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K324 / 2027
Redox Chemistry

Lesson 1 of 6

Recognise oxidation and reduction

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

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can define oxygen/hydrogen redox.Syllabus

    Syllabus K324, 7.1(a). Define oxygen/hydrogen redox

  • I can define electron/oxidation-state redox.Syllabus

    Syllabus K324, 7.1(b). Define electron/oxidation-state redox

  • I can identify redox reactions.Syllabus

    Syllabus K324, 7.1(c). Identify redox reactions

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.