IB HL Chemistry Summary: Depletion of the Ozone Layer and UV absorption in the Atmosphere (from Structure& Bonding topic)

Depletion of the Ozone Layer and UV absorption in the Atmosphere


Ozone = an allotrope of oxygen gas

- It is made naturally in the atmosphere
- It can also be made by photocopiers and laser printers

- Ozone formation (In the stratosphere)
Oxygen gas molecules are bombarded by high energy UV light from the sun
This causes them to break up to form Oxygen radicals
The oxygen radicals then react with more oxygen gas molecules to make ozone
O2  2O.
2O. + 2O2 2O3
3O2  2O3

- Ozone Destruction
The ozone gas molecules are hit by other UV radiation from the Sun
They break up to form oxygen gas molecules and oxygen radicals
2O3 2O. + 2O2
Oxygen radicals join up to form O2 molecules
2O. O2
2O3 3O2



UV Information

- Can be divided into 3 components
- UV-C(highest energy, shortest wavelength), UV-B, UV-A

- Ozone and O2 absorb UV as they undergo dissociation and form free radicals


Free Radicals

= atoms or groups of atoms with unpaired electrons

- An oxygen free radical is simply an oxygen atom (it has 2 unpaired electrons)
- Shown as O.



CFCs

Chlorofluorocarbon (CFC) = is an organic molecule containing chlorine, fluorine and carbon
atoms

- Used as:
Refrigerants
Bubbles in expanded polystyrene
Inert atmosphere
Aerosol propellants

- Chlorine Radicals
Remove ozone, turn it back into oxygen gas without absorbing UV light
The Cl. are reformed
Very long lasting (up to 30 years) >> react with lots of oxygen

How do CFCs remove ozone?
Cl• + O3 → ClO• + O2 O3 + ClO• + O• → ClO• + O2 + Cl• + O2

Simplified O3 + O• → 2O2