding

Is H+ + OH- -> H2O a redox reaction?

  1. No. Hydrogen stays +1 and oxygen stays -2, so nothing is oxidised or reduced.
  2. No. Redox needs a metal, and neither H+ nor OH- contains a metal atom.
  3. Yes. H+ gains a pair of electrons from OH-, so the hydrogen ion is reduced.
Show the answer

No. Hydrogen stays +1 and oxygen stays -2, so nothing is oxidised or reduced.

Check each oxidation state before and after. Hydrogen is +1 and oxygen is -2 on both sides, so no element changes. A reaction can happen without any electron transfer; neutralisation is the classic example.

02

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.

Test an unknown redox agent
ReagentObservationInference
Aqueous potassium iodideInitially colourless solution becomes brown as iodine formsUnknown oxidises I- to I2; it acts as an oxidising agent
Acidified potassium manganate(VII)Purple solution becomes colourlessUnknown 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 understanding

An unknown decolourises acidified potassium manganate(VII). Is the unknown oxidised or reduced?

  1. Oxidised. It gives electrons to the manganate(VII), so it acts as a reducing agent.
  2. Reduced. It removes the purple colour, so it acts as the oxidising agent here.
  3. Neither. Losing the purple colour is a dilution effect, not an electron transfer.
Show the answer

Oxidised. It gives electrons to the manganate(VII), so it acts as a reducing agent.

Purple MnO4- is reduced to almost colourless Mn2+. The electrons for that reduction come from the unknown, so the unknown loses electrons: it is oxidised and is the reducing agent.

Quick revision

Revisit the essentials, then return to an explanation when you need it.

Exam definitions: learn these in mark-scheme words
TermDefinition to write
OxidationGain of oxygen, loss of hydrogen, loss of electrons, or an increase in oxidation state.
ReductionLoss of oxygen, gain of hydrogen, gain of electrons, or a decrease in oxidation state.
Oxidising agentA substance that oxidises another substance; it is itself reduced (it gains electrons).
Reducing agentA substance that reduces another substance; it is itself oxidised (it loses electrons).
Keep the roles straight
TermMeaning
OxidationElectron loss; oxidation state rises
ReductionElectron gain; oxidation state falls
Oxidising agentAccepts electrons and is reduced
Reducing agentSupplies electrons and is oxidised

Scope

Learning outcomes

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

  1. 7(a) Define oxygen/hydrogen redox

    • Oxygen gain/loss
    • Hydrogen loss/gain

    Recognise oxidation and reduction

  2. 7(b) Define electron/oxidation-state redox

    • Electron loss/gain
    • Oxidation-state increase/decrease

    Recognise oxidation and reduction

  3. 7(c) Test redox agents

    • KI to brown iodine
    • Acidified KMnO4 purple to colourless

    Identify the agent from what it does