Question

The reaction that is not involved in the ozone layer depletion mechanism in the stratosphere is:
(a)$HOCl(g)$ hv→ O$H(g) + $Cl$(g)$
(b)$C{F_2}C{l_2}(g)$ UV→ Cl$(g) + $C${F_2}Cl(g)$
(c)$C{H_4} + 2{O_3} \to 3C{H_2} = O + 3{H_2}O$
(d)$Cl$O $(g) + O(g) \to $Cl$(g) + {O_2}(g)$

Answer 1

Hint: The upper stratosphere consists of several layers of ozone which protects us from the UV radiations coming from the sun. The depletion of the ozone layer is mainly due to CFCs. CFCs mix with the normal atmospheric gases and reach the stratosphere. These gases get broken down into powerful UV radiations releasing chlorine free radicals in the stratosphere. The chlorine free radicals react with the ozone to form chlorine monoxide radicals (ClO) and molecular ${O_2}$. The reaction of ClO with an atom produces more Cl radicals.

Complete answer:
In the stratosphere, ozone is formed from the reaction of UV on dioxygen.
${O_{2(g)}}$ UV→ ${O_{(g)}} + {O_{(g)}}$
${O_{2(g)}} + {O_{(g)}} \leftrightarrow {O_{3(g)}}$
Equation (ii) indicates the dynamic equilibrium between the production and decomposition of ozone molecules. Any disturbances caused in this reaction may lead to depletion of the ozone layer and this will lead to the release of CFCs.
Once these CFCs are released, they get mixed with the atmospheric gases and reach the atmosphere, where they are decomposed by UV radiations.
$C{F_2}C{l_2}(g)$ hv→ Cl$(g)$$ + $C${F_2}Cl(g)$
Now the chlorine free radical produced in (iii) reacts with ozone:
$Cl + {O_3}(g) \to Cl$O$(g) + {O_2}$$(g)$
The chlorine free radicals again react with atomic oxygen to produce more chlorine radicals.
$Cl$O$ + O(g) \to $Cl$(g) + {O_2}(g)$
The regeneration of chlorine in (v) causes a continuous breakdown of ozone present in the stratosphere, causing more depletion.
Therefore, from the above steps we found that option (c) $C{H_4} + 2{O_3} \to 3C{H_2} = O + 3{H_2}O$is not involved in ozone layer depletion mechanism.

Note:
The stratosphere is composed of ozone gas, which consists of three atoms of oxygen. Ozone layer absorbs the UVB portion of UV light which has many harmful effects like skin cancer, cataracts and harm to crops as well as marine life. The ozone-depleting substances or ODS that release chlorine, include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), carbon tetrachloride, and methyl chloroform. Some ODS release bromine that include halons and methyl bromide.