OCR A-Level Chemistry AS Breadth in chemistry (01), June 2025: Question 16

1 mark · Easy difficulty · Multiple Choice

Identify which equation represents a propagation step in the catalytic breakdown of ozone.

Practise this question

Question

Multiple-choice question asking: 'Which equation is a propagation step in the catalytic breakdown of ozone?' Four options are given: A: C2F2Cl2 -> C2F2Cl• + Cl•, B: O3 + O -> 2O2, C: Cl• + Cl• -> Cl2, D: ClO• + O -> Cl• + O2.
Question text

16 Which equation is a propagation step in the catalytic breakdown of ozone?

A C2F2Cl2 C2F2Cl• + Cl•

B O3 + O 2O2

C Cl• + Cl• Cl2

D ClO• + O Cl• + O2

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme snippet showing the correct answer for question 16 is D, awarded 1 mark.

16 D 1

How to answer it

Propagation Steps in Ozone Breakdown

📌 What this question tests

This question assesses your understanding of radical reaction mechanisms in the stratosphere, specifically distinguishing between initiation, propagation, and termination steps, as well as the overall equation for the catalytic destruction of ozone (O₃) by chlorine radicals (Cl•).

Question 16 Analysis

Multiple Choice (1 Mark)

✅ Correct Answer: D

ClO• + O → Cl• + O₂

In a propagation step, one radical reacts with a non-radical molecule or atom to generate a new radical and a stable molecule. Here, the radical ClO• reacts with an oxygen atom O to regenerate the chlorine radical Cl• and produce O₂ .

Award 1 mark for answer D.

💡 Key Knowledge: Ozone Depletion Mechanism

The catalytic breakdown of ozone by CFC-derived chlorine radicals consists of two key propagation steps:

  • Propagation Step 1:
    Cl• + O₃ → ClO• + O₂
  • Propagation Step 2:
    ClO• + O → Cl• + O₂
  • Overall Equation:
    O₃ + O → 2O₂ (adding steps 1 & 2 together; Cl• acts as a catalyst)

🧠 Exam Technique: Classifying Radical Steps

Count the radicals on both sides of the arrow:

  • Initiation: 0 radicals → 2 radicals (bond homolysis by UV light).
  • Propagation: 1 radical → 1 radical (radical is preserved and regenerates).
  • Termination: 2 radicals → 0 radicals (radicals collide and combine).

In this question, only D has 1 radical on the left ( ClO• ) and 1 radical on the right ( Cl• ).

❌ Breakdown of Distractors & Errors

  • A ( C₂F₂Cl₂ → C₂F₂Cl• + Cl• ): Incorrect. This is an initiation step caused by UV photolysis of a CFC (zero radicals become two radicals).
  • B ( O₃ + O → 2O₂ ): Incorrect. This is the overall equation for the catalysed reaction, not an individual elementary propagation step.
  • C ( Cl• + Cl• → Cl₂ ): Incorrect. This is a termination step (two radicals combine to form a stable molecule).

Topics

Module 4: Core organic chemistry · 4.2 Alcohols, haloalkanes and analysis

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.