Question

What are the reactions involved for ozone layer depletion in the stratosphere?

Answer 1

In the stratosphere, ozone is a product of the action of UV radiations on dioxygen as:

i) \[\ce{O_{2(g)} ->[UV] O_{(g)} + O_{(g)}}\]

ii) \[\ce{O_{2(g)} + O_{(g)} <->[UV] O_{3(g)}}\]

Reaction (ii) indicates the dynamic equilibrium existing between the production and decomposition of ozone molecules. Any factor that disturbs the equilibrium may cause depletion of ozone layer by its decomposition. One such factor is the release of chlorofluorocarbon compounds (CFCs). These are non-reactive, non-flammable molecules that are used in refrigerators, air conditioners, plastics, and electronic industries.

Once released CFCs mix with atmospheric gases and reach the stratosphere, where they are decomposed by UV radiations.

iii) \[\ce{CF2Cl_{2(g)} ->[UV] \overset\bullet{\text{Cl}}_{(g)} + \overset\bullet{\text{C}}\text{F2Cl}_{(g)}}\]

The chlorine free radical produced in reaction (iii) reacts with ozone as:

iv) \[\ce{\overset\bullet{\text{Cl}}_{(g)} + O_{3(g)} -> Cl\overset\bullet{\text{O}}_{(g)}}\]

The \[\ce{\overset\bullet{\text{Cl}}_{(g)}}\] radicals further react with atomic oxygen to produce more chlorine radicals as:

v) \[\ce{Cl\overset\bullet{\text{O}} + O_{(g)} -> \overset\bullet{\text{C}}l_{(g)} + O_{2(g)}}\]

The regeneration of \[\ce{\overset\bullet{\text{Cl}}_{(g)}}\] causes a continuous breakdown of ozone present in the stratosphere, damaging the ozone layer.