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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

3.3 Halogenoalkanes

3.3.1 Nucleophilic Substitution 3.3.2 Elimination 3.3.3 Ozone Depletion

Ozone Depletion

Specification Reference Organic chemistry, Halogenoalkanes 3.3.3.3

Quick Notes

  • Ozone (O3) is naturally formed in the upper atmosphere and absorbs harmful UV radiation.
  • CFCs (chlorofluorocarbons) break down in the upper atmosphere under UV light, releasing chlorine radicals.
  • Chlorine radicals catalyse the breakdown of ozone, contributing to depletion of the ozone layer.
      • Key reactions:
    • Cl• + O3 → ClO• + O2
    • ClO• + O3 → 2O2 + Cl• (Catalyst regenerated).
  • Scientific research led to the ban of CFCs and the development of chlorine-free alternatives.

Full Notes

Importance of Ozone in the Atmosphere

Ozone (O3) forms naturally in the upper atmosphere and absorbs UV radiation, which is harmful to many living organisms (including humans).

AQA A-Level Chemistry diagram illustrating the ozone layer absorbing harmful UV radiation in the upper atmosphere

Without ozone, harmful UV rays would reach Earth's surface, increasing risks of skin cancer and genetic damage.

How CFCs Lead to Ozone Depletion

CFCs are molecules mainly made up of carbon, chlorine and fluorine atoms and were widely used as refrigerants, solvents and aerosols.

AQA A-Level Chemistry uses of CFCs including refrigerants, solvents and aerosols

They are stable in the lower atmosphere making them useful for these purposes but decompose in the upper atmosphere under UV radiation.

AQA A-Level Chemistry photodissociation of a CFC molecule by UV light to form chlorine radicals

This releases chlorine radicals (Cl•), which destroy ozone in a catalytic cycle.

AQA A-Level Chemistry radical mechanism: Cl• reacts with ozone to form ClO• and O2, then ClO• reacts again to regenerate Cl• and form more O2

Step 1: UV light breaks the C–Cl bond in CFCs (Initiation)

Step 2: Cl• reacts with ozone, forming chlorine monoxide (ClO•):

Step 3: ClO• reacts with another ozone molecule, regenerating Cl•:

Since Cl• is regenerated, it continues to destroy ozone. One chlorine radical can cause the breakdown of over 10,000 ozone molecules.

Banning CFCs and Developing Alternatives

Scientific research provided evidence for the damage caused by CFCs.

This led to international agreements, such as the 1987 Montreal Protocol, banning CFCs.

Chemists have developed safer alternatives, such as hydrofluorocarbons (HFCs), which do not contain chlorine.

Summary

Concept Key points
Role of ozone Ozone (O3) is naturally formed in the upper atmosphere and absorbs harmful UV radiation.
CFCs and radicals CFCs break down in the upper atmosphere under UV light, releasing chlorine radicals.
Radical cycle Cl• + O3 → ClO• + O2
ClO• + O3 → 2O2 + Cl• (Catalyst regenerated).