Chapter 14, Problem 136AP

In recent years, ozone in the stratosphere has been depleted at an alarmingly fast rate by chlorofluorocarbons (CFCs). A CFC molecule such as ${\text{CFC1}}_{\text{3}}$ is first decomposed by UV radiation:

${\text{CFCl}}_{\text{3}}\underset{}{\to }{\text{CFCl}}_{\text{2}}\text{+ Cl}$

The chlorine radical then reacts with ozone as follows:

$\begin{array}{l}{\text{Cl + O}}_{\text{3}}\underset{}{\to }{\text{ ClO + O}}_{\text{2}}\\ \text{ClO + O}\underset{}{\to }{\text{Cl + O}}_{\text{2}}\end{array}$

(a) Write the overall reaction for the last two steps. (b) What are the roles of $\text{ Cl and ClO}$ ? (c) Why is the fluorine radical not important in this mechanism? (d) One suggestion to reduce the concentration of chlorine radicals is to add hydrocarbons such as ethane ${\text{(C}}_{\text{5}}{\text{H}}_{\text{6}}\text{)}$ to the stratosphere. How will this work? (e) Draw potential-energy

versus reaction progress diagrams for the uncatalyzed and catalyzed (by Cl) destruction of ozone: ${\text{O}}_{\text{3}}\text{+ O}\underset{}{\to }{\text{ 2O}}_{\text{2}}\text{.}$

Use the thermodynamic data in Appendix 2 to determine whether the reaction is exothermic or endothermic.