
Ozone Chemistry The naturally occurring ozone (O3 ) layer in the upper atmosphere is beneficial as it filters out much of the sun’s harmful UV radiation Ozone in the lower atmosphere is a pollutant and contributes towards the formation of smog Man-made chlorofluorocarbons (CFC’s) caused a hole to form in the ozone layer. Chlorine atoms are formed in the upper atmosphere when energy from ultra-violet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break Cl. + O3 ClO. + O2 ClO. + O3 2O2 + Cl. Overall equation 2 O3 3 O2 The chlorine free radical atoms catalyse the decomposition of ozone due to these reactions because they are regenerated. (They provide an alternative route with a lower activation energy) They contributed to the formation of a hole in the ozone layer. CF2Cl2 → CF2Cl + Cl Legislation to ban the use of CFCs was supported by chemists and that they have now developed alternative chlorine-free compounds HFCs (Hydro fluoro carbons) e.g.. CH2FCF3 are now used for refrigerators and air-conditioners. These are safer as they do not contain the C-Cl bond The regenerated Cl radical means that one Cl radical could destroy many thousands of ozone molecules The C-F bond is stronger than the C-Cl bond and is not affected by UV. chloroalkanes and chlorofluoroalkanes can be used as solvents Uses of halogenoalkanes Halogenoalkanes have also been used as refrigerants, pesticides and aerosol propellants CH3CCl3 was used as the solvent in dry cleaning Many of these uses have now been stopped due to the toxicity of halogeno alkanes and also their detrimental effect on the atmosphere
3.3.3.3 Ozone depletion
Ozone, formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation.
Chlorine atoms are formed in the upper atmosphere when ultraviolet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break.
Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer.
Appreciate that results of research by different groups in the scientific community provided evidence for legislation to ban the use of CFCs as solvents and refrigerants.
Chemists have now developed alternative chlorine-free compounds.
Students should be able to use equations, such as the following, to explain how chlorine atoms catalyse decomposition of ozone: Cl• + O3 → ClO• + O2 and ClO• + O3 → 2O2 + Cl•