Question

(a) Sometime ago the formation of polar stratospheric clouds was reported over Antarctica. Why were these formed? What happens when such clouds break up by warmth of sunlight?

(b) Ozone is a gas heavier than air. Why does ozone layer not settle down near the earth?

Answer 1

(a) In summer season, nitrogen dioxide and methane react with chlorine monoxide and chlorine atoms forming chlorine sinks, preventing much ozone depletion, whereas in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica. These polar stratospheric clouds provide surface on which chlorine nitrate gets hydrolysed to form hypochlorous acid. It also reacts with hydrogen chloride to give molecular chlorine.

ClO (g) + NO₂ → ClONO₂(g)

Cl (g) + CH₄ (g) → CH₃ (g) + HCl (g)

ClONO₂ (g) + H₂O (g) → HOCl (g) + HNO₃

ClONO₂ (g) + HCl (g) → Cl₂ (g) + HNO₃ (g)

When sunlight returns to the Antarctica in spring, the sun’s warmth breaks up the clouds and HOCl, Cl are photolysed by sunlight.

HOCl (g) \(\overset{hv}{\rightarrow}\) OH (g) + Cl (g)

Cl₂ (g) \(\overset{hv}{\rightarrow}\) 2Cl (g)

(b) In stratosphere, the formation of O₃ gas goes on continuously, but O₂ is also decomposed by UV radiation between 240-360 nm.

The O-atom reacts with second O₃ molecule.

O₃ + O → 2O₂

Net reaction 2O₃ → 3O₂

Thus, the reaction forms a delicate balance in which the rate of O₃ decomposition matches the rate of O₃ formation, i.e., a dynamic equilibrium exists and maintains a constant concentration of O₃.