Mechanisms and ozone

Ozone (O₃) is formed in the upper atmosphere when oxygen atoms are split apart. This can be caused by ultraviolet light, lightening, or any other source of energy. The separate oxygen atoms, assuming they don’t just recombine, may attach to other oxygen molecules forming ozone.

Here is a possible mechanism for this process:

Ozone is destroyed in the following way: An ozone molecule is split apart by energy (ultraviolet light, etc.). If two free atoms of oxygen meet, they form an oxygen molecule. The mechanism is here:

Here is a graphical representation of this process:

Of course, most of the time a free oxygen atom will simply rejoin with another O₂ recreating the ozone that was lost, so this process is fairly slow. In fact for a long time, the rates of production and destruction of ozone in the upper atmosphere were essentially equal.

Unfortunately...we came along. 

Vocabulary Alert: Something that is produced in one step of a mechanism and then used in another (so that it is NOT in the overall reaction) is called an intermediate. In the example above, the oxygen atoms (O) are intermediates.

What we have done to the ozone layer

In the late 1920's and early 1930's we began creating and using compounds called chlorofluorocarbons (CFCs) as refrigerants and aerosol propellants (the gas in the can that pushes out the spray paint or hair spray etc.).

CFC's are molecules with carbons in the middle and some chlorine atoms and/or fluorine atoms around the outside.

When these compounds find themselves in the upper atmosphere, bombarded by ultraviolet light, they break up into separate atoms.

Lone chlorine atoms (as opposed to chlorine molecules - Cl₂) are highly reactive, allowing the following process to occur.

A chlorine atom collides with an ozone, removing one of the oxygens, forming ClO (the intermediate in this mechanism). This reaction happens readily because chlorine atoms are very unstable (since they do not have a filled electron shell).

The problem is that, although the chlorine atom then has an "octet", the oxygen atom in ClO has an unpaired electron. As a result ClO reacts readily with ozone as well producing oxygen gas and releasing the chlorine atom.

Here is a graphical representation of this process. The chlorine atom is in green, so that you can follow it through the process.

Notice that the overall reaction does not include the chlorine atom. Once again, the catalyst is still present, unchanged at the end of the process.

Vocabulary Check: An intermediate is something that is produced and then used in a mechanism. A “get-involved” catalyst is first used, and then reproduced in the mechanism. Neither catalysts nor intermediates show up in the overall reaction.

The result of all of this, is that ozone is now being destroyed faster than nature can replace it. Thus, more ultraviolet light now reaches the surface of the earth than in previous centuries. Unfortunately, that means that the risk of skin cancer is much higher now than it was for your parents and grandparents.