OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 12

1 mark · Medium difficulty · Multiple Choice

Identify the overall cell reaction equation for a hydrogen-oxygen fuel cell in alkaline solution given two half-equations with their standard electrode potentials.

Practise this question

Question

Multiple choice question 12 showing two redox equilibria with standard electrode potentials: 2H2O(l) + 2e- <=> H2(g) + 2OH-(aq) with E = -0.83 V, and 1/2 O2(g) + H2O(l) + 2e- <=> 2OH-(aq) with E = +0.40 V. Below this, the question asks 'What is the equation for the overall cell reaction?' with options A, B, C, and D representing different combinations of reactants and products, followed by an answer box and [1] mark allocation.
Question text

12 The redox equilibria for a hydrogen–oxygen fuel cell in alkaline solution are shown below.

2H O(l) + 2e– H (g) + 2OH–(aq) Eө = –0.83 V

1/ O (g) + H O(l) + 2e– 2OH–(aq) Eө = +0.40 V

22 2

What is the equation for the overall cell reaction?

A H (g) + 4OH–(aq) 3H O(l) + 1/ O (g)

22 2 2

B 3H O(l) + 1/ O H (g) + 4OH–(aq)

22 2 2

C H O(l) H (g) + 1/ O (g)

22 2 2

D H (g) + 1/ O (g) H O(l)

22 2 2

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme table for question 12 showing the correct answer as D, with 1 mark awarded.

12 D 1 AO2.5

How to answer it

Hydrogen-Oxygen Fuel Cell Overall Equation

What this question tests

This question assesses your ability to combine two half-equations into an overall redox equation for an alkaline fuel cell. You must apply your knowledge of electrode potentials, identify oxidation and reduction processes, balance electron transfer, and cancel out common spectator species (like water molecules and hydroxide ions) to derive the simplest overall chemical equation.

Question Analysis

Overall Cell Reaction in an Alkaline Fuel Cell

✅ Correct Answer

Option D

H₂(g) + 0.5O₂(g) → H₂O(l)

💡 Key Knowledge

  • Fuel cells convert the energy from the reaction of a fuel (hydrogen) with oxygen cleanly into electrical energy.
  • The overall reaction for any hydrogen-oxygen fuel cell—regardless of whether the electrolyte is acidic or alkaline—is always the combustion of hydrogen: H₂ + 0.5O₂ → H₂O .

🧠 Exam Technique

Don't waste time fully writing out and balancing the half-equations if you spot the chemistry shortcut: a fuel cell essentially "burns" hydrogen to form water. Check the overall equation options for this stoichiometry first! To prove it rigorously, reverse the more negative half-equation (oxidation of H₂) and add it to the reduction half-equation (reduction of O₂), ensuring electrons cancel out.

❌ Common Errors

Students often select options like A or B by failing to cancel species that appear on both sides of the combined equation (such as H₂O and OH⁻), or by incorrectly reversing the direction of electron flow.

Mark Scheme Allocation: 1 mark available for selecting D (AO2.5: Applying chemical knowledge to unfamiliar contexts / constructing equations).

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 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.