F332 The atmosphere

A CFC is a covalently bonded relatively small molecule of carbon, chlorine and fluorine atoms (chlorofluorocarbon).

• If enough energy is supplied by heat or by visible/uv electromagnetic radiation, or the is weak enough, a covalent bond can break in two ways (heterolytically or homolytically)
• CFCs are very useful low boiling organic liquids or gases
• until recently, extensively used in refrigerators and aerosol sprays
• They are relatively unreactive, non-toxic and have low flammability
• It is their chemical stability in the environment that eventually causes the ozone problem (depletion)

Ozone is formed in the stratosphere by free radical reactions.

-Ordinary' stable oxygen O₂ (dioxygen) is split (dissociates) into two

-By high energy ultraviolet electromagnetic radiation from the Sun

- It splits in to two oxygen atoms which are radicals.

- a 'free' oxygen atom combines with an oxygen molecule (dioxygen) to form ozone (trioxygen).

O₂ + uv ==> O^{.  +} O^.

O^. + O₂ ==> O₃ 

• The ozone is a highly reactive and unstable molecule and decomposes into dioxygen when hit by other uv light photons

O₃ + uv ==> O₂ + O^.

• This last reaction is the main uv screening effect of the upper atmosphere and the ozone absorbs a lot of the harmful incoming uv radiation from the Sun 

• If the ozone levels are reduced more harmful uv radiation reaches the Earth's surface

• can lead to medical problems: 

• increased risk of sunburn 

• skin cancer

• accelerates skin aging processes.

The chemically very stable CFCs diffuse up into the stratosphere

• Decompose when hit by ultraviolet light (uv)
• Produce free radicals, including free chlorine atoms, which themselves are highly reactive free radicals.

 CCl₂CF₂ ==> CClF₂^. + Cl^.

• The formation of chlorine atom radicals is the root of the problem
• They readily react with ozone and change it back to much more stable ordinary oxygen.

O₃ + Cl^. ==> O₂ + ClO^. and then:

ClO + O ==> Cl + O₂ -'destructive' Cl atom free radical is still around!

• The two reactions above involving chlorine atoms are known as a catalytic cycle because the chlorine atoms from CFC's etc. act as a catalyst in the destruction of ozone.
• Therefore many countries are banning the use of CFCs,
• but not all despite the fact that scientists predict it will take many years for the depleted ozone layer to return to its 'original' O₃ concentration
• alternatives to CFC's are already being marketed.

The idea is to use replacement compounds that are less harmful to the ozone layer.

• Hydrochlorofluorohydrocarbons
• Hydrofluorocarbons
• Alkanes

However, all of these molecules are greenhouse gases and will contribute to global warming!