Depletion of the Ozone Layer – Revision Pack (C6)
The problem with the Ozone Layer:
The ozone layer is located in the stratosphere.
While there are only very small amounts of
ozone in this layer, it still absorbs most of the
ultraviolet (UV) radiation from the Sun.
The more depleted the ozone layer becomes,
the more UV light can get through to the
earth’s surface.
When chlorofluorocarbons (CFCs) were first discovered, it was thought that they
were safe to use. However, since then scientists have discovered that CFC
molecules slowly move upwards into the stratosphere where they attack the ozone.
Society agreed with the view of the scientists’ that CFCs had been depleting the
ozone. This is why the use of CFCs has been banned in the UK, however it is a global
issue, and one country alone cannot solve it.
CFCs can be replaced with alkane or hydrofluorocarbons (HFCs) which DO NOT
damage the ozone layer when used.
How does the Ozone work?
UV radiation is part of the electromagnetic spectrum. Visible light is NOT absorbed
by the ozone layer and passes through it very easily. However, UV radiation is
absorbed by it:
-
The UV part of the electromagnetic spectrum has exactly the right frequency
to make ozone molecules vibrate
The energy of the UV radiation is converted into movement energy within
each ozone molecule
The thicker the ozone layer, the more UV radiation is absorbed
When CFCs were discovered in the 1930s, scientists were VERY excited because
these substances were inert (unreactive).
However, in the 1970s, scientists began to link the ozone depletion with CFCs.
CFCs, Ozone and Radicals:
In the stratosphere, the UV radiation from the sun breaks down the CFC molecules.
This makes highly reactive chlorine atoms. One of these reactive chlorine atoms is
known as a chlorine radical.
1) These chlorine radicals react with the ozone molecules, turning the ozone
back into oxygen gas and depleting the ozone layer
Depletion of the Ozone Layer – Revision Pack (C6)
2) The highly reactive chlorine atoms are regenerated (made again) so can
react with more ozone molecules
3) UV light break down the CFCs very slowly, so they last for a very long time
CF 2 Cl 2  CF 2 Cl + Cl
NOTE – the dot means that there is an unpaired electron, making the atom very
reactive.
The main alternatives to CFCs are alkanes and hydrofluorocarbons. HFCs do NOT
contain chlorine, so cannot make chlorine radicals, and so are safer.
When a covalent bond is broke, it can split into equal halves to make radicals.
UV radiation causes radicals. Each radical sets off a chain reaction. One single
chlorine radical can cause the breakdown of 100,000 ozone molecules.
The chain reaction happens in three steps:
STEP 1 – UV light breaks a bond in the CFC molecules to form chlorine radicals (see
above)
STEP 2 – Chlorine radicals react with ozone molecules, creating more chlorine
radicals, for example:
Cl + O 3  OCl
+ O2
OCl + O 3  Cl
+ 2O 2
If you combine these two equations, you get 2O 3 + 3O 2.
STEP 3 – A possible termination reaction (that ends the depletion) is:
Cl + Cl  Cl 2
It is very common for CFCs to last for between 20 and 50 years before they are
completely broken down by UV radiation. For this reason, CFCs will continue to
deplete the ozone long after they have been banned.
Depletion of the Ozone Layer – Revision Pack (C6)
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Depletion of the Ozone Layer – Revision Pack (C6)
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Depletion of the Ozone Layer – Revision Pack (C6)
Depletion of the Ozone Layer – Revision Pack (C6)
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