CHAPTER 12 KINETICS THE PRINCIPLES THAT GOVERN THE RATE AT WHICH CHEMICAL REACTIONS OCCUR Kinetics About rates in processes about timing Chemical Reactions and Molecular Collisions Bimolecular reaction involves the collision of two molecules that are the reactants Because a reaction requires a collision a chemical reaction cannot occur at a rate faster than the rate at which reactant molecules collide Zmolecule rate of collisions taking place number of collisions sec Not every collision results in a reaction wrong orientation o Probability of reacting Rate observed Zmolecule The Overall Reaction vs the Elementary Reaction Elementary reaction associated with a specific potential energy surface o Not a combination or a sequence of reactions Determination of the Rate of a Chemical Reaction Can measure in variety of ways o Concentrations Determination of the Reaction Rate Constant kobs The Reaction Rate Order Determination of the Effect of Concentration on Reaction Rate Coefficient exponent first order normally Rate R k A m B n o M reaction order with respect to A o N reaction order with respect to B Overall order sum of m n The Behavior of Zero Order First Order Second Order and Third Order Kinetics Zero Order o Follows R d A dt k A 0 k First Order o Rate k A 1 Second Order o Rate k A 2 Third Order o Rate k A 3 SUMMARY o Integration of the Rate Law Defining the Concentration as a Function of Time Steady State Approximation takes advantage of the fact that w in chemical mechanisms reside elementary reactions that rapidly produce and remove reactive intermediates for which the loss rate of the species is limited by its production rate Set intermediate concentration rate it is consumed rate it is removed equal to 0 Arrhenius Expression for Temperature Dependence kobs Ae Ea RT