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Acid rain is the “atmospheric deposition of acidified rain, snow, sleet, hail, acidifying gases and particles, as well as acidified fog and cloud water”.It also refers to precipitation that is much more acidic than natural, unpolluted rain.”The pH scale is used to measure the acidity or alkalinity of an aqueous solution and is determined by the hydrogen ion (H+) content” (“Measuring Acid Rain”). This scale ranges from 0 to 14, moving from strongly acid to strongly alkaline, respectively, with the point 7 representing neutral.”The pH scale is logarithmic rather than linear”. Hence, there is a tenfold increase in acidity with each pH point. Natural rain is mildly acidic because of the presence of carbon dioxide present in atmospheric moisture, which react together to form weak carbonic acid (H2CO3):H2CO3 is a weak acid and so it partially dissociates to release H+ (aq), resulting in the reduction of pH of the system. This partial ionization occurs in equilibrium, since carbonic acid only ionizes 1.7% (“Sources of Acid Rain”):The two dominant acids found in acid rain are sulfuric acid (H2SO4) and nitric acid (HNO3). These acids are created from the primary pollutants sulfur dioxide SO2 and nitrogen oxides such as NO2. These pollutants are usually carried over long distances from their primary source, where they finally result in acid rain. The chemistry of both these pollutants from this source to the creation of acid rain is shown below:Sulfur dioxide is mainly released into the atmosphere through combustion of fossil fuels. The world over, sulfur dioxide is also released by volcanoes and also by the oxidation of sulfur gases released by the decomposition of plants. This natural sulfur dioxide is released very high into the atmosphere, and hence the concentration of the gas is very minimal in unpolluted air. But the sulfur dioxide produced from the combustion of fossil fuels, for example in electric power plants and mined coal containing 1 to 5% sulfur, is released into the ground level air. The concentration of sulfur dioxide in the ground level air is, thus, quite high in certain areas, particularly in the northern hemisphere. This contributes to acid rain over large areas.

2. Nitrogen monoxide is produced as a byproduct of the burning of gasoline in car engines using air as the source of oxygen. The nitrogen present in air, when exposed to high temperatures inside car engines, is converted into the pollutant nitrogen monoxide (NO). A series of secondary reactions in the presence of sunlight produces nitrogen dioxide oxide and ground level ozone, which further precipitate acid rain reactions.

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