Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Acid Rain Acid rain is formed when acidic air pollutants such as sulphur dioxide and nitrogen dioxide dissolve in rainwater. In the presence of oxygen in the air, this acid is slowly oxidised to sulphuric acid (H 2 SO 4 ). SO 2 (g) + H 2 O(l)  H 2 SO 3 (aq) Sulphur dioxide dissolves in water to form sulphurous acid (H 2 SO 3 ).

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Acid Rain Oxides of nitrogen also contribute to acid rain. In the presence of oxygen and water, nitrogen dioxide is converted to nitric acid. 4NO 2 (g) + 2H 2 O(l) + O 2 (g)  4HNO 3 (aq) The pH value of unpolluted rainwater is usually slightly below 7. This is because carbon dioxide in the air dissolves in rainwater to form carbonic acid, which is a weak acid.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What are the effects of acid rain on buildings, plants and aquatic life? Acid rain reacts with metals and with carbonates in marble and limestone. When this happens, metal bridges and stone buildings are damaged. Acid rain can reduce the pH value of natural water bodies to below 4, which will kill fish and other aquatic life. Acid rain also leaches important nutrients from the soil and destroys plants.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Reducing the Effects of Acid Rain The major contributors of acid rain are sulphur dioxide and the oxides of nitrogen. We can reduce acid rain through the use of catalytic converters and flue gas by desulphurisation.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How is Air Pollution Controlled in Singapore Prohibition of the use of open fires for the disposal of domestic and industrial wastes Introduction of unleaded petrol and phasing out of leaded petrol Reduction of the permissible level of sulphur in diesel Fitting of all petrol-driven vehicles with catalytic converters

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Catalytic Converters A catalytic converter is attached to the exhaust system of a car. It contains the catalysts platinum and rhodium.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. The following redox reactions take place in a catalytic converter: - Carbon monoxide is oxidised to carbon dioxide. - Oxides of nitrogen are reduced to nitrogen. - Unburnt hydrocarbons are oxidised to carbon dioxide and water. Catalytic Converters

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What other measures can be used to reduce air pollution caused by motor vehicles? In the long run, the best way to control air pollution is to burn less fossil fuels. Future control measures may include - the use of alternative fuels, such as methanol and hydrogen, - the development of electric-powered motor vehicles.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. There are 2 ways of minimising the effects of sulphur dioxide pollution. Flue Gas Desulphurisation The most direct approach is to remove sulphur from fossil fuels before burning. However, this method is too expensive and difficult. A cheaper way is to remove sulphur dioxide from the waste gases from burning. The waste gases are called flue gases. The process of removing sulphur dioxide from flue gases is called desulphurisation.

Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What Happens in Desulphurisation? As sulphur dioxide passes through the plant, it reacts with an aqueous suspension of calcium carbonate to form solid calcium sulphite. CaCO 3 (s) + SO 2 (g)  CaSO 3 (s) + CO 2 (g) The calcium sulphite is further oxidised to calcium sulphate by atmospheric oxygen. CaSO 3 (s) + O 2 (g)  2CaSO 4 (s)