OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 7

1 mark · Medium difficulty · Multiple Choice

Calculate the enthalpy change for the combustion of methane using the provided bond enthalpies.

Practise this question

Question

Multiple choice question 7 featuring a table of bond enthalpies for C-C, C-H, O-H, C-O, C=O, O-O, and O=O bonds. Students must calculate the enthalpy change for the combustion of methane: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g), with four multiple-choice options A (-730), B (-544), C (+544), and D (+730).
Question text

7 Bond enthalpies are shown in the table.

Bond C–C C–H O–H C–O C=O O–O O=O

Bond

enthalpy 347 435 464 358 805 144 498

/ kJ mol–1

What is the enthalpy change, in kJ mol–1, for the reaction below?

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

A –730

B –544

C +544

D +730

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 7 is A.

7 A 1 2.2

How to answer it

Calculating Enthalpy Change from Bond Enthalpies

What this question tests

This question assesses your understanding of enthalpy changes as a reflection of energy required to break bonds versus energy released when forming bonds. You must correctly identify the number and type of covalent bonds broken and formed in a balanced chemical equation, apply the correct mathematical formula, and handle signs carefully.

Question 7: Multiple Choice Analysis

Enthalpy Change Calculation for Combustion of Methane

Consider the reaction: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

✅ Correct Answer: A (-730 kJ mol⁻¹)

Option A is the correct choice. The calculation yields an overall negative enthalpy change, reflecting an exothermic combustion reaction.

💡 Key Knowledge

  • Bonds broken (Reactants): Energy is absorbed (Endothermic, +).
  • Bonds formed (Products): Energy is released (Exothermic, -).
  • Formula: Enthalpy change = Σ(Bonds broken) − Σ(Bonds formed)

🧠 Exam Technique

Always draw out or explicitly list every single covalent bond in the balanced equation. Watch out for coefficients like the 2 in front of O₂ and H₂O so you do not miss multiplier totals.

❌ Common Errors

Students frequently confuse the sign convention by subtracting reactant bond enthalpies from product bond enthalpies, resulting in a positive value (+730 kJ mol⁻¹, Option D), or they forget to multiply individual bond values by stoichiometric coefficients.

📐 Step-by-Step Calculation

  1. Identify bonds broken in reactants ( CH₄ + 2O₂ ):
    • 1 × C−H bond in CH₄? Careful! Methane has 4 C−H bonds: 4 × 435 = +1740 kJ
    • 2 × O=O bonds in 2O₂: 2 × 498 = +996 kJ
    • Total energy to break bonds = 1740 + 996 = +2736 kJ
  2. Identify bonds formed in products ( CO₂ + 2H₂O ):
    • 2 × C=O bonds in CO₂: 2 × 805 = 1610 kJ
    • 4 × O−H bonds in 2H₂O (2 per water molecule × 2): 4 × 464 = 1856 kJ
    • Total energy released by forming bonds = 1610 + 1856 = 3466 kJ
  3. Calculate overall enthalpy change (ΔH):
    • ΔH = Σ(Bonds broken) − Σ(Bonds formed)
    • ΔH = 2736 − 3466 = −730 kJ mol⁻¹
OCR Mark Scheme Note: 1 mark awarded for correct selection of option A.

Topics

Module 3: Periodic table and energy · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.