Elementary Reactions

Transcription

1 Elementary Reactions Elementary reactions occur in a single encounter Unimolecular: A Rate = k[a] Bimolecular: A + B Rate = k[a][b] Termolecular: A + B + C Rate = k[a][b][c] Termolecular reactions are rare; higher molecularities are unknown. For elementary reactions, reaction order equals molecularity

2 ConcepTest 1 Which of the following reactions is not an elementary reaction? A. H 2 S + O 2 H 2 O + SO B. CH 4 + F HF + CH 3 C. NO + NO 3 2NO 2 D. He + + N 2 N + + N + He

3 ConcepTest 1 Which of the following reactions is not an elementary reaction? A. H 2 S + O 2 H 2 O + SO B. CH 4 + F HF + CH 3 C. NO + NO 3 2NO 2 D. He + + N 2 N + + N + He

4 Composite Reactions Composite reactions involve two or more elementary steps Composite reactions are likely when: 1. Complex rearrangements occur 2. More than two molecules of reactants are involved 3. The rate equation does not correspond to the stoichiometric equation 4. Reaction intermediates are detected

5 ConcepTest 2 Is the rate of an overall composite reaction lower, higher, or equal to the average rate of the individual steps in the mechanism? A. Lower B. Higher C. Equal

6 ConcepTest 2 Is the rate of an overall composite reaction lower, higher, or equal to the average rate of the individual steps in the mechanism? A. Lower B. Higher C. Equal

7 Temperature Dependence of k Many reactions have a rate cons t that shows a temperature dependence as on the left. This corresponds to the form k Ae E*/RT where A and c are simply empirical constants. E* has units of energy, and k has units of the rate constant. Might A and E* have physical significance?

8 Arrhenius Equation temperature dependence of k k Ae E /RT a A = pre-exponential factor E a = activation energy Potential energy difference between products and reactants should be related to H reaction Let s try to develop a 2 nd order rate constant along these lines

9 A + B C + D an elementary reaction k should be proportional to number of times A and B collide per second multiplied by fraction with sufficient energy for rxn multiplied by fraction with that energy contained in the appropriate degrees of freedom for rxn multiplied by fraction of collisions that occur with a geometry appropriate for rxn There might or might not be a barrier. How do we think about this? k

10 The Arrhenius equation Insight from collision theory k Ae E /RT a A = frequency factor (collisions with proper orientation) E a /RT e = f = fraction of collisions with sufficient energy to surmount barrier

11 Pre-exponential Factor, A A = PZ AB Z AB = collision density (calculated earlier) P = steric factor (the probability that colliding molecules have the proper orientation) Values of P vary from 1 for atoms to 10 6 for biomolecules

12 Steric Factor, P molecular orientation NO + NO 3 NO 2 + NO 2 More in a few minutes

13 The Exponential Factor, E a e RT

14 A + B C + D an elementary reaction Consider possible activation energies (reaction profiles) and steric effects for the following reaction: CH 4 + D CH 3 D +H Steric hindrance: not much Activation energy: yes How much? C-H bond 140 kj/mol (but maybe a lot less!)

15 A + B C + D an elementary reaction Consider possible activation energies (reaction profiles) and steric effects for the following reaction: OH + D HOD Steric hindrance: Activation energy: some no

16 Back to integrated rate laws Now that we know about elementary reactions, we can look at how the integrated rate laws might apply to elementary processes. First, we would write the three 2 nd order rxns as A + A P A + B P A + B + B P The first two are 2 nd order from this perspective, while the third is a threebody (3 rd order) process, and much less common.

17 Rate laws A + A P A + B P A + B + B P d A dt Case 1: 1 1 kt A A d A dt d A dt 0 k k k A A A 2 B B Case 2: if [B] >> [A], then [B] is essentially constant, and A A [B] e k t 0 Pseudo first order reaction 2

18 Reaction Mechanism A detailed sequence of steps for a reaction Reasonable mechanism: 1. Elementary steps sum to the overall reaction 2. Elementary steps are physically reasonable 3. Mechanism is consistent with rate law and other experimental observations (generally found from rate limiting (slow) step(s) A mechanism can be supported but never proven

19 NO 2 + CO NO + CO 2 Observed rate = k [NO 2 ] 2 Deduce a possible and reasonable mechanism NO 2 + NO 2 NO 3 + NO slow NO 3 + CO NO 2 + CO 2 fast Overall, NO 2 + CO NO + CO 2 Is the rate law for this sequence consistent with observation? Yes Does this prove that this must be what is actually happening? No!