15.2 Organohalogen Compounds in the Environement Flashcards
What is an organohalogen?
Molecules that contain at least one halogen joined to a carbon chain.
What are some uses of organohalogens?
Pesticides
General and dry cleaning solvents
Making polymers
Refrigerants
How are oxygen radicals produced?
High energy UV breaks down oxygen molecules into oxygen radicals in the stratosphere
O2 -> 2O
How is ozone formed and broken down?
A steady state is formed invoking O2 and the oxygen radicals in which ozone forms and then breaks down. The rate of formation and the rate of being broken down is the same.
O2 + O (reversible sign) O3
What were the uses of CFCs and HFCs?
Refrigerants in air conditioning units
Aerosol propellants
Talk about the stability of CFCs
They are very stable as they have strong carbon - halogen bonds.
How was ozone depletion discovered?
2 chemists, Frank Sherwood Rolland and Mario Molina concluded that CFCs remain stable until they reach the stratosphere. Here the CFCs begin to break down, forming chlorine radicals , which are thought to catalyse the breakdown of the ozone layer.
They were awarded a Nobel Prize for this.
How do CFCs deplete the ozone layer?
In the stratosphere, UV radiation provides enough energy to break a carbon - halogen bond in CFCs by homolytic fission to form radicals. The C-Cl bond has the lowest bond enthalpy and so is the bond that breaks.
State the initiation, propagation and overall equations for the break down of CFCs and interaction with ozone
Initiation ( photodisskciation ) : CF2Cl2 -> CF2Cl• + Cl•
Propagation step 1 : Cl• + O3 -> ClO• + O2
Propagation step 2 : ClO • + O -> Cl• + O2
Overall : O3 + O -> 2O2
Propagation step 2 regenerates a chlorine radical, which can attack and remove another molecule of ozone in propagation step 1. These two steps can repeat in a cycle over and over again in a chain reaction.
Show the steps in how other radicals eg nitrogen oxide can also break down ozone
Propagation 1 : NO• + O3 -> NO2• + O2
Propagation 2 : NO2 • + O -> NO• + O2
Overall : O3 + O -> 2O2
What makes sure CFCs are in limited use?
The Montreal Protocol
It introduced steps for the complete removal of CFCs in all but a limited number of products where no suitable alternative can be found.
