Question
Explain the mechanism of ozone layer depletion by CFCs. What is the Montreal Protocol, and why is it significant?
(NCERT Class 11, Chapter 14 — Environmental Chemistry)
Solution — Step by Step
The ozone layer exists in the stratosphere (15-35 km altitude). Ozone () absorbs harmful UV-B and UV-C radiation from the sun, protecting life on Earth from skin cancer, cataracts, and DNA damage.
The natural ozone cycle:
This cycle maintains a steady-state ozone concentration — until CFCs entered the picture.
CFCs (chlorofluorocarbons, like CCl₂F₂ — Freon-12) are extremely stable in the lower atmosphere. They rise to the stratosphere where intense UV radiation breaks them down:
The free chlorine radical then destroys ozone in a catalytic cycle:
Net reaction:
The chlorine radical is regenerated — a single Cl atom can destroy up to 100,000 ozone molecules before being deactivated.
The most severe depletion occurs over Antarctica during spring (September-October), forming the “ozone hole.” Polar stratospheric clouds provide surfaces for the chemical reactions that release reactive chlorine. When sunlight returns in spring, massive ozone destruction occurs.
Signed in 1987, the Montreal Protocol is an international treaty to phase out the production of ozone-depleting substances (ODS), primarily CFCs, halons, and carbon tetrachloride.
Key outcomes:
- CFC production has been reduced by over 99%
- The ozone layer is slowly recovering (expected to return to 1980 levels by ~2066)
- It is considered the most successful environmental treaty ever signed
- CFCs have been replaced by HFCs (hydrofluorocarbons) and HCFCs, which have much lower ozone-depleting potential
Why This Works
The destructive power of CFCs lies in two properties: (1) they are chemically inert in the troposphere, so they survive long enough to reach the stratosphere, and (2) the Cl radical acts as a catalyst — it’s not consumed in the reaction and keeps destroying ozone molecules in a chain reaction.
The Montreal Protocol worked because it was based on clear scientific evidence, had near-universal ratification (197 countries), and included financial support for developing countries to transition away from CFCs.
Alternative Method — Quick Summary
| Aspect | Detail |
|---|---|
| Ozone layer location | Stratosphere (15-35 km) |
| Main pollutant | CFCs (Freon) |
| Mechanism | Cl radical catalytic cycle |
| One Cl atom destroys | ~100,000 O₃ molecules |
| Worst depletion | Antarctica (ozone hole) |
| Treaty | Montreal Protocol (1987) |
| CFC replacements | HFCs, HCFCs |
For NEET, memorise the two reactions of the catalytic cycle and the fact that Cl is regenerated (making it a catalyst). Also know that the Montreal Protocol was signed in 1987 and targets CFCs. These three facts cover 90% of the questions asked on this topic.
Common Mistake
Students often confuse the ozone hole with the greenhouse effect. CFCs deplete ozone (stratospheric problem), while CO₂ and CH₄ cause global warming (tropospheric problem). CFCs do contribute to global warming too (they’re greenhouse gases), but their primary environmental damage is ozone depletion. NEET questions test whether you can correctly associate the pollutant with the right environmental problem.