Lecture33A - Reaction Mechanisms(Chapter 29.1-3) We can...

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Lecture 33 1 Reaction Mechanisms (Chapter 29.1-3) We can find rate laws experimentally, but we also want to use our basic understanding of chemistry so that the observed rate laws make sense. Recall my examples from lecture 31 ( 29 ( 29 ( 29 ( 29 [ ] [ ] ( 29 ( 29 ( 29 ( 29 [ ] [ ] [ ] [ ] 2 2 2 2 1/ 2 2 2 2 2 1 2 2 , v t ' while 2 , v t 1 '' H g I g HI g k H I k H Br H g Br g HBr g k HBr Br - + = + = + Something different is happening at a fundamental level that leads to these two different observations. How is the chemistry different? ( 29 ( 29 ( 29 ( 29 [ ] ( 29 ( 29 ( 29 ( 29 [ ] [ ] ( 29 ( 29 ( 29 [ ] [ ] [ ] 2 2 3/ 2 2 2 2 2 3 3 2 2 3 2 2 v t but v t ! and 2 3 v t !! ' '' NOCl g NO g Cl g k NOCl Cl g CO g Cl CO g k Cl CO k O O g O g k O k O g + = + = = +
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Lecture 33 2 How can we make sense of such complicated expressions? Need to realize that complex reactions are made up of a number of single steps. Somehow, these hidden (at least at present) steps must lead to the observed rate laws. So what are these hidden steps? We call them elementary reactions. There are only three types of elementary reactions. [ ] [ ] [ ] [ ] [ ] [ ] Products v unimolecular Products v ' bimolecular Products v '' termolecular A k A A B k A B A B C k A B C = + = + + = Reaction Rate Molecularity " " designates elementary reaction For an elementary reaction, you can write the rate law directly from the stoichiometry. e
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Lecture 33 3 Note for bimolecular and termolecular reactions, we must have collisions that allow products to form immediately! That is, there are no reaction intermediates (a metastable species that lasts “long enough” to be detected that is not a reactant or product). Also, the highest molecularity observed for an elementary reaction is
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Lecture33A - Reaction Mechanisms(Chapter 29.1-3) We can...

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