Environmental chemistry
E. 9 ozone
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.
ozone depletion
http://www.theozonehole.com/
photodissociation
photodissociation of O2
equation
UV
wavelength, ,
nm
frequency, f, s1
(higher energy
radiation)
bond enthalpy,
kJ mol-1
covalent bond
types
O2 + uv  O
+ O
(shorter than) 242
f=c/
= 3.00 x 108 m s-1
/242 x 10-9 m
= 1.24 x 1015 s-1
498
one + one  bond
photodissociation of O3
equation
UV wavelength, ,
nm
O3 + uv 
O2 + O
330
frequency, f, s-1
f=c/
= 3.00 x 108 m s-1
/330 x 10-9 m
= 9.10 x 1014 s-1
bond enthalpy,
kJ mol-1
363
covalent bond
types
one + 2
delocalized 
electrons between
a double and
single bond
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
498 000 J mol-1 /6.02 x 1023 mol-1 = 8.27 x 10-19 J
E = hf and f = c / 
E = h c / 
=hc/E
= (6.63 x 10-34 J s x 3.00 x 108 m s-1 ) /8.27 x 10-19 J
= 2.41 x 10-7 m = 241 nm
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 10-34 J s x 3.00 x 108 m s-1 ) /300 x 10-9
m
= 6.03 x 10-19 J (one photon/one bond)
for 1 mole of bonds:
6.63 x 10-19 J x 6.02 x 1023 mol-1
= 399126 J mol -1 = 399 kJ mol-1
Catalytic depletion CFCs
CCl2F2  CClF2 + Cl
Cl
+
ClO
+
O +
O3 
O 
O3 
ClO
+
Cl +
2O2
O2
O2
Catalytic depletion
NO
NO2
+
+
O +
O3 
O 
O3 
NO2 +
NO
+
2O2
O2
O2
ozone in october in antarctica
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.