OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 8

1 mark · Medium difficulty · Multiple Choice

Identify the propagation step in the mechanism for the breakdown of ozone catalysed by NO radicals.

Practise this question

Question

Multiple choice question 8 asking to identify the propagation step in the ozone breakdown mechanism catalysed by NO radicals. Four options are given: A (NO + O2 -> N + O3), B (NO + O2 -> NO2 + O), C (N + O3 -> NO + O2), and D (NO2 + O -> NO + O2). There is an answer box provided below the options.
Question text

8 The breakdown of ozone is catalysed by NO radicals.

Which equation is a propagation step in the mechanism for this process?

A NO + O2 → N + O3

B NO + O2 → NO2 + O

C N + O3 → NO + O2

D NO2 + O → NO + O2

Your answer

[1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 8 is D, worth 1 mark.

8 D 1

How to answer it

Catalytic Breakdown of Ozone by NO Radicals

What this question tests

This question assesses your understanding of radical substitution/propagation mechanisms in atmospheric chemistry, specifically the catalytic destruction of ozone (O₃) by nitrogen monoxide (NO) radicals. You must be able to identify the defining characteristics of propagation steps—where a radical is consumed and a new radical is generated—within a multi-step catalytic cycle.

Question Part 8

Identifying a Propagation Step

Multiple Choice Question Analysis

✅ Correct Answer: D

NO₂ + O ➔ NO + O₂

This is a propagation step. It consumes a radical ( NO₂ ) and generates a radical ( NO ) to continue the chain reaction.

💡 Key Knowledge

  • Propagation: Takes a reactant and a radical, producing a product and another radical.
  • Catalysts: Species like NO are regenerated overall. The two key propagation steps in the NO-catalysed breakdown of ozone are:
    1) NO + O₃ ➔ NO₂ + O₂
    2) NO₂ + O ➔ NO + O₂

🧠 Exam Technique

Count your radicals! Look at both sides of the equation. A propagation step must always feature at least one radical on the left and at least one radical on the right.

❌ Common Errors

Students often incorrectly choose option B ( NO + O₂ ➔ NO₂ + O ). While this equation is chemically plausible, it is not part of the standard ozone depletion mechanism taught in OCR A-Level chemistry. Always learn the standard two-step propagation sequence for ozone depletion.

Mark Allocation: [1 mark] awarded for selecting option D.

Topics

Module 4: Core organic chemistry · 4.1 Basic concepts and hydrocarbons

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.