Explain the formation of acid rain due to:

(i) oxides of sulphur

(ii) oxides of nitrogen

(i) Sulphur is a non-metallic element found in coal and other fossil fuels. 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

(ii) Oxides of nitrogen are produced naturally during thunder and lightning. They are also produced by internal combustion engines.

N₂ + O₂ $\xrightarrow[(3000°\text{C})]{\text{high temperature}}$ 2NO

Nitrogen oxide then gets oxidized in the atmosphere of nitrogen dioxide.

2NO + O₂ ⟶ 2NO₂

Nitrogen dioxide combines with water to form a mixture of nitrous acid and nitric acid which causes acid rain.

2NO₂ + H₂O ⟶ HNO₂ + HNO₃