Environmental Chemistry - Result Question 18
####18. The reaction that is not involved in the ozone layer depletion mechanism in the stratosphere is
(2019 Main, 10 Jan II)
(a) $CH _4+2 O _3 \longrightarrow 3 CH _2=O+3 H _2 O$
(b) $Cl \dot{O}(g)+O(g) \longrightarrow \dot{C} l(g)+O _2(g)$
(c) $HOCl(g) \xrightarrow{h \nu} \dot{O} H(g)+\dot{C} l(g)$
(d) $CF _2 Cl _2(g) \xrightarrow{h \nu} \dot{C} l(g)+\dot{C} F _2 Cl(g)$
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Answer:
Correct Answer: 18. (d)
Solution:
- $CH _4$ is not present in the stratosphere and also it cannot diffuse or escape into the stratosphere like freon- $12\left(CF _2 Cl _2\right)$ from the atmosphere.
In the stratosphere, ozone layer depletion take place mainly by chlorofluorocarbons (CFCs) like $CF _2 Cl _2$ and the mechanism of ozone layer depletion can be shown as:
(i) $CF _2 Cl _2(g) \xrightarrow{h \nu} Ci(g)+\dot{C} F _2 Cl(g)$ [Option, (d)]
(ii) $Cl(g)+O _3(g) \longrightarrow ClO^{\bullet}(g)+O _2$
(iii) $ClO^{\bullet}(g)+O(g) \longrightarrow Cl^{\bullet}(g)+O _2(g)$ [Option (b)] (iv) $Ci(g)+\underset{\begin{array}{c}\text { [Present in the } \ \text { stratosphere] }\end{array}}{H _2 O(g)} \longrightarrow HOCl(g)+H^{\bullet}(g)$
(v) $HOCl(g) \xrightarrow{h \nu} \dot{O} H(g)+Cl^{*}(g)$ [Option (c)]
$\Rightarrow$ One $Cl^{*}$ can destroy or deplete $10^{5} O _3$ molecules.
As (i) reaction is involved in the formation of photochemical smog, not in ozone layer depletion. So option (a) is correct.
