Environmental Concerns of Organohalogen Use (OCR AS Chemistry A)
Revision Note
Breakdown of the Ozone Layer
Fluorohalogenoalkanes
The most common halogenoalkanes are chlorofluorocarbons (CFCs)
These compounds contain carbon atoms with chlorine and fluorine atoms attached to them
E.g. CCl3F and CCl2F2
CFCs have many uses due to their chemical inertness as they are non-flammable and non-toxic
They are used as refrigerators
Propellants for aerosols
As solvents for dry cleaning
Hydrofluorocarbons
Hydrofluorocarbons (HFCs) are compounds that contain a carbon attached to hydrogen and fluorine atoms only
E.g. CH2F-CF3
As with CFCs, HFCs are chemically inert and are therefore used for a wide range of purposes
Though chlorofluorocarbons (CFCs) have many beneficial uses, they also have devastating effects on the ozone layer:
CFCs absorb a lot of UV radiation in the upper atmosphere
The CFCs are then broken down by the UV light causing the formation of chlorine radicals
These radicals react with ozone and break down the ozone layer as shown:
Chlorine radicals and the ozone layer
This is why hydrofluorocarbons (HFCs) are often used as an alternative to CFCs
As these compounds don’t contain any chlorine atoms, they will not have adverse effects on the ozone layer
Examiner Tips and Tricks
There is another set of accepted propagation steps for the breakdown of ozone by chlorine radicals:
Propagation 1: Cl• + O3 → Cl• + O2
Propagation 2: ClO• + O → Cl• + O2
The difference is actually in the second propagation step where this version reacts with a lone oxygen atom rather than another ozone molecule
Other radicals
CFCs are not the only radicals that catalyse the breakdown of the ozone
One of the other sets of radicals linked to ozone breakdown are the nitrogen oxide radicals which can be formed:
Naturally during lightning strikes
As a result of air travel in the stratosphere
Nitrogen monoxide is formed by the direct combination of nitrogen and oxygen at high temperatures inside a combustion engine
The nitrogen monoxide formed is a free radical as it has an odd number of electrons / unpaired electron
The nitrogen monoxide radical reacts with one molecule of ozone to form the nitrogen dioxide molecule
NO• + O3 → NO2• + O2
The nitrogen monoxide radical is then reformed
NO2• + O → NO• + O2
Overall, the breakdown equation is O3 + O → 2O2
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