f i ~ '218 Chapter Five Gases Acid Rain: An Expensive ProbLem Rainwater, even in pristine wilderness areas, is slightly acidic because some of the carbon dioxide present in the atmosphere dissolves in the raindrops to produce H+ ions by the following reaction: H 2 0(/) + CO 2 (g)----+ W(aq) + HCOi(aq) This process produces only very small concentrations of H+ ions in the rainwater. However, gases such as N0 2 and S02, which are by-products of energy use, can produce sig-nificantly higher H+ concentrations. Nitrogen dioxide reacts with water to give a mixture of nitrous acid and nitric acid: 2N0 2 (g) + H 2 0(l)----+ HN0 2 (aq) + HN0 3 (aq) Sulfur dioxide is oxidized to sulfur trioxide, which then re-acts with water to form sulfuric acid: 2S0 2 (g) + 02(g)----+ 2S0 3 (g) S03(g) + H 2 0(l)----+ H 2 S0 4 (aq) The damage caused by the acid formed in polluted air is a growing worldwide problem. Lakes are dying in Norway, Molecules of unburned 0.5 fuel (petroleum) '[ 0.4 3-.9 0.3 g 0.2 c " g
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