4.1.4Chemical Changes
Oxidation and Reduction in Terms of Electrons — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 8 parts Lesson 5 of 21
Part 1 of 6Electron definitions (HT only)
Oxidation
- Oxidation A substance loses electrons when it is oxidised, so its atoms or ions become more positive.
- For example, when calcium atoms form Ca2+ ions they lose two electrons each.
Reduction
- Reduction A substance gains electrons when it is reduced, so its atoms or ions become less positive or more negative.
- For example, when chlorine molecules form chloride ions each chlorine atom gains one electron.
Remembering it
- Remembering it OIL RIG: Oxidation Is Loss, Reduction Is Gain, of electrons.
- Metals usually lose electrons to form positive ions, so they are normally oxidised.
Link to oxygen
- Link to oxygen Magnesium gaining oxygen to form MgO is also loss of electrons, because each Mg atom becomes an Mg2+ ion.
- The two definitions describe the same change.
Part 2 of 6Iron displacing copper (HT only)
1
Word equation: iron + copper sulfate → iron(II) sulfate + copper.
2
Full ionic equation: Fe + Cu2+ + SO42− → Fe2+ + SO42− + Cu.
3
Cancel the sulfate ions, which appear unchanged on both sides; these are spectator ions.
4
Ionic equation: Fe + Cu2+ → Fe2+ + Cu.
5
Split into half equations: Fe → Fe2+ + 2e− (oxidation) and Cu2+ + 2e− → Cu (reduction).
Part 3 of 6Redox in electrolysis (HT only)
| Electrode | Ions attracted | Process | Example |
|---|---|---|---|
| Cathode (negative) | Positive ions (cations) | Reduction: gain of electrons | Pb2+ + 2e− → Pb |
| Anode (positive) | Negative ions (anions) | Oxidation: loss of electrons | 2Br− → Br2 + 2e− |
Part 4 of 6Reading a worked question
1Compare charges
- Compare charges Look at each species before and after, such as V3+ becoming V4+.
2Count electrons
- Count electrons A higher positive charge means electrons were lost;
- a lower charge means electrons were gained.
3Apply OIL RIG
- Apply OIL RIG V3+ to V4+ is loss of an electron, so it is oxidation.
4Check the partner
- Check the partner Fe3+ to Fe2+ gains an electron and is reduced, completing the redox pair.
Part 5 of 6Worked examples of displacement (HT only)
| Reaction | Oxidised | Reduced |
|---|---|---|
| Zn + Cu2+ → Zn2+ + Cu | Zn → Zn2+ + 2e− | Cu2+ + 2e− → Cu |
| Mg + 2Ag+ → Mg2+ + 2Ag | Mg → Mg2+ + 2e− | Ag+ + e− → Ag (twice) |
| Cl2 + 2Br− → 2Cl− + Br2 | 2Br− → Br2 + 2e− | Cl2 + 2e− → 2Cl− |
Part 6 of 6Applications and exam context (HT only)
Redox with electrons
Reactivity
- Reactivity A more reactive metal loses electrons more easily, so it displaces a less reactive metal from its salt solution.
- The bigger the difference in reactivity, the more vigorous the displacement reaction.
Halogen displacement
- Halogen displacement A more reactive halogen gains electrons more easily and takes electrons from the halide ions of a less reactive halogen.
Exam technique
- Exam technique Name the species oxidised and reduced, quote the half equation, and state electron loss or gain to earn full marks.
Key terms
| Term | Meaning |
|---|---|
| Oxidation | Loss of electrons by an atom, ion or molecule. |
| Reduction | Gain of electrons by an atom, ion or molecule. |
| Half equation | An equation showing only the oxidation or only the reduction, with electrons included. |
| Spectator ion | An ion present in a reaction mixture that does not change and is left out of the ionic equation. |
| Ionic equation | An equation showing only the particles that actually take part in a reaction. |
| Displacement | A reaction in which a more reactive element takes the place of a less reactive element in a compound. |
Avoid these mistakes
1
Mixing up the directionoxidation is loss of electrons, even though oxidation in terms of oxygen is gain.
2
Leaving spectator ions in an ionic equation.
3
Forgetting that half equations must balance in charge as well as atoms.
4
Writing electrons on the wrong sidethey appear on the right for oxidation and on the left for reduction.
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