February 20th 2017 Chapter 16 Chapter 16 HW due Fri Exam 2 Chapters 16 17 next Friday 16 5 Theories of Chemical Kinetics Concentration and temperature have major effects on rxn rate Two models to explain these effects 1 Collision theory particles must collide in order to react Increase concentration more collisions Increase temperature more collisions with E Ea Effective collisions 2 Transition state theory Formation of transition state or activated complex at Ea Transition State Theory What Ea is used for An effective collision between particles leads to the formation of a transition state or activated complex Unstable transitional species that contains partial bonds Reaction energy diagram for the reaction of CH3Br and OH 16 6 Rxn Mechanisms The Steps from Reactant to Product The reaction mechanism is the sequence of single reaction steps that make up the overall equation Elementary steps describe single molecular event molecularity A B C C A D D E F 2A B E F Reaction intermediate substance formed and used up during rxn Rate determining step r d s slowest step in rxn limits overall rxn Sample Problem 16 10 Determining Molecularity and Rate Laws for Elementary Steps The following elementary steps are proposed for a reaction mechanism 1 NO2Cl g NO2 g Cl g 2 NO2Cl g Cl g NO2 g Cl2 g a Write the overall balanced equation 1 NO2Cl g NO2 g Cl g 2 NO2Cl g Cl g NO2 g Cl2 g 2NO2Cl g 2NO2 g Cl2 g b Determine the molecularity of each step Step 1 is unimolecular Step 2 is bimolecular c Write the rate law for each step rate1 k1 NO2Cl rate2 k2 NO2Cl Cl What is the molecularity of the following elementary reaction H g N2O g HO g N2 g a unimolecular b bimolecular c termolecular d None of these is correct Mechanisms with a Slow Initial Step The overall reaction 2NO2 g F2 g 2NO2F g follows the accepted mechanism 1 NO2 g F2 g NO2F g F g 2 NO2 g F g NO2F g slow r d s fast 2NO2 g F2 g 2NO2F g rate1 k1 NO2 F2 rate2 k2 NO2 F Step 1 is the slow step and rate1 correlates with the observed rate law Rate k NO2 F2 Mechanisms with a Fast Initial Step The overall reaction 2NO g O2 g 2NO2 g has an experimental rate law Rate k NO 2 O2 A proposed mechanism is 1 NO g O2 g D NO3 g 2 NO3 g NO g 2NO2 g fast slow r d s 2NO g O2 g 2NO2 g rate1 fwd k1 NO O2 When equilibrium 1 has been established rate1 rev k 1 NO3 rate1 fwd rate1 rev rate2 k2 NO3 NO k1 NO O2 k 1 NO3 rate2 k2 NO3 NO r d s rate1 fwd rate1 rev k1 NO O2 k 1 NO3 k1 NO3 k NO O2 1 rate2 k2 NO3 NO rate2 k2 k1 NO O2 NO k 1 rate2 k NO 2 O2 For any mechanism only reactants involved up to and including the r d s appear in the overall rate law A reaction between NO g and Br2 g has the following elementary steps NO g NO g N2O2 g fast N2O2 g Br2 g 2NOBr g slow Write the rate expression for the reaction between NO g and Br2 g a Rate k N2O2 Br2 rate2 k2 N2O2 Br2 b Rate k NO 2 rate1 f k1 NO NO c Rate k N2O2 d Rate k NO Br2 e Rate k NO 2 Br2 rate1 f rate1r N2O2 is the intermediate rate1r k 1 N 2O2 k1 NO NO k 1 N2O2 k1 N 2O2 NO NO k 1 k1 rate2 k2 NO NO Br2 k 1 rate k NO Br2 2 Let k k 2 k1 k 1