36_562ln08

36_562ln08 - MIT OpenCourseWare http:/ocw.mit.edu 5.62...

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MIT OpenCourseWare http://ocw.mit.edu 5.62 Physical Chemistry II Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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5.62 Spring 2008 Lecture #36 Page 1 Rates of Unimolecular Reactions: RRKM Consider a unimolecular reaction: A products isomerization CH 3 NC CH 3 CN decomposition (with barrier to recombination) C 2 H 5 Cl C 2 H 4 + HCl In order to occur, these reactions must overcome a barrier, E 0 . They can be activated to E* > E 0 by collision, overtone pumping, infrared multiphoton excitation, optical excitation followed by Internal Conversion, or Stimulated Emission Pumping. A molecule becomes activated, either by absorption of a photon or by a collision. The activated molecule has a definite E and J. If E > E 0 , where E 0 is the energy of the zero- point-energy-dressed barrier for the unimolecular process: We want to predict the rate of the reaction. reaction coordinate A(bound) v = 0 E 0 products E* Standard mechanism (from 5.60) activation A + M A* + M k 1 deactivation A* + M A + M k –1 revised 4/24/08 11:40 AM
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5.62 Spring 2008 Lecture #36 Page 2 irreversible decay into products A* products k 2 Steady state for A* d A* [ ] dt = k 1 [A][M] ! k ! 1 [A*][M] ! k 2 [A*] = 0 [A*] SS = k 1 [A][M] k ! 1 [M] + k 2 d[products] dt = k uni [A] = k 2 [A*] = k 1 k 2 [A][M] k ! 1 [M] + k 2 " k uni = k 1 k 2 [M] k ! 1 [M] + k 2 = k 1 k 2 k ! 1 1 + k 2 k ! 1 [M] ( ) use this form of k uni below ! " ######### “Unimolecular” rate is actually pressure-dependent. k
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This note was uploaded on 11/27/2011 for the course CHEM 5.43 taught by Professor Timothyf.jamison during the Spring '07 term at MIT.

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36_562ln08 - MIT OpenCourseWare http:/ocw.mit.edu 5.62...

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