AQA A-Level Chemistry Paper 3, June 2022: Question 12

1 mark · Easy difficulty · Multiple Choice

Identify the correct statement about an electrochemical cell made of zinc and lead half-cells given their standard electrode potentials.

Practise this question

Question

Question 12 displays electrode potential data: Zn2+(aq) + 2e- to Zn(s) with standard electrode potential of -0.76 V, and Pb2+(aq) + 2e- to Pb(s) with standard electrode potential of -0.13 V. The cell representation is Zn(s) | Zn2+(aq) || Pb2+(aq) | Pb(s). Four multiple-choice options are given: A, Electrons travel in the external circuit from zinc to lead; B, The concentration of lead(II) ions increases; C, The maximum EMF of the cell is 0.89 V; D, Zinc is deposited.
Question text

12 Some electrode potential data are shown.

Zn2+(aq) + 2 e− → Zn(s) Eɵ = − 0.76 V

Pb2+(aq) + 2 e− → Pb(s) Eɵ = − 0.13 V

Which is a correct statement about this cell?

Zn(s) Zn2+ (aq) Pb2+ (aq) Pb(s)

[1 mark]

A Electrons travel in the external circuit from zinc to lead.

B The concentration of lead(II) ions increases.

C The maximum EMF of the cell is 0.89 V

D Zinc is deposited.

Mark scheme

Show the mark scheme Mark scheme table for question 12 showing the correct answer is option A (AO1), worth 1 mark, with text 'Electrons travel in the external circuit from zinc to lead.'

12 A (AO1) 1 Electrons travel in the external circuit from zinc to lead.

How to answer it

Electrochemical Cells: Electrode Potentials & Electron Flow

📋 What this question tests

This question assesses understanding of standard electrode potentials (E°) and electrochemical cell notation:

  • Comparing standard electrode potentials to determine relative oxidising and reducing power.
  • Predicting spontaneous half-cell reactions and the overall feasible redox process.
  • Deducing the direction of electron flow in the external circuit.
  • Calculating cell EMF using standard electrode values: E°cell = E°RHS − E°LHS .
Multiple Choice • Question 12

Electrochemical Cell Evaluation

1 Mark • Assessment Objective 1 (AO1)

✅ Correct Answer

A: Electrons travel in the external circuit from zinc to lead.

Award 1 mark for option A.

💡 Key Knowledge

  • More negative E° value ( −0.76 V ): Zinc half-cell has a stronger tendency to lose electrons (undergo oxidation). Reaction:
    Zn(s) → Zn²⁺(aq) + 2e⁻
  • Less negative E° value ( −0.13 V ): Lead half-cell has a stronger tendency to gain electrons (undergo reduction). Reaction:
    Pb²⁺(aq) + 2e⁻ → Pb(s)
  • Electron Flow: Always flows through the wire from the negative electrode (oxidation site / zinc) to the positive electrode (reduction site / lead).

📐 Step-by-Step Option Elimination

  1. Check Option A (Correct): Zn is oxidised, releasing electrons into the external circuit. These travel towards the lead electrode where Pb²⁺ ions are reduced. Statement is correct.
  2. Check Option B (Incorrect): Pb²⁺(aq) is consumed to form Pb(s), so the concentration of Pb²⁺(aq) decreases, not increases.
  3. Check Option C (Incorrect): Calculate cell EMF:
    E°cell = E°(reduced) − E°(oxidised)
    E°cell = (−0.13 V) − (−0.76 V) = +0.63 V
    (0.89 V is obtained incorrectly by adding absolute values: 0.76 + 0.13).
  4. Check Option D (Incorrect): Zn(s) is oxidised to Zn²⁺(aq); zinc dissolves, it does not deposit. Lead is the metal being deposited.

🧠 Exam Technique & Cell Notation

The standard IUPAC cell representation is given:

Zn(s) | Zn²⁺(aq) || Pb²⁺(aq) | Pb(s)

  • Left-hand side (LHS): Standard convention places oxidation on the left: ROOR (Reduction, Oxidation, Oxidation, Reduction is NOT used; remember L-O-A-N: Left = Oxidation, Anode, Negative).
  • Electrons always flow from left to right in a correctly written conventional cell diagram.

❌ Common Errors & Pitfalls

  • Sign subtraction trap in EMF calculation: Forgetting that subtracting a negative number yields a positive: −0.13 − (−0.76) = +0.63 V . Many students carelessly calculate 0.76 + 0.13 = 0.89 V and choose option C.
  • Confusing the role of ions vs metals: Stating that "zinc is deposited" (Option D) because they misread the half-equation in the forward direction as written in the data table. Standard reduction potentials are always tabulated as reductions, but the more negative one reverses in reality.

Topics

Physical Chemistry · 3.1.11 Electrode Potentials

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.