1. Environmental chemistry
E. 9 ozone

2. ozone
Explain the dependence of O2 and O3 dissociation on the wavelength of light.
Describe the mechanism in the catalysis of O3 depletion by CFCs and NOx.
Outline the reasons for greater ozone depletion in polar regions.

3. ozone depletion http://www.theozonehole.com/

4. one + 2 delocalized  electrons between a double and single bond
photodissociation
photodissociation of O2
photodissociation of O3
equation
O uv  O O
UV wavelength, , nm
(shorter than) 242
frequency, f, s-1
(higher energy radiation)
f = c / 
= 3.00 x 108 m s-1 /242 x 10-9 m
= x 1015 s-1
bond enthalpy,
kJ mol-1
498
covalent bond types
one + one  bond
equation
O uv  O O
UV wavelength, , nm
330
frequency, f, s-1
f = c / 
= 3.00 x 108 m s-1 /330 x 10-9 m
= x 1014 s-1
bond enthalpy,
kJ mol-1
363
covalent bond types
one + 2 delocalized  electrons between a double and single bond

5. sample calculation
Calculation of wavelength of one photon to dissociate O2 when the bond enthalpy is 498 kJ mol-1
E of one photon is
J mol-1 /6.02 x 1023 mol-1 = 8.27 x J
E = hf and f = c / 
E = h c / 
 = h c / E
= (6.63 x J s x x 108 m s-1 ) /8.27 x J
= 2.41 x 10-7 m = nm

6. sample calculation
Calculation of bond enthalpy in kJ mol-1 for ozone when a wavelength of 300 nm is needed
E = h c / .
= (6.63 x J s x x 108 m s-1 ) /300 x 10-9 m
= x J (one photon/one bond)
for 1 mole of bonds:
6.63 x J x x 1023 mol-1
= J mol -1 = 399 kJ mol-1

7. Catalytic depletion CFCs
CCl2F2  CClF2 + Cl
Cl O3  ClO O2
ClO O  Cl O2
O O3  O2

8. Catalytic depletion NO + O3  NO2 + O2 NO2 + O  NO + O2
O O3  O2

9. ozone in october in antarctica

11. greater ozone depletion in polar regions
very cold winter temperatures over the polar regions
ice crystals form in the stratosphere and act as heterogeneous catalysts for reactions which produce chlorine oxides (e.g. HOCl) and Cl2 from CFCs.
during winter time a huge reservoir of HOCl and Cl2 molecules builds up until spring arrives.
spring brings UV light which causes chlorine free radicals to split from the HOCl or Cl2 molecules.
greater catalytic depletion of the ozone in the polar regions. UV also melts the ice crystals and stops the formation of HOCl and Cl2 slowing down this process.