CHM 104 1st Edition Lecture 6 Outline of Last Lecture I Arrhenius Equation Outline of Current Lecture II Reaction Mechanism Current Lecture Reaction Mechanism Exploring A rate determining steps and energy diagrams A p x z A frequency factor z collision factor p orientation factor number of collisions that have correct orientation For which reaction is the orientation factor closest to 1 100 of the collisions have the correct orientation H Cl H Cl H2 Cl2 Cl Cl Cl2 p 1 Sphere all collisions are geometrically identical p 1 2 NO2 g Cl2 g 2 ClNO2 g This reaction doesn t occur as a single step where 2 NO2 molecular simultaneously collide with a Cl2 molecule Mechanism shows how it might happen 1 NO2 g Cl2 ClNO2 Cl 2 NO2 g Cl ClNO2 Notice the steps ADD up to the overall reaction Notice the reaction intermediate species created in step 1 is used up in step 2 Each step in a mechanism by definition is an elementary step These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Rate law equations for Elementary steps have orders that match stoichiometric coefficients in that step Reaction Type Molecularity Rate Law Equation A Products unimolecular 1 A A Products bimolecular 2 A B Products bimolecular 2 A A A Products termolecular 3 A B B Products termolecular 3 1 2 k A B A B C Products termolecular 3 1 1 1 k A B C k A k A 1 2 1 1 k A B k A 3 Each step in the mechanism has its own rate law equation and rate constant Usually one step will be much slower than all of the other steps rate limiting determining step To a good approximation the rate law equation for the entire mechanism will be equal to the rate equation for the rate determining step Evaluation a Mechanism 1 Elementary steps of the proposed mechanism must add up to overall reaction 2 Rate Law equation for the proposed mechanism must match that observed by experiment data 3 If both criteria are met then the reaction might happen that way
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