Environmental chemistry
description
Transcript of Environmental chemistry
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Environmental chemistry
E. 9 Ozone
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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.
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Ozone depletionOzone depletion http://www.theozonehole.com/
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PhotodissociationPhotodissociationphotodissociation of O2
equation O2 + 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 = 1.24 x 1015 s-1
bond enthalpy, kJ mol-1 498
covalent bond types
one + one bond(bond order = 2)
photodissociation of O3
equation O3 + uv O2 + O
UV wavelength, , nm
shorter than 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 (bond order 1.5)
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Sample calculationCalculation 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 JE = hf and f = c / E = h c / = h c / 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
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Sample calculationCalculation 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 = 6.63 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
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Catalytic depletion CFCs
CCl2F2 CClF2 + Cl
Cl + O3 ClO + O2
ClO + O Cl + O2
O + O3 2O2
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Catalytic depletion by NO
NO + O3 NO2 + O2
NO2 + O NO + O2
O + O3 2O2
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Catalytic depletion by NO2
NO2 NO + O
NO + O3 NO2 + O2
NO2 + O NO + O2
Net: O + O3 2O2
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Ozone in October in Antarctica
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Greater ozone depletion 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.
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