AQA A-Level Chemistry AS Paper 2, June 2025: Question 15

1 mark · Easy difficulty · Multiple Choice

Identify the equation showing a step in the decomposition of ozone by free radicals formed from CFCs.

Practise this question

Question

Multiple-choice question 15 asks which equation shows a step in the decomposition of ozone by free radicals formed from CFCs in the upper atmosphere. Four options are provided: A shows Cl• + O3 -> O2 + ClO•; B shows Cl• + O3 -> ClO2 + O•; C shows Cl• + O2 -> ClO• + O•; D shows ClO• + O3 -> ClO2 + O2.
Question text

15 Which equation shows a step in the decomposition of ozone by free radicals formed

from CFCs in the upper atmosphere?

[1 mark]

A Cl● + O3 → O2 + ClO●

B Cl● + O3 → ClO2 + O●

C Cl● + O2 → ClO● + O●

D ClO● + O3 → ClO2 + O2

Mark scheme

Show the mark scheme Mark scheme for question 15 shows the correct answer is option A (1 mark, AO1), which corresponds to the reaction Cl• + O3 -> O2 + ClO•.

15 A 1 (AO1) Cl● + O3 → O2 + ClO●

How to answer it

Ozone Depletion by Chlorine Radicals

What this question tests

This question assesses your recall and understanding of haloalkanes and atmospheric chemistry (AQA Specification 3.3.3.2):

  • The catalytic destruction of the ozone layer by chlorine free radicals formed from chlorofluorocarbons (CFCs).
  • The specific propagation equations that make up the catalytic cycle.
  • Distinguishing genuine atmospheric radical propagation steps from plausible-looking distractors.
Question 15 (1 Mark)

Multiple Choice Analysis

Decomposition of Ozone by CFC-derived Free Radicals

✅ Correct Answer

A: Cl• + O₃ → O₂ + ClO•

Mark Scheme: Award 1 mark (AO1) for selecting A.

This is the first propagation step of the mechanism. The chlorine free radical reacts with ozone to form an oxygen molecule and a chlorine monoxide radical.

💡 Key Knowledge: The Ozone Mechanism

You must memorise the two-step propagation cycle required by the AQA specification:

  • Step 1: Cl• + O₃ → ClO• + O₂
  • Step 2: ClO• + O₃ → Cl• + 2O₂
  • Overall: 2O₃ → 3O₂

Because the Cl• radical is consumed in Step 1 and regenerated in Step 2, it acts as a homogeneous catalyst. A single chlorine radical can destroy thousands of ozone molecules.

🧠 Exam Technique: Eliminating Distractors

  • Check the radical balance: In a propagation step, one radical reacts with one non-radical to yield one radical and one non-radical. Notice in D ( ClO• + O₃ → ClO₂ + O₂ ), there is a radical on the left but none on the right! That is impossible for a propagation step.
  • Check reactant identity: The prompt specifies the decomposition of ozone ( O₃ ). Option C has oxygen ( O₂ ) as a reactant, not ozone.
  • Know the intermediates: The only chlorine-containing species in the specification are Cl• and ClO• . Species like ClO₂ (seen in options B and D) do not feature in the standard AQA mechanism.

❌ Common Errors

  • Writing O• instead of O₂: In option B, students incorrectly think an oxygen atom radical is generated. The bond cleavage directly yields stable O₂ gas.
  • Forgetting the radical dot: In written exams, omitting the unpaired electron dot (•) on Cl• or ClO• results in an immediate loss of marks.
  • Confusing Initiation and Propagation: Initiation produces the radical via homolytic fission of a C-Cl bond using UV light ( C-Cl → C• + Cl• ), not by reaction with ozone.

Topics

Organic Chemistry · 3.3.3 Halogenoalkanes

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.