---
title: "Chlorofluorocarbons and photolytic reactions"
subject: "Engineering Chemistry"
chapter: 3
key: "semester_2/engineering_chemistry:cfc-ozone-depletion"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/cfc-ozone-depletion
site: BCT Engineering
---

# Chlorofluorocarbons and photolytic reactions

Engineering Chemistry · Chapter 3: Environmental Chemistry

- Asked in: 2075 Baisakh, 3 marks; 2073 Magh, 1+2 marks; 2069 Poush, 3 marks (asked 3 times)
- Video: [Ozone Depletion and CFCs: Free Radical Breakdown of O3 into O2](https://www.youtube.com/watch?v=uAFAddAmVS4) — Chemistry Student
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/cfc-ozone-depletion
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-3

## Question

What do you mean by chlorofluorocarbons? Mention their photolytic reactions in the upper atmosphere.

## Short answer

Chlorofluorocarbons (CFCs) like CCl3F and CCl2F2 are inert refrigerants/propellants. In the stratosphere, UV photolysis cleaves C-Cl bonds: e.g., CCl2F2 + hv -> •CClF2 + Cl•. The released chlorine atoms catalytically destroy ozone: Cl• + O3 -> ClO• + O2; ClO• + O -> Cl• + O2; net O3 + O -> 2O2.

## Answer

#### Chlorofluorocarbons
**Chlorofluorocarbons (CFCs)** are organic compounds containing only carbon, chlorine and fluorine (Freons), for example CCl₃F (CFC-11) and CCl₂F₂ (CFC-12). They are non-toxic, non-flammable and chemically inert in the lower atmosphere, so they were widely used as refrigerants, aerosol propellants, foam-blowing agents and cleaning solvents.

#### Photolytic reactions in the upper atmosphere
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 (Cl• + CH₄ → HCl + •CH₃) or as chlorine nitrate (ClO• + NO₂ → ClONO₂).

Thus CFCs are the main cause of the thinning of the ozone layer and the ozone hole over Antarctica.

## Check yourself

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

1. What are chlorofluorocarbons?
   A. Compounds of carbon, hydrogen and chlorine only
   B. Organic compounds containing only carbon, chlorine and fluorine, e.g. CCl3F and CCl2F2
   C. Inorganic gases of nitrogen and fluorine
   D. Compounds containing carbon and bromine
   Answer: B. CFCs (Freons) contain only C, Cl and F. (Key point: What are CFCs)

2. Why can CFCs reach the stratosphere?
   A. They are lighter than all other gases
   B. They are very reactive and rise by combustion
   C. They are chemically inert and stable in the lower atmosphere
   D. They are dissolved in rain and carried up
   Answer: C. Because they are stable they are not destroyed in the troposphere. (Key point: Why reach stratosphere)

3. 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. Converts it into ozone
   C. Does nothing, since CFCs absorb no UV
   D. Breaks the C-Cl bond, releasing a chlorine atom (Cl.)
   Answer: D. The C-Cl bond is the weakest and breaks, giving Cl radicals. (Key point: CFC photolysis)

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

## Related in Engineering Chemistry

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