Lecture35A - Chain Reactions and Catalysis(29.7-9 Chain...

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Lecture 35 1 Chain Reactions and Catalysis (29.7-9) Chain reactions are common to chemistry and primarily involve free radicals. From our earlier discussions, what looked like a simple reaction: ( 29 ( 29 ( 29 2 2 2 H g Br g HBr g + had an unexpectedly complicated rate law: [ ] [ ] [ ] [ ] [ ] 1/2 2 2 1 2 1 2 1 ' d HBr k H Br dt k HBr Br - = + Now that we have a little experience, we see two things immediately. 1) The term in the denominator involves a back reaction since it depends on [HBr]. 2) The [Br 2 ] 1/2 dependence is something new. It suggests (at least to me) that the concentration of a reactive intermediate [R] 2 is proportional to [Br 2 ]. A good guess would be [Br] 2 !
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Lecture 35 2 As you may have guessed, this involves a chain reaction mechanism which has 4 basic parts: ( 29 ( 29 ( 29 ( 29 1 2 Initiation: 2 k Br g M g Br g M g + + ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 2 3 2 2 Propagation: k k Br g H g HBr g H g H g Br g HBr g Br g + + + + ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 2 3 2 2 Inhibition: k k HBr g H g Br g H g HBr g Br g H g Br g - - + + + + ( 29 ( 29 ( 29 ( 29 1 2 Termination: 2 k Br g M g Br g M g - + + The reaction is triggered by the collisional dissociation of Br 2 . But the propagation reactions are the primary producers of product. They work in tandem to consume each reactant and regenerate a reactive species (H or Br).
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Lecture 35 3 The inhibition reactions reverse the formation of product. These counterproductive reactions are studied in great detail in order to minimize their effects if possible. ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 ( 29 2 3 2 2 Note 70 / while 170 / k r k r HBr g H g H g Br g H kJ mol HBr g Br g Br g H g H kJ mol - - + + 2245 - + + 2245 + It should come as no surprise that the latter reaction is very unlikely
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This note was uploaded on 04/08/2010 for the course CHEM 444 taught by Professor Jameslis during the Spring '08 term at University of Illinois at Urbana–Champaign.

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Lecture35A - Chain Reactions and Catalysis(29.7-9 Chain...

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