Ozone Depletion (AQA A Level Chemistry): Revision Note

Uses

Chlorofluorocarbons (CFCs)

• CFCs (chlorofluorocarbons) are halogenoalkanes in which all the hydrogen atoms have been replaced by chlorine and fluorine atoms.

• These compounds contain carbon atoms with chlorine and fluorine atoms attached to them

  • E.g. CCl₃F and CCl₂F₂

• CFCs have many uses due to their chemical inertness, as they are non-flammable and non-toxic

  • They are used as refrigerants

  • Propellants for aerosols

  • As solvents for dry cleaning

Hydrofluorocarbons

• Hydrofluorocarbons (HFCs) are compounds that contain a carbon attached to hydrogen and fluorine atoms only

  • E.g., CH₂F-CF₃

• As with CFCs, HFCs are chemically inert and are therefore used for a wide range of purposes

Halogenoalkanes & The Ozone Layer

• The ozone layer is beneficial for life on Earth because it absorbs most of the sun's harmful ultraviolet radiation

  • Ultraviolet radiation is linked to skin cancer, damaging DNA, and harming marine life

• Though chlorofluorocarbons (CFCs) have many beneficial uses, they also have devastating effects on the ozone layer:

  • CFCs absorb a lot of UV radiation in the upper atmosphere

  • The CFCs are then broken down by the UV light, causing the formation of chlorine radicals

  CF₂Cl₂ ^•CF₂Cl + Cl^•

  • It is the C–Cl bond (not C–F) that breaks because it is weaker

  • These radicals react with ozone, O₃, and break down the ozone layer

Cl^• + O₃ → ClO^• + O₂

ClO^• + O₃ → + 2O₂ + Cl^•

• The chlorine radical is regenerated in the second step, so a single Cl^• can destroy many thousands of ozone molecules

• The two steps sum to the overall reaction with Cl^• unchanged:

2O₃ → + 3O₂

• Results of research by different groups in the scientific community provided evidence for legislation to ban the use of CFCs as solvents and refrigerants

• This ban was the Montreal Protocol (1987)

• Chemists have now developed alternative chlorine-free compounds

• This is why hydrofluorocarbons (HFCs) are often used as an alternative to CFCs

• As these compounds do not contain any chlorine atoms, they will not have adverse effects on the ozone layer

Related topics

• Reactions of halogenoalkanes

• Free radical substitution

• Bond enthalpy