OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 11

1 mark · Easy difficulty · Multiple Choice

Calculate the standard enthalpy change of combustion of methanol from standard enthalpy changes of formation.

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Question

Question 11 shows the equation for the complete combustion of methanol: CH3OH(l) + 1 1/2 O2(g) → CO2(g) + 2H2O(l). A table gives values of standard enthalpy change of formation in kJ mol⁻¹: CH3OH(l) is −239, O2(g) is 0, CO2(g) is −393, and H2O(l) is −286. The question asks for the enthalpy change of combustion of CH3OH(l) in kJ mol⁻¹, with options A: +726, B: +440, C: −440, and D: −726.
Question text

11 The equation for the complete combustion of methanol, CH3OH, is shown below.

CH3OH(l) + 1 O2(g) CO2(g) + 2H2O(l)

The table shows standard enthalpy changes of formation, Δ Ho, in kJ mol–1.

f

Substance CH3OH(l) O2(g) CO2(g) H2O(l)

ΔΔ Hoo / kJ mol–1 –239 0 –393 –286

f

What is the enthalpy change of combustion of CH OH(l), in kJ mol–1?

A +726

B +440

C –440

D –726

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme entry for question 11 showing the correct answer is D, worth 1 mark.

11 D 1

How to answer it

Calculating Enthalpy of Combustion from Enthalpies of Formation

📋 What this question tests

This question assesses your ability to apply Hess’s Law to calculate standard enthalpy changes of reaction (specifically combustion) using standard enthalpies of formation (ΔfH⦵), balancing stoichiometric coefficients, handling positive/negative signs accurately, and identifying the exothermic nature of combustion reactions.

Question 11 (Multiple Choice)

Enthalpy Change of Complete Combustion of Methanol

✅ Correct Answer: D (−726 kJ mol⁻¹)

The standard enthalpy change of combustion of liquid methanol is −726 kJ mol⁻¹.

Mark Awarded: 1 mark for selecting D.

💡 Key Knowledge

  • Hess's Law Expression:
    ΔrH⦵ = Σ ΔfH⦵(products) − Σ ΔfH⦵(reactants)
  • Elements in Standard States: Standard enthalpy of formation for an element in its standard state is zero (e.g., ΔfH⦵[O₂(g)] = 0 kJ mol⁻¹).
  • Stoichiometry Matters: Multiply each ΔfH⦵ value by its stoichiometric coefficient from the balanced equation.
  • Sign Sanity Check: Combustion reactions release energy; ΔcH⦵ must be negative!

📐 Step-by-Step Calculation

Balanced chemical equation provided:

CH₃OH(l) + 1½ O₂(g) → CO₂(g) + 2H₂O(l)

  1. Sum the enthalpies of formation for the products:
    Products = 1 × CO₂(g) + 2 × H₂O(l)
    Σ ΔfH⦵(products) = [1 × (−393)] + [2 × (−286)]
    Σ ΔfH⦵(products) = −393 + (−572) = −965 kJ mol⁻¹
  2. Sum the enthalpies of formation for the reactants:
    Reactants = 1 × CH₃OH(l) + 1½ × O₂(g)
    Σ ΔfH⦵(reactants) = [1 × (−239)] + [1.5 × 0]
    Σ ΔfH⦵(reactants) = −239 kJ mol⁻¹
  3. Apply the formula [Products − Reactants]:
    ΔcH⦵ = Σ ΔfH⦵(products) − Σ ΔfH⦵(reactants)
    ΔcH⦵ = (−965) − (−239)
    ΔcH⦵ = −965 + 239 = −726 kJ mol⁻¹

🧠 Exam Technique & Rapid Elimination

  • Immediate 50:50 rule-out: Combustion is always exothermic (releases heat to the surroundings). Therefore, ΔcH must be negative (< 0). You can immediately eliminate A (+726) and B (+440) without calculating!
  • Arrow Direction in Cycles: If drawing a Hess's cycle, constituent elements go at the bottom with arrows pointing upwards towards both reactants and products. Following the alternative route means going against the reactant arrow (hence: Products − Reactants).

❌ Common Distractor Traps

  • Distractor C (−440 kJ mol⁻¹): Forgetting the balancing number of 2 for H₂O(l).
    (−393 − 286) − (−239) = −440
  • Distractor A (+726 kJ mol⁻¹): Doing Reactants − Products (confusing the formation formula with the mean bond enthalpy formula: Bonds Broken − Bonds Made ).
  • Distractor B (+440 kJ mol⁻¹): Combining both errors (omitting stoichiometry AND inverting the sign).

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 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.