Chlorofluorocarbons (CFC’s) CO2 is now used as a blowing agent for producing expanded polymers instead of CFC’s Legislation to ban the use of CFCs was supported by chemists and chemists have now developed alternative chlorine-free compounds N Goalby chemrevise.org 3 The Ozone Layer 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 Ozone is continuously being formed and broken down in the stratosphere by the action of ultraviolet radiation Ozone formation UV light causes an O2 molecule to split into free radicals O2 + UV-light → O + O When the free radical hits another O2 molecule ozone forms O + O2 → O3 Ozone depletion This is the reverse of the formation reaction.. The energy is supplied by ultraviolet light O3 + ultraviolet light O2 + O There is a continuous cycle of formation and depletion of ozone rate of ozone formation = rate of ozone removal So there is a constant amount of ozone in the atmosphere The frequency of ultra-violet light absorbed equals the frequency of biologically damaging ultra-violet radiation. These reactions therefore filter out harmful UV and allow life to survive on earth. UV light can increase risk of skin cancer and increase crop mutation. Radicals from CFCs, and NOx from thunderstorms or aircraft, may catalyse the breakdown of ozone O + O2 O3 Destruction of Ozone Layer Chlorine radicals are formed in the upper atmosphere when energy from ultra-violet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break CF2Cl2 → CF2Cl + Cl Cl. + O3 ClO. + O2 ClO. + O. O2 + Cl. Overall equation O3 + O. 2 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. The regenerated Cl radical means that one Cl radical could destroy NO + O3 NO2 + O2 many thousands of ozone molecules NO2 + O. O2 + NO Overall equation O3 + O. 2 O2 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 CFC’s still concern us because CFCs are still entering the atmosphere from disused items and are still used for some purposes and by some countries. CFCs have a long lifetime in the atmosphere and it takes a long time for CFCs to reach upper atmosphere. Many of these uses have now been stopped due to the toxicity of halogenoalkanes and also their detrimental effect on the ozone layer in the atmosphere C. CFC’s were developed for use as aerosols, refrigerants, and in air-conditioning due their low reactivity, volatility and non-toxicity. 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
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4.2.2 Haloalkanes
Environmental concerns from use of organohalogen compounds (e) production of halogen radicals by the action of ultraviolet (UV) radiation on CFCs in the upper atmosphere and the resulting catalysed breakdown of the Earth’s protective ozone layer, including equations to represent: (i) the production of halogen radicals (ii) the catalysed breakdown of ozone by Cl• and other radicals e.g. •NO. Simple equations of the breakdown process are required, e.g. CF2Cl 2 → CF2Cl• + •Cl •Cl + O3 → •ClO + O2 •ClO + O → •Cl + O2 Learners could be expected to construct similar equations for other stated radicals. HSW9,10,11,12 Benefits of CFCs; acceptance of scientific evidence explaining ozone depletion leading to government legislation against CFC use.