Explain how a stable refrigerant can become an ozone-depleting substance, and prove catalytic behaviour by adding radical equations.
Where ozone is helpful
Stratospheric ozone absorbs harmful ultraviolet radiation. Depleting that protective ozone increases the UV reaching the surface. Ozone near ground level is instead an air pollutant; the effect of a substance depends on where it is present.
CFCs contain carbon, chlorine and fluorine. Their low reactivity and useful physical properties led to use as refrigerants, aerosol propellants and solvents. Stability in the lower atmosphere also gives them time to reach the stratosphere, where higher-energy UV can break C–Cl bonds.
UV releases chlorine radicals
For dichlorodifluoromethane, homolysis of a C–Cl bond produces a chlorine radical and an organic radical. Each fragment receives one electron from the bond. The radical dot matters: Cl• is not Cl⁻.
Show that chlorine is regenerated
Use the two-step cycle specified here. The first reaction consumes Cl•; the second reforms it. Add both equations and cancel Cl• and ClO• to obtain the net equation. One chlorine radical can participate in many cycles before being removed by another process.
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Chlorine radical ozone cycle
Labels to include:
- Cl• consumed in step 1
- ClO• intermediate
- Cl• regenerated in step 2
- two O₃ consumed
- three O₂ formed
Show two linked stages using the displayed equations. Label the return of Cl• after the second step and cancel it when writing the net reaction. Keep the organic CFC photolysis outside the catalytic cycle: it supplies the initial radical.
Evidence, controls and alternatives
Laboratory chemistry, atmospheric measurements and observations of ozone loss together supported restrictions on ozone-depleting substances. This illustrates why independent evidence matters when connecting a molecular mechanism to an environmental effect.
A chlorine-free replacement cannot release chlorine radicals by C–Cl photolysis, but that does not make it impact-free. A replacement must also be judged for greenhouse effect, persistence, toxicity, flammability and suitability for the job. Some HFCs avoid chlorine-driven ozone depletion while still being strong greenhouse gases.
Reducing emissions does not remove all previously released CFCs immediately. Long atmospheric lifetimes mean environmental recovery is delayed. Keep ozone depletion, which concerns UV protection, distinct from the enhanced greenhouse effect, which concerns infrared radiation and energy balance.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Explain why CFC stability was useful commercially but harmful environmentally.Show answer
Low reactivity made them useful in applications such as refrigeration, but also allowed them to persist long enough to reach the stratosphere. UV there can release ozone-destroying chlorine radicals.
Q2. Identify the bond broken and the type of fission in CCl₂F₂ → •CClF₂ + Cl•.Show answer
A C–Cl bond breaks by homolytic fission. Each fragment takes one electron from the shared pair.
Q3. Add Cl• + O₃ → ClO• + O₂ and ClO• + O₃ → 2O₂ + Cl•.Show answer
Cancel Cl• and ClO• from opposite sides. The net equation is 2O₃ → 3O₂.
Q4. Why can one chlorine radical destroy more than one ozone molecule?Show answer
It is regenerated at the end of the cycle and can start another cycle. It is not consumed in the net reaction, though other reactions can eventually remove it.
Q5. Does “chlorine-free refrigerant” mean “no environmental impact”?Show answer
No. It avoids this chlorine-radical route to ozone loss, but its greenhouse effect and other environmental properties still need assessment.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise: Halogenoalkanes — Coverage checklist, pp1–5; original explanations and questions below.
- AQA 7405 specification — 3.3.3.1–3.3.3.3.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.