AQA A-Level Chemistry Paper 2, 2021: Question 9

7 marks · Medium difficulty · State/Explain/Describe

Ozone Layer Chemistry and Radical Mechanisms This question focuses on the role of the ozone layer in protecting life from harmful UV radiation and how chlorofluorocarbons (CFCs) decompose to form chlorine radicals. It tests understanding of radical chain reactions, including catalytic ozone depletion and propagation steps in radical substitution reactions involved in forming hydrochlorofluorocarbons (HCFCs).

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Question

AQA A-Level Chemistry Paper 2, 2021: Question 9
Question text

09 This question is about the ozone layer in the upper atmosphere.

09.1 State why the ozone layer is beneficial for living organisms.

[1 mark]

09.2 State how chlorofluorocarbons (CFCs) form chlorine atoms in the

upper atmosphere.

[1 mark]

09.3 Give equations to show how chlorine atoms catalyse the decomposition of ozone.

[2 marks]

09.4 Hydrochlorofluorocarbons (HCFCs) have been used in place of CFCs.

In the mechanism to make an HCFC from a fluoroalkane, two incomplete steps are

shown.

Complete each step in the mechanism.

Give the name of the type of step shown by both these equations.

[3 marks]

→ ●CHF2 + HCl

●CHF2 + Cl2 →

Type of step

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 2, 2021: Question 9

Question Answers Additional Comments/Guidelines Mark

09.1 Absorbs/prevents harmful uv Allow reduced risk of skin cancer from uv 1

09.2 C-Cl bonds broken (homolytically) Could show in an equation showing the bond 1

. .

Cl + O3 → ClO + O2 M1

09.3 . .

ClO + O3 → Cl + 2O2 M2

.

Cl + CH2F2 → Penalise missing dot once only M1

.

09.4 → CHClF2 + Cl M2

Propagation M3

How to answer it

Study Guide: Ozone Decomposition and Radical Mechanisms

Q1. Why Is the Ozone Layer Important?

Key point: It absorbs harmful UV radiation .

This protects living organisms from UV-related issues such as skin cancer.

Always specify “UV radiation” — just saying “radiation” or “sunlight” is not enough!

Q2. How Do CFCs Form Chlorine Radicals?

Answer: C–Cl bonds are broken homolytically by UV light.

This forms chlorine radicals (Cl•), which then initiate reactions in the upper atmosphere.

Many students missed this because they talked about the whole CFC molecule reacting, rather than bond-breaking.

Q3. Chlorine-Catalysed Ozone Breakdown

Step 1: Cl• + O3 → ClO• + O2

Step 2: ClO• + O3 → Cl• + 2O2

Overall: 2O3 → 3O2

Chlorine is regenerated → shows it's a catalyst. Radical symbols are essential (•).

Q4. Free Radical Propagation with HCFCs

Step 1: Cl• + CH2F2 → •CHF2 + HCl

Step 2: •CHF2 + Cl2 → CHClF2 + Cl•

Type of step: Propagation

In propagation, a radical is used and a radical is regenerated. That's the key feature!

Topics

Organic Chemistry · 3.3.2 Alkanes · 3.3.3 Halogenoalkanes

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.