OxfordAQA International GCSE Chemistry: Redox reactions (9202) – Revision Notes

These notes condense section 3.8.4 Redox reactions (statements a, b and c) of the OxfordAQA International GCSE Chemistry (9202) specification, in its Version 6.2 form for exams May/June 2018 onwards. With no tiers, every line here is for you, and the specification says content from any part may come up on either paper. Section 3.8.4 has no required practical.

For full explanations and six worked examples, read the Redox reactions study guide first. Then check yourself with the Redox reactions practice questions. Other topics sit on the OxfordAQA chemistry hub. Tick statements off on the course checklist, and use a free diagnostic to spot gaps.

The definitions (3.8.4 a and b)

Term	Oxygen definition (3.8.4 a)	Electron definition (3.8.4 b)
Oxidation	Gain of oxygen by a substance	Loss of electrons
Reduction	Loss of oxygen from a substance	Gain of electrons
Redox (3.8.4 c)	Oxidation and reduction happening at the same time	Electrons lost by one particle are gained by another

Shortcut: OIL RIG, meaning Oxidation Is Loss and Reduction Is Gain.

Which definition should you use?

• Oxygen visible in the equation, and a metal oxide or a fuel involved? Use the oxygen definition.
• Ions, halogens, displacement or electrolysis? Use the electron definition.
• Carbon reducing a metal oxide (3.3.1.1 d)? The specification limits reduction there to removal of oxygen, so use the oxygen definition.

Oxygen transfer: quick reminders

ZnO + CO → Zn + CO₂

• Zinc oxide loses oxygen: reduced.
• Carbon monoxide gains oxygen: oxidised.

2Ca + O₂ → 2CaO

• Calcium gains oxygen: oxidised.

Hydrocarbon combustion (3.10.1.2 e): the carbon and the hydrogen in the fuel are both oxidised.

Ethanol → ethanoic acid (3.10.3.1 c): ethanol is oxidised, by a chemical oxidising agent or by microbes.

The specification’s own example (3.8.4 c), from the blast furnace:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

• Fe₂O₃ → Fe is reduction (oxygen lost).
• CO → CO₂ is oxidation (oxygen gained).

Electron transfer: half equations

Method in steps: writing a half equation

1. Write the particle before and after, e.g. Ni²⁺ and Ni.
2. Balance the atoms. Remember halogens are diatomic: 2Br⁻ makes Br₂.
3. Add electrons until the charges on the two sides match.
4. Read the result: electrons taken in (left side) mean reduction; electrons given out (right side) mean oxidation.

Ni²⁺ + 2e⁻ → Ni (charge +2 − 2 = 0 on the left, 0 on the right; reduction)

2O²⁻ → O₂ + 4e⁻ (charge −4 on the left, 0 − 4 = −4 on the right; oxidation)

The specification also shows the subtraction style, 2Cl⁻ − 2e⁻ → Cl₂. It is the same oxidation written differently.

Method in steps: from full equation to ionic equation

1. Write the full balanced equation.
2. Split soluble ionic compounds into their ions.
3. Cross out ions that appear unchanged on both sides (spectator ions).
4. Write what is left. Check atoms and charge.

Example: zinc powder in lead(II) nitrate solution.

Zn + Pb(NO₃)₂ → Zn(NO₃)₂ + Pb

Nitrate ions are spectators, so the ionic equation is

Zn + Pb²⁺ → Zn²⁺ + Pb

Half equations:

• Zn → Zn²⁺ + 2e⁻ (oxidation)
• Pb²⁺ + 2e⁻ → Pb (reduction)

Method in steps: combining two half equations

1. Write each half equation.
2. Multiply so both contain the same number of electrons.
3. Add them and cancel the electrons.

Sodium burning in bromine vapour:

• Na → Na⁺ + e⁻ (×2 gives 2Na → 2Na⁺ + 2e⁻)
• Br₂ + 2e⁻ → 2Br⁻

Overall: 2Na + Br₂ → 2NaBr. Sodium is oxidised; bromine is reduced.

Redox in other parts of the specification

Topic	Oxidised	Reduced
Metal + dilute acid or water (3.3.1.1 a)	The metal	Hydrogen ions or water
Metal displacement and scrap iron (3.3.1.1 b, h)	More reactive metal	Ions of the less reactive metal
Carbon reduction of oxides (3.3.1.1 d)	Carbon or carbon monoxide	The metal oxide
Electrolysis (3.3.2 d to f)	Negative ions, at the anode	Positive ions, at the cathode
Group 1 with water or halogens (3.7.1 a)	Group 1 metal	Water or the halogen
Halogen displacement (3.7.1 e)	Halide ions	More reactive halogen
Combustion (3.10.1.2 e)	Carbon and hydrogen in the fuel	Oxygen

Reactivity and electrons (3.7.1 f):

• Group 1: lower down, outer electrons are in a higher energy level, so they are lost more easily. Each metal is oxidised more readily than the one above it, and reactivity rises.
• Group 7: lower down, outer electrons are in a higher energy level, so electrons are gained less easily. The halogen is reduced less readily, so reactivity decreases.

Must-know distinctions

• Anode vs cathode. Anode: oxidation (negative ions lose electrons). Cathode: reduction (positive ions gain electrons).
• The ion vs the compound. In Mg + CuSO₄, it is Cu²⁺ that is reduced, not “copper sulfate”. Sulfate does nothing.
• Redox vs not redox. Neutralisation, precipitation and thermal decomposition of carbonates do not involve electron transfer. They are not redox.
• Atom vs molecule for halogens. Cl₂ gains two electrons and forms two Cl⁻ ions.
• Losing electrons vs charge going down. Losing negative electrons makes a particle more positive. Fe²⁺ → Fe³⁺ + e⁻ is oxidation.

Model answers to learn

“Describe the reaction between zinc and lead(II) ions in terms of oxidation and reduction.”

Each zinc atom gives up two electrons and becomes a Zn²⁺ ion, so zinc is oxidised. Lead(II) ions gain those two electrons to form lead atoms, so Pb²⁺ is reduced. Because both happen at once, the reaction is redox.

Notice the pattern: name the particle, say whether it loses or gains electrons, then give the word oxidised or reduced. Three short sentences cover it.

“Explain why the reaction of a metal oxide with carbon is a redox reaction.”

The metal oxide loses oxygen, so it is reduced. Carbon gains that oxygen, so it is oxidised. Oxidation and reduction happen together, so the reaction is redox.

Electrolysis of molten compounds: half equations at a glance

Molten compound	Cathode (reduction)	Anode (oxidation)
Nickel(II) chloride, NiCl₂	Ni²⁺ + 2e⁻ → Ni	2Cl⁻ → Cl₂ + 2e⁻
Calcium oxide, CaO	Ca²⁺ + 2e⁻ → Ca	2O²⁻ → O₂ + 4e⁻
Sodium iodide, NaI	Na⁺ + e⁻ → Na	2I⁻ → I₂ + 2e⁻

To build a row: the metal ion goes to the cathode and gains as many electrons as its charge. The non-metal ion goes to the anode, pairs up into a molecule and gives out electrons.

Quick self-test

1. State the meaning of oxidation in terms of oxygen.
2. State the meaning of reduction in terms of electrons.
3. In Fe₃O₄ + 4H₂ → 3Fe + 4H₂O, which substance is oxidised?
4. Complete the half equation: Al³⁺ + __ → Al. Is it oxidation or reduction?
5. Balance: __I⁻ → I₂ + __e⁻.
6. Write the ionic equation for magnesium reacting with nickel(II) sulfate solution.
7. At which electrode does oxidation happen during electrolysis?
8. Is MgO + 2HCl → MgCl₂ + H₂O a redox reaction? Explain.
9. In Br₂ + 2I⁻ → 2Br⁻ + I₂, which particle loses electrons?
10. In Ca + Br₂ → CaBr₂, which particle gains electrons?
11. Why is potassium oxidised more easily than lithium?

Answers

1. Gain of oxygen by a substance.
2. Gain of electrons.
3. Hydrogen, H₂ (it gains oxygen to become H₂O).
4. Al³⁺ + 3e⁻ → Al. Electrons are gained, so it is reduction.
5. 2I⁻ → I₂ + 2e⁻.
6. Mg + Ni²⁺ → Mg²⁺ + Ni (sulfate ions are spectators).
7. The anode (positive electrode).
8. No. Mg²⁺, Cl⁻ and H⁺ ions only change partners; no particle gains or loses electrons and no substance gains oxygen.
9. The iodide ion, I⁻ (2I⁻ → I₂ + 2e⁻).
10. Bromine, Br₂. Each molecule gains two electrons and forms two Br⁻ ions.
11. Potassium has its outer electron in a higher energy level than lithium does, so the nucleus holds it less strongly and potassium gives it up more readily.

Where marks are usually lost

• Defining oxidation as “gain of electrons”. OIL RIG prevents this; write it in the margin.
• Naming the compound instead of the ion that is oxidised or reduced, e.g. “silver nitrate is reduced” instead of “silver ions are reduced”.
• Writing Br⁻ → Br₂ + e⁻ without balancing atoms or charge.
• Writing electrons on the wrong side and then calling the change by the wrong name.
• Leaving spectator ions in an ionic equation when the question asks for the ionic form.
• Stating only that a substance is oxidised when the question says “in terms of electrons”. You need to say it loses electrons.
• Calling a carbonate decomposition or a neutralisation redox.
• Mixing up the electrodes: oxidation is at the anode, reduction at the cathode.
• Explaining halogen displacement with “more reactive” only. A full answer says which particle loses electrons and which gains them.

Next steps

• Redox reactions practice questions
• Redox reactions study guide
• Reactivity series and extraction notes
• Notes on electrode reactions (electrolysis)

Official syllabus

International GCSE Chemistry (9202) specification published by OxfordAQA; Version 6.2, for exams May/June 2018 onwards. Unit covered: 3.8.4 Redox reactions, statements a to c, with links to 3.3.1.1, 3.3.2, 3.7.1 and 3.10.