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chemistry open book paper 2003

             Photochemical smog is a mixture of primary and secondary pollutants; primary pollutants are those released directly into the atmosphere from human activities such as burning fuels. Secondary pollutants are formed when primary pollutants undergo further reactions due to the Sun's rays effect (from that comes the name "photochemical smog-.) .
             There are 2 main types of primary pollutants:.
             • Oxides of nitrogen (NO and NO ) and carbon monoxide (CO) formed when fossil fuels are burnt under high temperatures.
             • Unburnt hydrocarbons, mainly alkanes (VOCs) which are released from cars' exhausts or from the evaporation of fuels inn petrochemical plants or petrol stations. .
             The formation of secondary pollutants involves free radicals. The main secondary pollutants are: .
             • Ozone (O ).
             • Nitrogen dioxide (NO ).
             • Hydrogen peroxide (H O ).
             • Peroxyacetyl nitrate (PAN).
             • Nitric acid (HNO ).
             • Partially oxidised hydrocarbons .
             Primary and Secondary pollutants produced by motor vehicles.
             Fuels contain compounds of nitrogen (NOx) these are oxidised when the fuel is burnt and make fuel NOx or thermal NOx when burnt under very high temperatures: .
             N + O + heat 2 NO .
             (From Chemistry in action by Michael Freemantle).
             Sulphur compounds are also present in coal as impurities, like nitrogen compounds they react with oxygen and make sulphur dioxide: .
             S + O SO .
             Carbon monoxide is produced when the fuel is burnt in a low concentration of oxygen. .
             2 C + O 2 CO .
             Photochemical smog is formed when primary pollutants undergo chemical reactions and form secondary pollutants. In order for these reactions to take place some conditions are required:.

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