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 Tip
- 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
