OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify the correct statements for an alkaline hydrogen-oxygen fuel cell using standard electrode potential data.

Practise this question

Question

Multiple choice question 15 featuring a table of four redox half-equations with their standard electrode potentials (E-theta in V) relevant to hydrogen-oxygen fuel cells. Below the table are three numbered statements about an alkaline hydrogen-oxygen fuel cell: 1. Positive electrode reaction, 2. Overall cell reaction, 3. Cell potential value. Four multiple-choice options A, B, C, D are provided.
Question text

15 Four redox systems relevant to hydrogen–oxygen fuel cells are shown below.

Eө / V

H O(l) + e− OH−(aq) + ½H (g) −0.83

H+(aq) + e− ½H (g) 0.00

½O (g) + H O(l) + 2e− 2OH−(aq) +0.40

½O (g) + 2H+(aq) + 2e− H O(l) +1.23

Which statement(s) is/are correct for an alkaline hydrogen–oxygen fuel cell?

1 The reaction at the positive electrode is: ½O (g) + 2H+(aq) + 2e− H O(l).

2 The overall cell reaction is: H2(g) + ½O2(g) H2O(l).

3 The cell potential is 1.23 V.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows the correct answer for question 15 is C, worth 1 mark.

15 C 1 AO2.1

Total 15

How to answer it

Alkaline Hydrogen-Oxygen Fuel Cells

What this question tests

This question assesses your understanding of redox equilibria in hydrogen-oxygen fuel cells, specifically distinguishing between acidic and alkaline operating conditions. You must apply knowledge of electrode potentials (E-theta values), half-equations, overall cell reactions, and cell potential calculations (AO2.1).

Multiple Choice Question 15 — Correct Answer: C

Analysis of Statement Validity

✅ Statement 2: Correct

The overall cell reaction for any hydrogen-oxygen fuel cell is always the formation of water from hydrogen and oxygen: H₂(g) + ½O₂(g) → H₂O(l) . This is independent of whether the electrolyte is acidic or alkaline because the internal ionic equations combine to give this exact stoichiometry.

✅ Statement 3: Correct

The cell potential is calculated using E(right) − E(left) or E(positive) − E(negative). For an alkaline fuel cell, the positive electrode (oxygen reduction) has E-theta = +0.40 V and the negative electrode (hydrogen oxidation) has E-theta = −0.83 V.
+0.40 V − (−0.83 V) = +1.23 V .

❌ Statement 1: Incorrect

Statement 1 describes the positive electrode reaction for an acidic fuel cell ( ½O₂(g) + 2H⁺(aq) + 2e⁻ → H₂O(l) ), which features H⁺ ions. Because this is an alkaline fuel cell, the electrolyte contains OH⁻ ions, meaning the correct positive electrode half-equation is ½O₂(g) + H₂O(l) + 2e⁻ → 2OH⁻(aq) .

🧠 Exam Technique & Examiner Insight

Multiple-choice questions combining multiple statements require you to check each statement systematically. A common pitfall is memorising only acidic fuel cell equations and mistakenly applying them to alkaline conditions. Always check the electrolyte context (acidic uses H⁺ /H₂O, alkaline uses OH⁻ /H₂O).

Mark breakdown: Correct option is C (Statements 2 and 3 only). Worth 1 mark.

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 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.