OZ5: Ozone
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Define homogeneous catalysis.
• Catalysis where the reactants and catalysts have the same physical state
How does homogeneous catalysis work?
• Intermediate compounds are formed - These break down to form products and reform catalysts
What does the enthalpy profile for homogeneous catalysis look like?
• Two humps for each step (first=intermediate compounds formed, second=intermediate compounds breaking down) • Combined activation enthalpy is lower than the same reaction without a catalyst
Where does photodissociation occur?
Stratosphere
What type of compound undergoes photodissociation to form radicals which contribute to the breakdown of ozone?
Haloalkanes e.g CH3Cl/CH3Br
What occurs in the photodissociation of haloalkanes?
• High energy UV radiation is absorbed by the haloalkane - This breaks the C-halogen bond homolytically to form 2 free radicals
What is the photodissociation of haloalkanes an example of?
Initiation
What does the ease of the photodissociation of haloalkanes depend on?
The carbon-halogen bond enthalpy
Why is the carbon-iodine bond more likely to break than the carbon-chlorine bond?
C-I has a lower bond enthalpy
Why are only a few haloalkanes broken down by UV in the troposphere?
Most of the high frequency UV has been absorbed by the ozone layer
Why are halogen radicals (e.g Cl.) bad for the atmosphere?
They are catalysts for the breakdown of ozone
Give the two steps in the homogenous catalysis of the breakdown of ozone by chlorine radicals.
• Cl.+ O3 ===> ClO. + O2 2. ClO. + O ===> Cl. + O2
What is the intermediate compound formed in the homogenous catalysis of the breakdown of ozone by chlorine radicals?
ClO. (Chlorine monoxide)
What is the overall reaction for the breakdown of ozone?
O3 + O ====> 2O2
Give the two steps in the homogenous catalysis of the breakdown of ozone by hydroxyl radicals.
• HO. + O3 ====> HO2 + O2 2. HO2 + O ====> HO + O2
What is the homogenous catalysis of the breakdown of ozone by chlorine radicals an example of?
Propagation
Why is homogenous catalysis of the breakdown of ozone by chlorine radicals particularly dangerous, despite there being a low concentration of Cl radicals?
• The reaction happens 1500 times faster than ozone can be reformed • Chlorine radicals are regenerated and can go on to degenerate more ozone
How is ozone formed?
By UV radiation from the sun hitting oxygen molecules
Where is ozone formed?
Stratosphere
Give the two equations for the formation of ozone.
• O2 + hv ====> O + O 2. O2 + O ====> O3
Give the equation for UV radiation reversing the formation of ozone.
O3 + hv ====> O2 + O
How is an equilibrium set up between the formation of ozone and its breakdown?
The ozone layer is constantly being replaced or destroyed by UV radiation either hitting ozone or oxygen molecules
Give the equation for the reversible reaction for ozone formation and destruction.
O2 + O O3
Why is ozone important?
It absorbs high energy UV radiation when it breaks down