Formation of Acid Rain


Rain is naturally acidic due to the dissolution of carbon dioxide forming weak carbonic acid. Acid rain refers to rain that has a higher hydrogen ion concentration than about 10-5 M. Acid rain occurs due to the dissolution of acidic oxides in the atmosphere. Sulfur dioxide is one such acidic oxide which is produced by natural means such as volcanoes and geothermal hot springs and industrial processes such as the burning of fossil fuels (brown coal) and metal extraction from sulfide ores.

Formation of Reactants

Burning of Fossil Fuels: S(in compounds) + O2(g) -> SO2(g)
Burning of Zinc Sulfide: 2ZnS(s) + 3O2(g) - > 2ZnO(s) + 2SO2(g)

The other major acidic oxide that contributes to the formation of acid rain is nitrogen dioxide. Nitric oxide is formed in high localised temperatures created by lightning strikes and naturally reacts in the atmosphere to produce nitrogen dioxide. Nitrogen dioxide is also produced in the high temperatures of combustion chambers of power stations and motor vehicles.

Chemical Equations for the Formation

Formation of Nitrogen Dioxide: N2(g) + 2O2(g) -> 2NO2(g)
Both sulfur dioxide and nitrogen dioxide are acidic oxides and react with water to form acids.

Sulfur dioxide reacts with water to form sulfurous acid.
SO2(g) + H2O(l) -> H2SO3(aq)

Substances in the upper atmosphere then catalyse the reaction between sulfurous acid and oxygen to form sulfuric acid.
2H2SO3(aq) + O2(g) -> 2H2SO4(aq)

Similarly, nitrogen dioxide reacts with water to form a mixture of nitric acid and nitrous acid.
2NO2(g) + H2O(l) -> HNO3(aq) + HNO2(aq)

Substances in the atmosphere then catalyse the reaction between nitrous acid and oxygen causing the formation of more nitric acid.
2HNO2(aq) + O2(g) -> 2HNO3(aq)

Both sulfuric acid and nitric acid are soluble in water and are the major acids present in acid rain. As this forms and falls onto the Earth's surface, these strong acids are also brought to the surface causing harmful effects on the built and the natural environment.

Effects of Acid Rain

Acid rain damages plant leaves and has cause mass defoliation of pine forests in Europe and USA. Acid rain changes acidity of lakes and rivers killing fish eggs and pH sensitive plants and organisms.
Soil pH is also affected which leads to the biological magnification of heavy and toxic metals such as aluminium as well as the leeching of minerals vital to the survival of flora. Acid rain also causes damage to concrete, marble, limestone and sandstone buildings and statues (Notre Dame).

Chemical Equations

  • Limestone and Marble
    • CaCO3(s) + H2SO4(aq) -> CO2(g) + H2O(l),, + Ca(NO3)2(aq)
  • Metal Corrosion
    • Metal + Acid -> Salt + Water
    • i.e. Fe(II)(s) + H2SO4(aq) -> FeSO4(aq) + 2H+

It is clear that acid rain has a wide range of negative effects on the natural and built environment. It is only through the regulation of the release of acidic oxides such as SO2 and NO2 that acid rain can be minimised.