In some places in world the rain water has become so polluted with chemicals that it is like vinegar. This type of rain is called acid rain.
Pure water is neutral and has a pH of 7.
Natural rain water is slightly acidic mainly because of dissolved CO2 which produces carbonic acid or H2CO3
H2O(l) + CO2(g) <==> H2CO3(aq)
The pH of unpolluted rainwater ranges from pH 5 to 6.
Acid rain is rain water with a pH of less than 5.
In some parts of the Northern Hemisphere the pH of the rain water has been as low as 2!
Acid rain is caused by caused by industrial pollutants.
The main industrial gases responsible are SO2 and NOx (a mixture of NO and NO2).
Major sources of industrial sulfur dioxide.
SO2(g) comes from mining smelters and the burning of coal.
i) The roasting of minerals releases SO2(g) from
Metal sulfide + oxygen ==> Metal oxide + SO2(g)
ii) Electrical power stations that burn coal produce sulfur dioxide from the sulfur impurities in the coal.
S(s) + O2(g) ==> SO2(g)
The SO2(g) combines with water to produce sulfurous acid.
H2O(l) + SO2(g) ==> H2SO3(g)
Note: Sulfur dioxide is not readily oxidized to sulfur trioxide in dry clean air. Water droplets and dust particles however, catalyse the reaction between O2 and SO2 in the air producing sulfur trioixde, SO3.This dissolves in water and produces sulfuric acid which is a much stronger acid. This can cause considerable damage to buildings, vegetation and fish populations by destroying fish eggs.
SO2(g) + ½O2(g) ==> SO3(g)
H2O(l) + SO3(g) ==> H2SO4(aq)
Source of nitrogen oxides
Sources of NOx are more widespread. Nitrogen is a diatomic molecule and is fairly inert because its triple bond. However, at temperatures over 1300°C, nitrogen combines with oxygen to form nitrogen monoxide.
N2(g) + O2(g) 2NO(g)
These high temperatures can be achieved by
i) the internal combustion engine (human activity)
ii) lightning in the atmosphere (natural source)
The nitrogen monoxide slowly combines with oxygen to form soluble nitrogen dioxide gas.
2NO(g) + O2(g) => 2NO2(g)
Nitrogen dioxide readily dissolves in water producing a mixture of nitric and nitrous acids.
2NO2(g) + H2O(l) ==> HNO3(aq) + HNO2(g)
Acidic rain is mainly caused by atmospheric pollutants of sulfur dioxide and nitrogen oxides.
The chemical formula of acidic rain is dependent upon the type of acids present. Acidic rain is a complex mixture of nitrous, nitric, sulfurous and sulfuric acids which all combine to lower the pH.