Halogenoalkanes: Ozone Depletion

Question 1

Why is ozone beneficial?

Answer 1

• Ozone (O3), formed naturally in the upper atmosphere, is beneficial because it absorbs ultraviolet radiation
• UV radiation can cause skin cancer, eye damage and possible damage to crops

Question 2

What do CFC’s do to the ozone layer and how do they reach the ozone layer?

Answer 2

• Ozone is easily destroyed by CFC’s
• CFC’s are chlorofluorocarbons
• They are less reactive than alkanes because the C-F bond is very strong
• So they diffuse unchanged into the upper atmosphere

Question 3

Explain how CFC’s destroy the ozone layer

Answer 3

• UV causes a chlorine atom to break away from the CFC molecule
• The free chlorine atom (radical) hits an ozone molecule
• The chlorine atom pulls one oxygen atom away
• A free oxygen atom hits the chlorine monoxide molecule
• The result is another free chlorine atom
• Free chlorine will continue to deplete ozone in the stratosphere

Question 4

Briefly describe how CFC’s destroy the ozone layer (SPEC)

Answer 4

• Chlorine atoms are formed in the upper atmosphere when ultraviolet radiation causes C–Cl bonds in chlorofluorocarbons (CFCs) to break
• Chlorine atoms catalyse the decomposition of ozone and contribute to the hole in the ozone layer

Question 5

What is the initiation step in the decomposition of ozone with 1,2,2-trichloro-1,1,2-trifluoroethane?

Answer 5

• CClF2CFCl2 → CClF2CFCl• + Cl•

- C―Cl breaks homolytically in the presence of UV radiation to produce chlorine free radicals Cl•

Question 6

What are the propagation steps in the decomposisiton of ozone?

Answer 6

• Part 1

• O3 + Cl• → O2 + ClO•
• Chlorine radicals attack ozone molecules

• Part 2

• O3 + ClO• → 2O2 + Cl•
• Reaction between another ozone and the newly produced ClO• radical

Question 7

What are the possible termination reactions for the decomposition of ozone?

Answer 7

• Cl• + Cl• → Cl2
• Cl• + ClO• → Cl2O
• ClO• + ClO• → Cl2O2

Question 8

What is the overall equation for the decomposition of ozone?

Answer 8

• Combine both propagation steps and eliminate molecules that don’t change
• O3 + Cl• → O2 + ClO•
• O3 + ClO• → 2O2 + Cl•
• 2O3 ⇌ 3O2
• Chlorine free radical is not used up
• It acts as a catalyst in the breakdown of ozone to oxygen

Question 9

What led to the ban of CFCs?

Answer 9

• In the 1970s, research by scientist groups showed that CFCs were causing damage to the ozone layer
• They were banned

Question 10

What alternatives have been made to CFC’s?

Answer 10

• Chemists have developed safer alternatives to CFCs which contain no chlorine
• HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons) are temporary alternatives until safer products are made

Question 11

Give other examples of alternatives to CFC’s

Answer 11

• Most aerosols have been replaced by pump spray systems, or use nitrogen as the propellant
• Many industrial fridges use ammonia or hydrocarbons as the coolant gas
• Carbon dioxide is used to make foamed polymers