Physical Chemistry. Lecture 5 Theoretical chemical kinetics

Transcription

1 Physical Chemistry Lecture 5 Theoretical chemical kinetics

2 Chemical kinetics Understand the nature of reactions Predict reaction outcomes based on Reactants Conditions Requires integration of theory and experimental results

3 Temperature dependence of rate constants k( T ) Aexp( E / RT ) a Empirical theory of Arrhenius gives a useful way to parameterize rate constants A pre-exponential factor E a activation energy Often seen in analysis of kinetic data

4 Non-Arrhenius behavior Arrhenius behavior is empirical For some reactions the temperature dependence of the rate constant is not exponential Theory does not easily predict Arrhenius form Amazing that Arrhenius behavior is so often seen

5 Elementary reactions Chemical reactions are often more complex than presented and do not occur as a single step Some reactions do occur in a single step -- elementary reactions Generally involve simple mono- or bimolecular interactions Order in elementary reactions is the stoichiometry number, called molecularity H 2 + 2O H 2 O 2 v k[ H 2 1 ] [ O] 2 H + Br HBr v 1 k[ H ] [ Br] 1

6 Simple collision theory of gasphase kinetics To participate in a bimolecular reaction, molecules must approach each other SCT: gas-phase reaction rate proportional to collision frequency SCT does not generally agree with experimental rates Points out how to think about theory of chemical reactions σ k v v max max < Z < Z σ σ > N 0 > σ 8kT C πµ 8kT πµ A C B < v > n 2 πd collision cross section N 0 * A n * B

7 Correcting simple collision theory SCT neglects two features Collision must be sufficiently energetic to cause reaction Molecules must have proper orientation to allow reaction Empirically add two factors to account for these features p steric factor v k < Z pσ pσ > p exp( E N N 0 0 min 8kT exp( E πµ 8kT exp( E πµ / RT ) min min / RT ) C C / RT ) A B E min minimum energy for reaction Like van der Waals s improvement of the idealgas law

8 Relation of SCT parameters to Arrhenius parameters Many experimental data reported as Arrhenius behavior Comparison with SCT necessary to connect theory and experiment Predicts a temperaturedependent activation energy and preexponential factor (which may not be the observation) Arrhenius ln k T E a A E min p e behavior E RT 1/ 2 a 2 By comparison of differentials RT + 2 k max

9 Experimental SCT and Arrhenius parameters Reaction Range ln A E a p H + D 2 HD + D H + HCl H 2 + Cl H + HBr H 2 + Br O + O 3 2 O N + NO N 2 + O ~ CH 3 + C 6 H 6 CH 4 + C 6 H BH 3 + BH 3 CO CO + B 2 H PH 3 + B 2 H 6 PH 3 BH 3 +BH CO + O 2 O + CO F 2 + ClO 2 F + FClO (a) The range of validity is expressed in kelvins. (b) A is in dm 3 mol -1 s -1. (c) E a is in kj mol -1.

10 Computer modeling of potential-energy surfaces Potential energy of configuration of molecules controls interaction Model time-dependent approach of molecules with (classical or quantum) simulation Example: D + H 2 HD + H Several parameters Distances Angles

11 Computer simulation of reaction Reactive encounters go through the reactive region Unreactive encounters go to the reactive region and return in the same channel M. Karplus, R. N. Porter, and R. D. Shamra, J. Chem. Phys., 43,3258 (1965).

12 Simulation of a reactive event Follow the time course of the approach Atoms exchange partners Time scale is very short Repeat many times and measure fraction of times that reactive events happen

13 Activated-complex theory Theory developed by H. Eyring, M. Evans, and M. Polanyi Proposed the existence of an activated complex, a precursor to products The activated complex forms product(s) by a first-order process

14 Activated-complex mathematics Reaction velocity proportional to activated-complex concentration Quasi-equilibrium between reactants and activated complex Statistical mechanics defines disappearance rate constant of activated complex, f v f C a K a a A B C θ C C C A fk v CAC θ B C RT S exp θ hn0c R B exp H RT C A C B

15 Eyring s equation Activated-complex theory gives a form for the biomolecular rate constant, k 2, in terms of the parameters of the activated complex k RT hn C 2 θ 0 S exp R exp H RT This is not Arrhenius-like behavior Difficult to distinguish from Arrhenius behavior under many circumstances Different way to parameterize the SCT rate constant

16 Evaluation of Eyring parameters Evaluate by plotting ln(k/t) versus 1/T Example CH4 + OH CH3 + H2O From slope and intercept H 12.0 kj/mol S J/K-mol Alternative to Arrhenius parameters T. Gierczak, R. Talukdar, S. Herndon, G. Vaghjiani, A. Ravishankara, J. Phys. Chem, 101A, 3125 (1997).

17 Relation of Eyring and Arrhenius parameters Need to be able to convert between the two parameterizations of kinetic data Use differential of Eyring form to show relationships Phase/Molecularity Activation Energy, E a Pre-exponential factor, A Solution H + RT ert S exp( ) θ hn0c R Gas, unimolecular H + RT ert S exp( ) hn0 R Gas, bimolecular H RT e RT S exp( ) θ hn0c R Gas, termolecular H RT e RT S exp( ) θ 2 hn C R 0

18 Summary Theories of simple reactions Simple collision theory Modified collision theory Computer simulation Activated-complex theory Parameterization of reaction dynamics Activation energy and pre-exponential factor Equilibrium thermodynamic properties of activated complex