Acid Deposition

Acidity of Rainwater

H₂O_(l) + CO_2(g) ⇌ H₂CO_3(aq)

H₂CO_3(aq) ⇌ H⁺_(aq) + HCO₃^-_(aq)

HCO₃^-_(aq) ⇌ H⁺_(aq) + CO₃^2-_(aq)

This gives a solution with a minimum pH of 5.6. 

Sulfur Oxide

S_(s) + O_2(g) → SO_2(g)

Sulfur dioxide is a colourless gas with a sharp smell. It dissolves in water to form sulfurous acid, H₂SO_3(aq).

• H₂O_(l) + SO_2(g) → H₂SO_3(aq)

Sulfur dioxide can also be oxidized to sulfur trioxide, SO₃, which then dissolves in water to form sulfuric acid, H₂SO₄.

• 2SO_2(g) + O_2(g) → 2SO_3(g)

• H₂O_(l) + SO_3(g) → H₂SO_4(aq)

Nitrogen Oxide

N_2(g) + O_2(g) → 2NO_(g) ΔH = +181 kJ mol⁻¹

Nitrogen dioxide also forms from the oxidation of nitrogen monoxide:

2NO_(g) + O_2(g) → 2NO_2(g)

A similar reaction gives rise directly to the brown gas, nitrogen dioxide, NO₂.

N_2(g) + 2O_2(g) → 2NO_2(g)

Nitrogen dioxide dissolves in water to form a mixture of nitrous acid (HNO₂) and nitric acid (HNO₃).

H₂O_(l) + 2NO_2(g) → HNO_2(aq) + HNO_3(aq)

Alternatively, nitrogen dioxide can be oxidized to form nitric acid.

2H₂O_(l) + 4NO_2(g) + O_2(g) → 4HNO_3(aq)

Acid deposition

broader term than acid rain and includes all processes by which acidic components as precipitates or gases leave the atmosphere. There are two main types of acid deposition:

• wet acid deposition: rain, snow, sleet, hail, fog, mist, dew fall to ground as aqueous precipitates;

• dry acid deposition: acidifying particles, gases fall to ground as dust and smoke, later dissolve in water to form acids.