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Gail Amort- Larson

The last few lectures we dealt with kinetics, straight lines and slopes … Dilbert by Scott Adams Read 16.7 Practice PS 2 is posted online. You can solve question 1. 1 2 The rate constant is given by k = p Z f Transition state model Assuming a favorable collision occurs, what is happening? p = steric factor only a proper orientation is . dewo l l a Plot reaction profile or activation energy diagram Z = # of collision per unit time per unit volume f = fraction of molecules with energy  E a  Energy vs. rxn progress –E aRT = e Upon collision So k = p Z e –E aRT  Bonds in the reactants stretch and become weaker.  New intermolecular bonds start to form (still weak The larger E a the smaller k and hence a slower rxn. at this stage). Ea = 0  k = pZ e = pZ  The transition species is known as the transition – RT/RT –1 Ea= RT  k = p Z e = pZ e = 0.37 pZ state or the activated complex. 3 4 See fig. 16.21 The transition state (TS) or activated complex has these characteristics - high energy - short–lived (unstable) - exact structure is unknown (can’t be isolated) - in equilibrium (low concentration) with reactants and products - can proceed in either direction When the potential energy reaches a maximum, we call it the transition state. When E reactantsEproductsexothermic reaction E < E endothermic reaction reactants products 5 6 Temperature effect on rxn rates See fig 16.16 All rxn’s go faster with temperature increase. Increase  Increase in number of molecules with energy > E a  k increases Rule of thumb If Ea 50 kJ/mol 10 degrees increase (room T)  doublreofn 7 8 Demo 1
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