L9_warming_O3_115b08

L9_warming_O3_115b08 - L9: Interaction of molecules with...

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L9: Interaction of molecules with light (Chapter 20) - General aspects of spectroscopy - Photochemistry - Stratospheric chemistry: the ozone hole - Tropospheric chemistry: smog - Global Warming Read:1. http://www.atm.ch.cam.ac.uk/tour/part3.html Chapter 20, Sections 1 and 5 and lecture notes Save the Environment! Jobs $350-550/week Ad: Entrance to PSA
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h ν In σ * state the H + H energy is lower than the H 2 energy. L9: Photochemical processes Light can induce σ−σ * transition -> reduces bond order to zero -> causes dissociation to H + H
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Example: Photodissociation of HCl at wavelength ~180 nm R H-Cl E Bond energy Photon energy h ν R eq H + Cl ( σ ) 2 Excited state configuration: ( σ ) 1 ( σ *) 1
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Demo: Photoinduced chain reaction of chlorine and hydrogen H 2 + Cl 2 -> 2HCl Δ H=- 92.3 kJ/mol (exothermic, but need light to initiate) Cl 2 + light -> 2Cl Cl + H 2 -> HCl + H H + Cl 2 -> HCl + Cl H + Cl -> HCl Cl+Cl -> Cl 2 H + H -> H 2 Bond energy in Cl 2 : 230 kJ/mol Bond energy in H 2 : 432 kJ/mol Visible light: 400-700 nm = 171-299 kJ/mol
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… for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. … clarified the mechanisms for the chemical reactions involved. The results have led to extensive limitations on the release of ozone-damaging substances. The Nobel Prize in Chemistry 1995 Sherwood Rowland UC Irvine USA Mario Molina UCSD USA Paul Crutzen MPI for Chemistry Mainz, Germany
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Structure of atmosphere
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Ozone is most abundant at 30-40 km height: in the stratosphere Maximum concentration: < 10 13 molecules/cm 3 Ozone (O 3 ) in stratosphere: Protects us from harmful UV light (skin cancer, melanoma)
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Chapman Mechanism: O 2 + radiation -> 2O need UV radiation 2O + 2O 2 + M <-> 2O 3 + M M = a species that removes energy by collision Overall reaction: 3O 2 -> 2O 3 How ozone is destroyed in the stratosphere? O 3 + UV light - > O 2 + O There is a very delicate balance for ozone. Can also be destroyed by chemical reactions with
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L9_warming_O3_115b08 - L9: Interaction of molecules with...

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