OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 10

1 mark · Medium difficulty · Multiple Choice

Identify the correct half-equation at the positive electrode of an alkaline hydrogen-oxygen fuel cell.

Practise this question

Question

Question 10 states: The overall equation for a hydrogen–oxygen fuel cell is H2(g) + 1/2 O2(g) -> H2O(l). The half-equation at the negative electrode is: H2(g) + 2OH-(aq) -> 2H2O(l) + 2e-. What is the half-equation at the positive electrode? Four options are given: A: 1/2 O2(g) + H2(g) -> 2H2O(l) + 2e-; B: 1/2 O2(g) + H2O(l) -> 2OH-(aq) + 2e-; C: 1/2 O2(g) + H2O(l) + 2e- -> 2OH-(aq); D: O2(g) + 2OH-(aq) + H2(g) + 2e- -> 2H2O(l).

Mark scheme

Show the mark scheme Mark scheme table row showing question number 10, correct answer C, and 1 mark allocated.

How to answer it

Alkaline Hydrogen–Oxygen Fuel Cell Half-Equations

📋 What this question tests

This question assesses your understanding of redox chemistry in fuel cells under alkaline conditions. Specifically, it tests your ability to deduce an electrode half-equation given the overall cell reaction and the complementary half-equation, while checking for charge and atom balance.

Question 10 (Multiple Choice) • [1 Mark]

Electrode Half-Equation at the Positive Electrode (Cathode)

Identifying the reduction half-equation for oxygen in an alkaline electrolyte

✅ Correct Answer

C: ½O₂(g) + H₂O(l) + 2e⁻ → 2OH⁻(aq)

[1 mark] awarded for selecting option C.

💡 Key Knowledge

  • Electrode polarities: In an electrochemical/fuel cell:
    • Negative electrode (anode): Site of oxidation (electron release).
    • Positive electrode (cathode): Site of reduction (electron uptake).
  • Alkaline conditions: The presence of OH⁻(aq) in the negative electrode equation confirms the electrolyte is alkaline, meaning the positive electrode must consume O₂ and produce OH⁻.
  • Conservation of mass & charge: Summing both half-equations must yield the overall equation with electrons cancelling out.

📐 Step-by-Step Deduction Method

  1. Identify the process at the positive electrode:
    The negative electrode undergoes oxidation: H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻ .
    Therefore, reduction must occur at the positive electrode, meaning electrons ( e⁻ ) must appear on the left-hand side (reactants). This immediately eliminates options A and B.
  2. Set up the algebraic cancellation:
    Let the unknown half-equation be added to the negative electrode half-equation to give the overall equation:

    Negative: H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻
    + Positive: [ ? ]
    Overall: H₂(g) + ½O₂(g) → H₂O(l)
  3. Determine required reactants and products:
    To eliminate 2e⁻ on the right, the positive half-equation must consume 2e⁻ on the left.
    To provide ½O₂ on the reactant side, ½O₂ must be on the left.
    To cancel 2OH⁻ on the left, 2OH⁻ must be produced on the right.
    To net H₂O(l) on the right (from 2H₂O ), one H₂O must be on the left:

    ½O₂(g) + H₂O(l) + 2e⁻ → 2OH⁻(aq)
  4. Verify atom and charge balance:
    Left: 1 O (from ½O₂) + 1 O (from H₂O) = 2 O; 2 H; charge = -2.
    Right: 2 O; 2 H; charge = 2 × (-1) = -2. (Perfect match → C).

🧠 Exam Technique & Speed Tactics

  • Quick charge check: Check which options are even balanced!
    • In B: LHS charge is 0; RHS charge is -2 + (-2) = -4. Not balanced!
    • In A: LHS charge is 0; RHS charge is -2. Not balanced!
  • By simply checking charge neutrality on both sides, options A and B are instantly eliminated in seconds without even looking at fuel cell chemistry!
  • Option D needlessly includes H₂ on the reactant side—hydrogen is already supplied and oxidised at the negative electrode, not the positive.

❌ Common Student Traps

  • Confusing Acidic vs Alkaline Fuel Cells: In acidic fuel cells, the positive electrode equation is ½O₂ + 2H⁺ + 2e⁻ → H₂O . Memorising only the acidic version leads to panic when seeing OH⁻ . Always inspect the electrolyte species provided!
  • Wrong side for electrons: Forgetting that reduction is gain of electrons (OIL RIG), which means electrons must be reactants at the positive cathode.
  • Mixing fuel inputs: Selecting options that combine H₂ and O₂ on the same electrode (as in A and D). Fuel cells keep fuel and oxidant separated at different electrodes.

Topics

Module 5: Physical chemistry and transition elements · 5.2 Energy

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.