A student is researching the history of ozone depletion and the international response to it. Explain how the use of CFCs led to ozone depletion, and why an international agreement was needed to solve this problem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In 1987, the Montreal Protocol was signed by nations around the world. It aimed to phase out the production and use of ozone-depleting substances, including CFCs. Scientists had detected a significant thinning of the ozone layer, particularly over Antarctica, since the 1970s.
Model answer (5 marks)
CFCs are broken down by UV radiation in the stratosphere, releasing chlorine atoms.
These chlorine atoms react with ozone (O₃) to form chlorine monoxide (ClO) and O₂, destroying the ozone molecule.
A single chlorine atom can catalyse the destruction of many ozone molecules before it is removed.
The ozone layer lies in the stratosphere; its depletion allows more UV‑B radiation to reach the surface, increasing skin cancer risk and damaging ecosystems.
Because CFCs emitted anywhere affect the global ozone layer, an international agreement was required to control and phase them out worldwide.
These chlorine atoms react with ozone (O₃) to form chlorine monoxide (ClO) and O₂, destroying the ozone molecule.
A single chlorine atom can catalyse the destruction of many ozone molecules before it is removed.
The ozone layer lies in the stratosphere; its depletion allows more UV‑B radiation to reach the surface, increasing skin cancer risk and damaging ecosystems.
Because CFCs emitted anywhere affect the global ozone layer, an international agreement was required to control and phase them out worldwide.
Examiner tips
- Use the exact sequence: breakdown → chlorine release → catalytic destruction → health/ecosystem impact → global nature of the problem.
- Show the catalytic cycle briefly to earn the catalyst point.
- Mention the stratosphere explicitly to hit the location mark.
Common mistakes
- Confusing the stratosphere with the troposphere; the ozone layer is in the stratosphere.
- Losing the catalytic nature of chlorine; saying it is consumed rather than reused.
- Failing to state that the problem is global and therefore needs an international treaty.
Mark scheme (5 marks)
- CFCs are broken down / decomposed by ultraviolet (UV) radiation in the upper atmosphere
- This releases chlorine (free radicals / atoms) which react with / destroy ozone molecules
- Chlorine acts as a catalyst / is not used up, so a single chlorine atom can destroy many ozone molecules
- The ozone layer is in the upper atmosphere / stratosphere and ozone depletion increases the amount of UV radiation reaching Earth's surface, causing harm (e.g. increased skin cancer / damage to ecosystems)
- An international agreement was needed because ozone depletion is a global problem — CFCs released in one country affect the ozone layer worldwide / no single country can solve it alone
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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