Organohalogen: Contain at least one halogen atom joined to carbon chain. Used in pesticides,
polymers and refrigerators.
• Ozone Layer: Ozone layer in the outer edge of the stratosphere. Only a fraction of gases making
up ozone layer is ozone. Ozone absorbs the harmful ultraviolet radiation from Sun’s rays. UV- B
radiation is linked to sunburn and higher risk of skin cancer. Depletion of ozone layer will allow
more UV- B radiation to reach Earth’s surface.
• Oxygen Radicals: O2 -> 2O. In stratosphere broken down by ultraviolet (UV) radiation into oxygen
radicals.
• Steady State: Steady state is set up where rate of formation of ozone is same as rate at which
decomposed. O2 + O.
-> O3 and vice versa. Give reversed equation as well.
• Chlorotrifluoromethane is CF3Cl.
• CFCs: CFCs are stable due to strength of carbon- halogen bonds. They are a problem because CFCs
have long residence time in troposphere and CFCs are still being used. When CFCs reach the
stratosphere however, UV radiation breaks C-Cl bond in CFCs by homolytic fission forming chlorine
radicals- which catalyse breakdown of ozone layer.
• Photodissociation:
• CF2Cl2 -> CF2Cl. + Cl. Initiation. Dot always next to the Cl.
• Chlorine Radical: It is reactive. It can react with an ozone molecule, breaking down the ozone into
oxygen molecule- catalysing breakdown of ozone layer.
• Propagation Step 1: Cl. + O3 -> ClO. + O2
• Propagation Step 2: ClO. + O -> Cl. + O2. Chlorine radical regenerated to remove another
molecule of ozone in propagation step 1- chain reaction.
• Overall Equation: O3 + O -> 2O2. Need all three steps in dark blue, even if does not ask for overall
equation.
• Catalytic Breakdown of Ozone Layer: Chlorine radical is not entirely responsible. Nitrogen oxide
radicals also catalyse breakdown of ozone. Nitrogen oxide radicals formed during lightning strikes
and air craft.
• Propagation Step 1: NO. + O3 -> NO2
. + O2. Propagation Step 2: NO2
. + O -> NO. + O2. Overall
Equation: O3 + O -> 2O2.
• Cl and NO act as catalyst.
Biodegradable alternative to CFCs are HCFCs (hydrocarbons).
• N2 + O2 -> 2NO Formation of NO in aircraft engine.