OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 11

1 mark · Medium difficulty · Multiple Choice

Identify the correct statement or equation corresponding to the standard enthalpy change of formation of water.

Practise this question

Question

Multiple choice question 11 states that the standard enthalpy change of formation of water is -286 kJ mol-1 and asks which statement or equation is correct. Option A shows H2(g) + 1/2O2(g) -> H2O(l) with delta H = -143 kJ mol-1. Option B shows 2H2(g) + O2(g) -> 2H2O(l) with delta H = -286 kJ mol-1. Option C states that the O-H bond enthalpy is -143 kJ mol-1. Option D states that the standard enthalpy change of combustion of hydrogen is -286 kJ mol-1. There is an answer box provided and the question is worth 1 mark.
Question text

11 The standard enthalpy change of formation of water is –286 kJ mol–1.

Which statement or equation is correct?

A H (g) + ½O (g) H O(l) ΔHo = –143 kJ mol–1

22 2

B 2H (g) + O (g) 2H O(l) ΔHo = –286 kJ mol–1

22 2

C The O–H bond enthalpy is –143 kJ mol–1.

D The standard enthalpy change of combustion of hydrogen is –286 kJ mol–1.

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme for question 11 indicates that the correct answer is D.

11 D 1 AO1.1

How to answer it

Standard Enthalpy Changes: Formation vs. Combustion

What this question tests

This question tests your precise definitions and stoichiometric understanding of enthalpy changes—specifically the standard enthalpy change of formation ( ΔfH ϴ ) versus the standard enthalpy change of combustion ( ΔcH ϴ ). It also examines your ability to link a numerical enthalpy value to the correct chemical equation and physical process.

Question 11

Analyzing the Options

✅ Correct Answer: D

The standard enthalpy change of combustion of hydrogen is −286 kJ mol⁻¹ .

Why it's correct: The equation for the combustion of 1 mole of hydrogen is H₂(g) + 0.5O₂(g) → H₂O(l) . Because formation of 1 mole of water from its elements has an enthalpy change of −286 kJ mol⁻¹ , and burning 1 mole of hydrogen produces 1 mole of water under standard conditions, its enthalpy of combustion is numerically identical to its enthalpy of formation.

💡 Key Knowledge

  • Enthalpy of Formation ( ΔfH ϴ ): The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.
  • Enthalpy of Combustion ( ΔcH ϴ ): The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
  • For hydrogen, the equation representing its enthalpy of combustion is identical to the equation representing the enthalpy of formation of water!

🧠 Exam Technique

Read multiple-choice options systematically and check both stoichiometry and the sign/magnitude of enthalpy values.

When an equation scales up the moles of product (like Option B multiplying everything by 2), the enthalpy change must also be multiplied by 2.

❌ Common Errors

  • Option A Trap: Students saw the correct equation H₂(g) + 0.5O₂(g) → H₂O(l) but fell for the halved enthalpy value ( −143 kJ mol⁻¹ ). Remember that the given value of −286 kJ mol⁻¹ applies to this exact equation as written for 1 mole of water formed.
  • Option B Trap: Equation has 2H₂O(l) produced, but the enthalpy value was left as −286 kJ mol⁻¹ instead of doubling to −572 kJ mol⁻¹ .
  • Option C Trap: Confusing overall enthalpy changes with individual mean bond enthalpies.
Mark Scheme Award: 1 mark for selecting D (AO1.1: Demonstrate knowledge and understanding of scientific definitions and equations).

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.