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Gail Amort- Larson

We covered several models and theories … Bizarro by Dan Piraro Read 16.7 Practice PS #2 You can solve Q 1 1 2 Rxn mechanisms Theoretical concept Elementary rxn = rxn that occurs in a single step. Chemical equation summary of overall process It has special properties: no information how it happens 1. rate law follows stoichiometry aA + bB  cC + dD Rxn mechanism = sequence of steps by which a rxn Rate = k [reactants]toich coeff= k [A] [B]b s r ucco For example, if the following is an elementary rxn It MUST reproduce the observed overall NO + CO  CO + NO 3 2 2 stoichiometry AND reproduce the observed rate law. Then Rate = k [NO ][CO] 3 Mechanism can involve: 2. The molecularity for an elementary rxn is  Several steps 1 unimolecular  Intermediate species = species that appear as a 2 bimolecular product in one step and being consumed in a 3 termolecular different step and that do not appear in the overall NO 4 or higher ! no fractions ! no zeros, no –ve chemical equation values! 3 4 What is molecularity? theoretical concept Most rxn’s are not elementary Molecularity = number of molecules that are but they can be broken into several elementary rxn’s. involved/collide in an elementary reaction having sufficient energy  Ea and the proper orientation to A + B  C overall rxnnot elementary (from ) sno i t av r e sbo form the transition state. A D elementar(theoretical) Obviously we can’t have fraction or negative D + B elementar(theoretical) molecules colliding. Also the probability of 4 molecules fulfilling all these requirements is too small. First we check that the net reaction of the proposed mechanism matches the observed reaction equation. So 2 A + 2 B  C + D can’t be elementary b/c 4 molecules are involved in the collision. A + D + B  C + D Also watch out for the reverse step in a mechanism. is equivalent to the net reactionA + B  C  Then we can derive a theoretical rate law based on A + B  3 C + D can’t be elementary b/c of the this proposed mechanism. reverse step. 5
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