The Chemistry behind Acid Rain

Acid Rain
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Acid Rain
Introduction
Air pollution is the main cause of acid rain (Turck, 1990). Different types of chemicals are normally produced when fuel is burnt and these chemicals are in most cases released to the atmosphere. Apart from the burning of fossil fuel, harmful gases are also released as fumes form a car exhaust. Other sources of polluting gases include factories and power stations. Sulphur dioxide and nitrogen oxides are the most notable gases that are released when fuel is burned and the reaction of these gases with rain droplets cause acid rain. The acid is normally very strong and is normally very dangerous to the environment (Turck, 1990). Any rainfall whose pH is below 5.6 is normally classified as acidic. This article will explain the chemistry behind acid rain and how acid rain affects everyday life.
The Chemistry behind Acid Rain
There are some naturally occurring oxides in the atmosphere and therefore normal rain is slightly acidic as a result of its mixture with the oxides (Petheram, 2002). The normal pH value of unpolluted rain is between 5 and 6. The acidity can increase to a pH value of 4 when the air becomes more polluted with sulphur dioxide and nitrogen oxides. In extreme cases, the pH value can increase to 2. The pH value for pure water is 7.0 and his means that pure water is neutral. The natural acidity in rain water is as a result of a high concentration of carbon dioxide in the atmosphere (Petheram, 2002). The lowest layer of the atmosphere which is commonly to as the troposphere contains the major oxides such as sulphur dioxide, nitrogen monoxide and carbon dioxide that are responsible for acidic rain (Petheram, 2002). Carbonic acid is formed when carbon dioxide reacts with water as illustrated in the following equation:
CO2(g) + H2O(l) ⇌ H2CO3(aq) ⇌ H+(aq) + HCO3‒(aq)
From the above equation, the dissociation of carbonic acid gives hydrogen ion (H+) and the hydrogen carbonate ion (HCO3-) (Petheram, 2002). Nitric oxide is formed when molecular nitrogen and molecular oxygen react at high temperatures in lightning discharges and internal combustion engines.
N2(g) + O2(g) → 2NO(g)
Nitrogen dioxide is then formed when nitric oxide rapidly reacts with excess oxygen. The brown color of smog is attributed to nitrogen dioxide (Morgan, 2009).
2NO(g) + O2(g) → 2NO2(g)
Finally, a mixture nitrous acid and nitric acid are form when nitrogen dioxide dissolves in water as illustrated in the following equation (Morgan, 2009).
2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)
The overall reaction becomes:
2N2(g) + 5O2(g) + 2H2O(l) → 4HNO3(aq)
Since nitrous acid is eventually oxidized by molecular oxygen to also form nitric acid (Morgan, 2009).
Sulphur dioxide is another common oxide in the atmosphere and it is normally...