Ozone catalysis

I know that catalysts don't change chemical *equilibrium* because they accelerate both the reactions in the same way.


I can't understand why CFC catalysts accelerate ozone destruction but not ozone formation.


Could you help me?




You make the assumption that the ozone concentration in the upper atmosphere is in equilibrium. It isn't.


$\ce{O3}$ is a much less stable molecule than $\ce{O2}$ (the heat of formation from $\ce{3/2 O2}$ is $143~\mathrm{kJ/mol}$) and the concentration at equilibrium would be *very* low. A significant concentration exists in the upper atmosphere because $\ce{O3}$ is continuously formed from the reaction of $\ce{O2}$ and UV light. Also, though it is a highly reactive molecule, there are still kinetic barriers to its conversion back to $\ce{O2}$. There is no simple chemical route back to normal oxygen and the UV absorption of the $\ce{O2}$ molecule that creates ozone is different to the UV absorption of the ozone molecule, so the driving force of ozone creation doesn't destroy it.


CFCs release $\ce{Cl}$ atoms in the upper atmosphere via a photochemical reaction. The $\ce{Cl}$ atoms react with ozone in a variety of reactions the simplest of which are:


$$\ce{Cl + O3 -> ClO + O2}$$


and 


$$\ce{ClO + O3 -> Cl + 2 O2}$$


So, strictly speaking, the $\ce{Cl}$ from CFCs is not directly catalysing the equilibrium between $\ce{O3}$ and $\ce{O2}$ but providing a new reaction pathway via several intermediates that, in a roundabout way, allows the "equilibrium" to be reestablished. Another way to think about this is that the presence of $\ce{Cl}$ facilitates a lower kinetic barrier route to ozone destruction. A lower kinetic barrier to the reaction allows the relative concentrations to get closer to the natural equilibrium. So even if there were a reverse pathway (which there isn't in these complex reactions) the result would be a much lower concentration of ozone.


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