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3.3.3Physical Chemistry

Ozone Depletion — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 75 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Ozone in the Stratosphere

Ozone, O₃, forms a layer in the stratosphere that absorbs harmful ultraviolet radiation.

Formation
  • Formation Ultraviolet radiation splits oxygen molecules, O₂ → 2O•, and an oxygen atom combines with O₂ to make ozone.
Protection
  • Protection Ozone absorbs UV radiation, which would otherwise cause skin cancer, cataracts and damage to crops and plankton.
Natural balance
  • Natural balance Formation and breakdown of ozone normally occur at similar rates, keeping the concentration roughly steady.
Human impact
  • Human impact Chlorine radicals from man-made halogenoalkanes upset the balance by destroying ozone.

Part 2 of 4Radical Catalysis by CFCs

Chlorofluorocarbons (CFCs) are stable enough to reach the stratosphere, where UV light breaks their C–Cl bonds.

1
UV radiation breaks the weakest bond homolytically, for example CCl₂F₂ → •CClF₂ + Cl•.
2
A chlorine radical reacts with ozone: Cl• + O₃ → ClO• + O₂.
3
The ClO• radical then reacts with another ozone molecule: ClO• + O₃ → Cl• + 2O₂.
4
The chlorine radical is regenerated, so it acts as a catalyst and one radical can destroy many thousands of ozone molecules.
5
The overall change is 2O₃ → 3O₂, so ozone is converted into ordinary oxygen.

Part 3 of 4Responses and Alternatives

Responses and Alternatives
Montreal Protocol
  • Montreal Protocol An international agreement of 1987 phased out CFC production, and ozone-layer recovery is now being monitored.
C–F and C–Cl bonds
  • C–F and C–Cl bonds The C–Cl bond is weaker than C–F and is broken by UV radiation, which is why chlorine is the culprit.
Replacements
  • Replacements Hydrofluorocarbons and hydrocarbons contain no chlorine and so do not deplete ozone, though some are greenhouse gases.
Other radicals
  • Other radicals Nitrogen oxides from aircraft engines can also catalyse ozone breakdown by a similar radical process.
Property of CFCsWhy usefulEnvironmental problem
Unreactive and stableSafe for refrigerants and aerosolsPersist and reach the stratosphere
Non-flammable and non-toxicConvenient in solvents and foamsRelease chlorine radicals under UV

Part 4 of 4Exam Checklist & Tips

3.3.3 – Key Points for the Exam

  • Ozone O₃, a molecule that absorbs UV radiation in the stratosphere.
  • CFC A chlorofluorocarbon, a compound of carbon, chlorine and fluorine that is stable and non-toxic.
  • Catalyst A substance that speeds up a reaction and is regenerated.
  • Free radical A species with an unpaired electron that is highly reactive.

Common Mistakes

  • Saying that CFCs cause ozone depletion by using up chlorine;
    • chlorine radicals are regenerated and act as catalysts.
  • Confusing the ozone layer (stratosphere) with ground-level ozone, which is a pollutant.
  • Writing the radicals without a dot, which loses the mechanism marks.
  • Stating that the C–F bond breaks in the stratosphere; the weaker C–Cl bond does.

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