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Acid rain is mostly caused by gaseous sulphur dioxide and gaseous nitrogen dioxide.

Acid rain is mostly caused by gaseous sulphur dioxide and gaseous nitrogen dioxide. These gases are produced as a by product from the combustion of fuels. In the atmosphere, sulphur dioxide is converted into sulphuric acid via an oxidation reaction. Nitrogen dioxide is transformed into nitric acid due to similar reactions in the atmosphere. Both these products (sulphuric acid and nitric acid) are responsible for the lowering of the rainwater pH, resulting in the environmental concern known as acid rain.

A large amount of fuel is subjected to combustion on a daily basis in order to facilitate the day-to-day activities of human beings. Many of these combustion processes result in the liberation of large quantities of nitrogen dioxide and sulphur dioxide into the atmosphere.

Gaseous sulphur dioxide undergoes oxidation when it participates in an intermolecular reaction with the hydroxyl free radicals present in the atmosphere. This chemical reaction results in the formation of a hydrogen sulphate (a chemical compound with the formula HSO 3 ) free radical. This HSO 3 radical goes on to react with atmospheric oxygen, forming sulphur trioxide (a chemical compound with the formula SO 3 ) as one of the products. Now, when reacted with the water vapour present in the atmosphere, sulphur trioxide forms sulphuric acid, which is a highly corrosive compound. The formation of sulphuric acid at this level of the atmosphere can greatly increase the acidity of the rainwater by forming a dilute solution with the rainwater and, therefore, lowering its pH value.

An alternate chemical pathway for the formation of sulphuric acid from atmospheric sulphur dioxide does exist. In this alternate pathway, the sulphur dioxide gas is initially subjected to hydrolysis by the atmospheric water vapour. Post the hydrolysis reaction, the product formed undergoes a series of equilibrium reactions to finally afford sulphuric acid as the product.

The formation of nitric acid in the atmosphere from the presence of excess nitrogen dioxide is fairly straightforward. The nitrogen dioxide gas participates in a chemical reaction with the hydroxide free radicals in the atmosphere to form nitric acid as the product. However, it can be noted that nitric acid can also be produced in the atmosphere due to certain natural phenomena such as lightning. The electric discharge into the atmosphere as a result of lightning can facilitate the formation of nitric acid from oxides of nitrogen and water.

Thus, the presence of gases like nitrogen dioxide and sulphur dioxide (in excess) in the atmosphere can contribute towards acid rain. Acid rain is known to have a negative impact on soil, aquatic life, and forests. Acid rain is also known to severely damage entire ecosystems like aquatic ecosystems and forest ecosystems. Man-made structures are also known to be affected by this phenomenon. For example, acid rain can contribute towards the corrosion of many steel structures (such as bridges) and also cause visible degradation to marble and stone constructs. Furthermore, acid rain is also known to have a negative impact on human health. Therefore, the emission of sulphur dioxide and nitrogen dioxide into the atmosphere must be controlled via the regulation of the human activities that cause this.

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