11th Class Chemistry Environmental Chemistry

  • question_answer 9) What are the reactions involved for ozone layer depletion in the stratosphere?


    Two types of the compounds have been found responsible for destruction of ozone. (i) Nitric oxide: Though, nitrous oxide is quite inert in the stratosphere, it is photo chemically decomposed into more reactive nitric oxide. \[N{{O}_{2}}+hv\to NO+N\] Nitric oxide, so formed, undergoes the following chain reactions: \[\underset{Net\,reaction}{\overset{{}}{\mathop{{}}}}\,\frac{\begin{align} & NO+{{O}_{3}}\to N{{O}_{2}}+{{O}_{2}} \\ & {{O}_{3}}+hv\to {{O}_{2}}+O \\ & N{{O}_{2}}+O\to NO+{{O}_{2}} \\ \end{align}}{2{{O}_{3}}\to 3{{O}_{2}}}\] Thus, in presence of oxides of nitrogen, the rate of decomposition of \[{{O}_{3}}\] increases. (ii) Chlorofluorocarbons: Chlorofluorocarbons absorb produce chlorine atoms by breaking carbon-chlorine bonds. \[C{{F}_{2}}C{{l}_{2}}+hv\to C{{F}_{2}}Cl+Cl\] \[CFC{{l}_{3}}+hv\to CFC{{l}_{2}}+Cl\] Chlorine atoms undergo the following reactions: \[\underset{Net\,reaction}{\overset{{}}{\mathop{{}}}}\,\frac{\begin{align} & Cl+{{O}_{3}}\to ClO+{{O}_{2}} \\ & ClO+O\to Cl+{{O}_{2}} \\ \end{align}}{{{O}_{3}}+O\xrightarrow{Cl}2{{O}_{2}}}\] Chlorine atoms catalyse the decomposition of\[{{O}_{3}}\].  

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