Question

Ozone depletion over Antarctica is due to the:
(A) Formation of Chlorine nitrate (\[ClON{O_2}\])
(B) Formation of HCl
(C) Formation of HOCl and \[C{l_2}\] which are converted back into reactive Cl atoms
(D) None of the above

Answer 1

Hint: Ozone depletion over Antarctica is due to the formation of chlorine radicals that are formed by cleavage of some species. Ozone depletion over Antarctica is due to the polar stratospheric clouds that give a platform for some species to form compounds that get cleaved in presence of sunlight.

Complete answer:
In Antarctica during winter, special types of clouds called polar stratospheric clouds are formed. Now, these polar stratospheric clouds give some compounds such a surface that they undergo some reaction in these conditions.
We know that Chlorine nitrate and hydrogen chloride are produced over there during summer season in the atmosphere.
Below are the reactions that they give to produce Hypochlorous acid and Chlorine gas.
\[ClON{O_2}_{(g)} + {H_2}{O_{(g)}} \to HOC{l_{(g)}} + HN{O_3}_{(g)}\]
\[ClON{O_2}_{(g)} + HC{l_{(g)}} \to C{l_2}_{(g)} + HN{O_3}_{(g)}\]
Now, when sunlight appears there, in presence of it, HOCl and \[C{l_2}\] break to give active chlorine radicals that are responsible for the ozone depletion.
\[HOC{l_{(g)}}{\xrightarrow{{hv}}^ \bullet }O{H_{(g)}}{ + ^ \bullet }C{l_{(g)}}\]
\[C{l_{2(g)}}\xrightarrow{{hv}}2C{l^ \bullet }_{(g)}\]
So, we can say that correct answer of this question is (C) Formation of HOCl and \[C{l_2}\] which are converted back into reactive Cl atoms.

Additional Information:
Let’s see how Chlorine nitrate and hydrogen chloride are produced in the atmosphere.
\[Cl{O^ \bullet }_{(g)} + N{O_2}_{(g)} \to ClON{O_{2(g)}}\]
\[C{l^ \bullet } + C{H_4} \to HCl{ + ^ \bullet }C{H_3}\]
So, we can say that nitrogen dioxide and methane actually act as chlorine sinks and make chlorine less available for the depletion of ozone layer, but this only happens during the summer and in winter in the presence of polar stratospheric clouds, side reactions occur and it helps in ozone depletion.

Note: Remember that even though Chlorine nitrate and hydrogen chloride is directly produced from polar stratospheric clouds, they are not responsible for the ozone depletion at all.
Remember that HOCl and Chlorine gas do not require any reactants to make active chlorine species, but they just get dissociated in presence of sunlight.