What is a CFC?
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
accelerates skin aging processes.
Back to CFCs
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.
Alternatives to CFCs
The idea is to use replacement compounds that are less harmful to the ozone layer.
However, all of these molecules are greenhouse gases and will contribute to global warming!