Question

Explain the formation of acid rain due to :

(i) oxides of sulphur

(ii) oxides of nitrogen

Answer 1

• Sulphur is a non-metallic element found in coal and fuel oil. When these fuels are burned, sulphur combines with oxygen in air to form its gaseous oxides, sulphur dioxide (SO₂) and sulphur trioxide (SO₃).
  S + O₂ → SO₂
  2SO₂ + O₂ → 2SO₃
• Sulphur dioxide and sulphur trioxide react with water to form H₂SO₄ which is the main cause of acid rain.
  2SO₂ + O₂ + 2H₂O → 2H₂SO₄
  SO₃ + H₂O → H₂SO₄
• Nitric acid is formed by the combination of nitrogen and oxygen. Nitrogen and oxygen combine in the presence of thunder and lightning. Oxides of nitrogen are also produced by internal combustion engines.
  N₂ + O₂ → 2NO
• Nitrogen oxide then gets oxidised in the atmosphere to nitrogen dioxide.
  2NO +O₂ → 2NO₂
• Nitrogen dioxide combines with water to form a mixture of nitrous acid and nitric acid.
  2NO₂ + H₂O → HNO₂ + HNO₃