2.13f: discuss and explain, including the mechanisms for the reactions, the science community’s reasons for recommending that CFCs are no longer used due to their damaging effect on the ozone layer

The ozone (O3) layer is found in the stratosphere

• formed when oxygen (O2) molecules absorb UV light and undergo homolytic fission to creat two free radicals
• O2 --UV→ O- + O-
• a free radical then reacts with an oxygen (O2) molecule to form ozone
• O2 + O- → O3
• ozone can also absorb UV light and split back into oxygen and a free radical
• O3 → O2 + O-
• in the absence of pollutants, ozone can be formed at the same rate as it is being destroyed

Reactions with CFCs:

• CFCs are inert so make their way into the stratosphere unchanged
• UV light can break the CFCs down into other chlorine-based substances
• the conditions are just right over the Antarctic
• Cl2 --UV→ Cl- + Cl-
• the chlorine radicals destroy ozone molecules and oxygen radicals
• propagation:
• O3 + Cl- → ClO- + O2
• ClO- + O2 → Cl- + O2
• O3 + O- → 2O2

Reactions with nitrous oxides:

• O3 → O2 + O-
• O3 + NO → NO2 + O2
• NO2 + O- → NO + O2
• 2O3 → 3O2

In the stratosphere, CFCs are broke down by absorption of UV radiation to form chlorine free radicals.

The following two reactions occur:

• Cl- + O3 → ClO- + O2
• ClO- + O → Cl- + O2

Combine these two equations to give the overall equation for the reaction of ozone in the stratosphere. State the role played by the chlorine free radical in the overall reaction. Hence explain why many scientists consider the effect of CFCs on ozone to be harmful.

• overall reaction = O + O3 → 2O2
• the chlorine free radical acts as a catalyst by reacting with oxygen free radicals, preventing O3 from forming
• CFCs are harmful because they decimate the number of ozone molecules in the stratosphere, so that less of the UV radiation from the sun is absorbed, allowing more to reach the Earth’s surface, which could result in more cases of skin cancer