OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 10

1 mark · Medium difficulty · Multiple Choice

Calculate the energy released during the breakdown of a given volume of ozone using the enthalpy change of the reaction and the molar gas volume.

Practise this question

Question

Multiple choice question 10 about the breakdown of the ozone layer. It provides the reaction equation 2 O3 -> 3 O2 with an enthalpy change of -284 kJ mol^-1, and a molar gas volume of 2.5 m^3 mol^-1. Four options A (56.8), B (113.6), C (355), and D (710) are listed for the energy released in kJ during the breakdown of 1.0 m^3 of ozone.
Question text

10 Radical reactions are responsible for the catalysed breakdown of the ozone layer.

The overall equation is shown below.

2O 3O ∆ H = −284 kJ mol−1

32 r

The molar gas volume in the ozone layer is approximately 2.5 m3 mol−1.

What is the energy released, in kJ, during the breakdown of 1.0 m3 of ozone in the ozone layer?

A 56.8

B 113.6

C 355

D 710

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme showing question number 10 with the correct answer A.

10 A 1 AO2.2

How to answer it

Ozone Breakdown Enthalpy Calculation

What this question tests

This question assesses your ability to combine moles calculations involving non-standard molar gas volumes with thermochemical enthalpy change data. You must correctly interpret stoichiometric ratios from a balanced chemical equation and scale enthalpy changes according to the reacting amount of a specific substance (ozone, O₃).

Question 10 Multiple Choice Breakdown

Full Worked Solution & Examiner Guidance

✅ Correct Answer: A (56.8)

Option A is the correct choice. When 1.0 m³ of ozone breaks down using the given molar gas volume and enthalpy change, the energy released is exactly 56.8 kJ.

💡 Key Knowledge

  • Molar Gas Volume: Defined as the volume per mole of gas ( m³ mol⁻¹ or dm³ mol⁻¹ ).
  • Enthalpy of Reaction ( Δr H ): The enthalpy change that accompanies a reaction in the stoichiometric quantities shown in the chemical equation.
  • Stoichiometry: The coefficients in 2O₃ → 3O₂ show that −284 kJ of energy is released when 2 moles of O₃ react.

🧠 Exam Technique

For multiple-choice calculation questions, always write down your numbered steps clearly on your working-out paper. Do not round intermediate values on your calculator, as this prevents rounding errors from altering your final selected option.

❌ Common Errors & Traps

  • Stoichiometry Trap: Forgetting to divide by 2 to find the enthalpy change per 1 mole of O₃ (distractor D leads to 710 kJ if students multiply instead of dividing, or use the 5:2 ratio incorrectly).
  • Volume Ratio Inversion: Multiplying volume by the molar gas volume instead of dividing (e.g., 1.0 × 2.5 ).

📐 Step-by-Step Calculation Guide

  1. Calculate the moles of ozone (O₃) in 1.0 m³:
    Moles = Volume / Molar gas volume
    Moles of O₃ = 1.0 m³ / 2.5 m³ mol⁻¹ = 0.40 mol
  2. Relate moles of O₃ to the enthalpy change of the reaction:
    The equation shows 2 mol of O₃ releasing 284 kJ .
    Therefore, 1 mol of O₃ releases 284 / 2 = 142 kJ mol⁻¹ .
  3. Calculate the total energy released for 0.40 moles:
    Energy released = Moles × Enthalpy change per mole
    Energy = 0.40 mol × 142 kJ mol⁻¹ = 56.8 kJ
Mark Allocation: [1] mark awarded for selecting correct option A (AO2.2 - Applying chemistry knowledge to numerical problem solving).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.1 Atoms and reactions · 3.2 Physical chemistry

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.