The Chemistry of Acid Rain
Key ConceptsRain from an unpolluted atmosphere has a pH close to 6.0 (slightly acidic).
This acidity is due to the reaction of water vapour and non-metal oxides in the atmosphere, such as carbon dioxide and nitrogen oxide, forming dilute acids.
- carbon dioxide reacts with water to form carbonic acid:
CO2(g) + H2O(l) H2CO3(aq)
Since carbonic acid is a weak acid it partially dissociates:
CO2(g) + H2O(l) H+(aq) + HCO3-(aq)
- nitrogen dioxide reacts with water to form a mixture of nitrous acid and nitric acid:
2NO2(g) + H2O(l) HNO2(aq) + HNO3(aq)
Acid rain has a pH below 5.6 due mainly to the reaction of water vapour with sulfur dioxide and the oxides of nitrogen.
- Sulfur dioxide reacts with water to form sulfurous acid (H2SO3):
SO2(g) + H2O(l) H2SO3(aq)
Sulfur dioxide (SO2) can be oxidised gradually to sulfur trioxide (SO3):
2SO2(g) + O2(g) → 2SO3(g)
Sulfur trioxide (SO3) reacts with water to form sulfuric acid (H2SO4):
SO3(g) + H2O(l) → H2SO4(aq)
- Oxides of nitrogen, particularly nitrogen dioxide (NO2) react with water to form nitrous acid (HNO2) and nitric acid (HNO3):
2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)