220 Chapter Five Gases Figure 5.32 An environmental officer in Wales tests the pH of water. Figure 5.33 A schematic diagram of the process for scrubbing sulfur dioxide from stack gases in power plants. concentration of ozone and other pollutants builds up. Current efforts to combat the forma-tion of photochemical smog are focused on cutting down the amounts of molecules from unburned fuel in automobile exhaust and designing engines that produce less nitric oxide, The other major source of pollution results from burning coal to produce electricity. Much of the coal found in the Midwest contains significant quantities of sulfur, which. when burned, produces sulfur dioxide: S(in coal) + 02(g) ~ S02(g) A further oxidation reaction occurs when sulfur dioxide is changed to sulfur trioxide in the air:* 2S0 2 (g) + 02(g) ~ 2S0 3 (g) The production of sulfur trioxide is significant because it can combine with droplets 0= water in the air to form sulfuric acid: S03(g) + H 2 0(l) ~ H 2 S0 4 (aq)
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