---
title: "Photochemistry behind ozone layer depletion"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:ozone-photochemistry"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/ozone-photochemistry
site: BCT Engineering
---

# Photochemistry behind ozone layer depletion

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2068 Bhadra, 2 marks
- Video: [A Level Chemistry Revision "Chlorofluorocarbons and the Ozone Layer"](https://www.youtube.com/watch?v=GhH77DcEv9M) — Freesciencelessons
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/ozone-photochemistry
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

What is the photochemistry behind ozone layer depletion?

## Short answer

CFCs photolyse under UV, releasing chlorine atoms. Chlorine catalytically destroys ozone: Cl + O3 -> ClO + O2; ClO + O -> Cl + O2, net O3 + O -> 2O2. One Cl atom can destroy about 10^5 ozone molecules before removal, depleting the layer.

## Answer

Because CFCs are so stable, they slowly rise to the stratosphere, where strong UV radiation (λ < 220 nm) breaks the C–Cl bond and releases chlorine atoms:

```text
CCl₃F  + hν → •CCl₂F + Cl•
CCl₂F₂ + hν → •CClF₂ + Cl•
```
The free chlorine atom then destroys ozone in a chain reaction:

```text
Cl• + O₃  → ClO• + O₂
ClO• + O  → Cl• + O₂
-------------------------------
Net:  O₃ + O → 2O₂        (Cl• is regenerated)
```
A single Cl atom can destroy about 10⁵ ozone molecules before it is removed (for example, as HCl by reaction with CH₄, or as ClONO₂ when ClO combines with NO₂).

Ozone is normally formed and destroyed in balance: O₂ + hν → 2O; O + O₂ → O₃; O₃ + hν → O₂ + O. The chlorine released from CFCs adds an extra, catalytic destruction cycle, so net ozone decreases.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does strong UV (lambda < 220 nm) do to a CFC such as CCl2F2 in the stratosphere?
   A. Breaks the C-F bond releasing fluorine atoms
   B. Breaks the C-Cl bond, releasing a chlorine atom (Cl.)
   C. Converts it into ozone
   D. Does nothing, since CFCs absorb no UV
   Answer: B. The C-Cl bond is the weakest and breaks, giving Cl radicals. (Key point: CFC photolysis)

2. What is the chain reaction of chlorine with ozone?
   A. Cl. + O2 -> ClO2, then ClO2 -> Cl. + O2
   B. Cl. + O3 -> Cl2 + O2 only
   C. Cl. + O3 -> ClO. + O2, then ClO. + O -> Cl. + O2
   D. Cl. + O3 -> ClO3 which is stable
   Answer: C. Chlorine is regenerated in the second step, so it acts as a catalyst. (Key point: Chain reaction)

3. What is the net reaction and role of Cl?
   A. O3 -> O + O2; Cl is consumed
   B. 2O3 -> 3O2; Cl is consumed
   C. O2 -> 2O; Cl is a product
   D. O3 + O -> 2O2; Cl is regenerated (catalyst)
   Answer: D. Cl is not used up, so one atom destroys many ozone molecules. (Key point: Net reaction)

4. Why does net ozone decrease?
   A. Chlorine adds an extra catalytic destruction cycle to the normal formation-destruction balance
   B. CFCs stop O2 photolysis
   C. CFCs produce ozone
   D. UV no longer reaches the stratosphere
   Answer: A. Extra Cl cycle breaks the balance. (Key point: Balance)

## Related in Engineering Chemistry

- [CFCs in ozone depletion and harmful effects](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acfc-ozone-harmful) — Engineering Chemistry, chapter 3
- [Short note: Ozone depletion](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aozone-depletion) — Engineering Chemistry, chapter 3, asked 2×

## Related in other subjects

- [Stratospheric ozone depletion mechanism and effects](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aozone-depletion-mechanism-effects) — Environment and Pollution Control, chapter 7
- [PN junction built-in potential and depletion layer](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Apn-junction-builtin-depletion) — Electrical Engineering Material, chapter 6, asked 2×
- [Built-in potential and depletion in forward bias](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aforward-bias-depletion) — Electrical Engineering Material, chapter 6