Oxidation and reduction in terms of electrons (HT only)
Practise 14 exam-style questions on Oxidation and reduction in terms of electrons (HT only) for AQA GCSE Chemistry. Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
14 questions
1–6 marks each
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14 questions, marked as you go
Sample questions with model answers
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Question 1
2 marks
The half equation shows what happens to zinc when it reacts with an acid: Zn → Zn²⁺ + 2e⁻
Is the zinc oxidised or reduced? Give a reason for your answer.
Model answer
The zinc is oxidised because each zinc atom loses two electrons to form a zinc ion.
Explanation
In a half equation, electrons shown on the right-hand side have been lost, so the species is oxidised. Electrons on the left-hand side are gained, which is reduction.
Mark scheme
1 mark
oxidised
1 mark
zinc (atoms) lose electrons
allow electrons are on the product side; do not accept 'zinc gains a positive charge' alone
Write the ionic equation for this reaction. Include state symbols.
Model answer
Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)
Explanation
An ionic equation shows only the particles that change. The sulfate ions are in solution at the start and at the end, so they are spectator ions and are left out. Check that the total charge is the same on both sides (2+ on each side here).
Mark scheme
1 mark
reactants: Mg + Cu²⁺
1 mark
products: Mg²⁺ + Cu
do not award if sulfate ions are included on both sides
1 mark
state symbols: Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)
allow state symbols mark only if the species are correct
Question 3
3 marks
A student wrote this about the reaction between magnesium and copper(II) ions: Mg + Cu²⁺ → Mg²⁺ + Cu "Magnesium is reduced because it gains a positive charge."
Explain what is wrong with the student's statement.
Model answer
Magnesium is oxidised, not reduced. Each magnesium atom loses two electrons to become a Mg²⁺ ion. The ion is positive because it has lost negatively charged electrons, not because it has gained anything positive.
Explanation
A common confusion is thinking that becoming more positive means gaining something. Atoms only gain or lose electrons in chemical reactions; losing negative electrons leaves a positive ion, and loss of electrons is oxidation.
Mark scheme
1 mark
magnesium is oxidised (not reduced)
1 mark
because magnesium atoms lose (two) electrons
allow Mg → Mg²⁺ + 2e⁻
1 mark
the positive charge is the result of losing (negative) electrons; it does not gain positive charge / protons
Question 4
4 marks
Chlorine reacts with iron(II) ions in solution: Cl₂ + 2Fe²⁺ → 2Cl⁻ + 2Fe³⁺
Write the two half equations for this reaction. Identify the species that is oxidised and give a reason.
Model answer
Fe²⁺ → Fe³⁺ + e⁻ Cl₂ + 2e⁻ → 2Cl⁻ The iron(II) ions are oxidised because each one loses an electron to become an iron(III) ion.
Explanation
An ion becoming more positive has lost electrons, so Fe²⁺ to Fe³⁺ is an oxidation. Each chlorine molecule gains two electrons, which is why two iron(II) ions are needed for each Cl₂ molecule in the overall equation.
Mark scheme
1 mark
Fe²⁺ → Fe³⁺ + e⁻
allow 2Fe²⁺ → 2Fe³⁺ + 2e⁻
1 mark
Cl₂ + 2e⁻ → 2Cl⁻
1 mark
iron(II) ions are oxidised
allow Fe²⁺; do not accept iron or Fe³⁺
1 mark
because they lose electrons
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