OCR A-Level Chemistry AS Breadth in chemistry (01), November 2021: Question 10

1 mark · Medium difficulty · Multiple Choice

Calculate the standard enthalpy change of combustion of propene using standard enthalpy changes of formation

Practise this question

Question

Multiple choice question showing the combustion equation for propene and a table of standard enthalpy changes of formation for C3H6(g), O2(g), CO2(g), and H2O(l). Four options A (-2060), B (-700), C (+700), and D (+2060) are listed with a box for the student's answer.
Question text

10 The equation for the complete combustion of propene, C3H6, is shown below.

C3H6(g) + 4½O2(g) → 3CO2(g) + 3H2O(l)

Standard enthalpy changes of formation, ∆ H o, are shown in the table.

f

Compound ∆ H o / kJ mol–1

f

C3H6(g) +20

O2(g) 0

CO2(g) –394

H2O(l) –286

What is the standard enthalpy change of combustion of C H (g), in kJ mol–1?

A –2060

B –700

C +700

D +2060

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is A.

10 A 1 AO2.6

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 an enthalpy change of combustion using standard enthalpy changes of formation values. You must correctly handle stoichiometric coefficients from a balanced equation, manage positive and negative algebraic signs, and perform multi-step thermochemical calculations under AS Level Chemistry exam conditions.

Question 10: Multiple Choice Solution & Breakdown

Enthalpy Change of Combustion Calculation

✅ Correct Answer

A (-2060 kJ mol⁻¹)

Option A is the correct choice because applying the formation cycle formula yields a negative total value corresponding to an exothermic combustion reaction.

💡 Key Knowledge

  • Hess's Law: The total enthalpy change of a reaction is independent of the route taken.
  • Formation Formula: ΔH = ΣΔfH(products) - ΣΔfH(reactants).
  • Elements in standard state: O₂(g) has an enthalpy of formation of exactly 0 kJ mol⁻¹.

🧠 Exam Technique

  • Always write out or visualize the cycle with arrows pointing upwards to elements when in doubt.
  • Double-check every stoichiometric multiplier from the balanced equation.
  • Watch out for double negatives when subtracting negative formation values.

❌ Common Errors

  • Sign inversion: Subtracting products from reactants instead of products minus reactants.
  • Forgetting stoichiometry: Omitting multiplication by coefficients like 3 for CO₂ or 4.5 for H₂O.
  • Sign confusion: Forgetting that combustion reactions must always be exothermic (negative enthalpy change). Positive values like C and D can be instantly ruled out by smart candidates.

📐 Step-by-Step Calculation

  1. Identify the formula:
    ΔH = [3 × ΔfH(CO₂) + 3 × ΔfH(H₂O)] - [1 × ΔfH(C₃H₆) + 4.5 × ΔfH(O₂)]
  2. Substitute the values from the table:
    ΔH = [ (3 × -394) + (3 × -286) ] - [ (1 × +20) + (4.5 × 0) ]
  3. Calculate products total:
    (-1182) + (-858) = -2040 kJ mol⁻¹
  4. Calculate reactants total:
    +20 kJ mol⁻¹
  5. Perform final subtraction:
    ΔH = (-2040) - (+20) = -2060 kJ mol⁻¹
Mark Scheme Allocation: 1 mark awarded for selecting A. (AO2.6: Applying chemical knowledge to quantitative problems).

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), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.