AQA A-Level Chemistry AS Paper 1, November 2021: Question 3

Mark scheme

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Question Marking guidance Additional Comments/Guidelines Mark

03.1 Electron acceptor Do not allow electron pair acceptor 1

SO 2– + H O → SO 2– + 2H+ + 2e– Allow multiples in each case 1

32 4

03.2 Cr O 2– + 14H+ + 6e– → 2Cr3+ + 7H O 1

27 2

3SO 2– + Cr O 2– + 8H+ → 3SO 2– + 2Cr3+ + 4H O 1

32 7 4 2

How to answer it

Redox Reactions & Half-Equations Study Guide

What this question tests

This question assesses your fundamental understanding of redox terminology and your ability to construct complex ion-electron half-equations in acidic conditions, culminating in combining them into a balanced overall redox equation.

Question Part 03.1

Defining an Oxidising Agent

State, in terms of electrons, the meaning of the term oxidising agent.

✅ Correct Answer

Electron acceptor

Award: 1 mark

❌ Common Errors

Writing "electron pair acceptor" (this refers to Lewis acids/coordinate bonding, not redox).
Stating that it "loses electrons" or "donates oxygen" (must strictly refer to electrons for this command term).

Question Part 03.2

Constructing Half-Equations & Overall Redox Equations

Deduce half-equations and combine them for the oxidation of SO₃²⁻ by Cr₂O₇²⁻ in acidic conditions.

✅ Correct Answers

Oxidation half-equation: SO₃²⁻ + H₂O → SO₄²⁻ + 2H⁺ + 2e⁻
Reduction half-equation: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Overall equation: 3SO₃²⁻ + Cr₂O₇²⁻ + 8H⁺ → 3SO₄²⁻ + 2Cr³⁺ + 4H₂O

Award: 3 marks (1 mark per correct equation; multiples allowed)

🧠 Exam Technique (The Ion-Electron Method)

Balance main elements: Balance atoms other than O and H first.
Balance Oxygen: Add H₂O to the side deficient in oxygen.
Balance Hydrogen: Add H⁺ ions to the opposite side to balance the hydrogens introduced by water.
Balance Charge: Add electrons ( e⁻ ) to make the total charge equal on both sides.
Scale and Combine: Multiply half-equations so electron numbers match, then cancel out electrons and simplify common species ( H⁺ and H₂O ).

❌ Common Errors & Examiner Pitfalls

Failing to multiply the sulfite oxidation half-equation by 3 before combining, leaving uncancelled electrons in the final equation.
Incorrectly adding OH⁻ instead of H⁺ despite the question explicitly stating "acidic conditions".
Algebraic slip-ups when cancelling out excess H⁺ and H₂O on opposite sides of the overall equation.

Topics

Physical Chemistry · 3.1.7 Oxidation, Reduction and Redox Equations