1. Chapter 13: Chemical Kinetics CHE 124: General Chemistry II Dr. Jerome Williams, Ph.D. Saint Leo University

2. Overview Reaction Mechanisms Catalysts Enzymes

3. Reaction Mechanisms We generally describe chemical reactions with an equation listing all the reactant molecules and product molecules But the probability of more than 3 molecules colliding at the same instant with the proper orientation and sufficient energy to overcome the energy barrier is negligible Most reactions occur in a series of small reactions involving 1, 2, or at most 3 molecules Describing the series of reactions that occurs to produce the overall observed reaction is called a reaction mechanism Knowing the rate law of the reaction helps us understand the sequence of reactions in the mechanism 3 Tro: Chemistry: A Molecular Approach, 2/e

4. An Example of a Reaction Mechanism Overall reaction: H 2(g) + 2 ICl (g)  2 HCl (g) + I 2(g) Mechanism: 1.H 2(g) + ICl (g)  HCl (g) + HI (g) 2.HI (g) + ICl (g)  HCl (g) + I 2(g) The reactions in this mechanism are elementary steps, meaning that they cannot be broken down into simpler steps and that the molecules actually interact directly in this manner without any other steps 4 Tro: Chemistry: A Molecular Approach, 2/e

5. H 2 (g) + 2 ICl(g)  2 HCl(g) + I 2 (g) 1)H 2 (g) + ICl(g)  HCl(g) + HI(g) 2)HI(g) + ICl(g)  HCl(g) + I 2 (g) Elements of a Mechanism Intermediates Notice that the HI is a product in Step 1, but then a reactant in Step 2 Because HI is made but then consumed, HI does not show up in the overall reaction Materials that are products in an early mechanism step, but then a reactant in a later step, are called intermediates 5 Tro: Chemistry: A Molecular Approach, 2/e

6. Molecularity The number of reactant particles in an elementary step is called its molecularity A unimolecular step involves one particle A bimolecular step involves two particles – though they may be the same kind of particle A termolecular step involves three particles – though these are exceedingly rare in elementary steps 6 Tro: Chemistry: A Molecular Approach, 2/e

7. Rate Laws for Elementary Steps Each step in the mechanism is like its own little reaction – with its own activation energy and own rate law The rate law for an overall reaction must be determined experimentally But the rate law of an elementary step can be deduced from the equation of the step H 2 (g) + 2 ICl(g)  2 HCl(g) + I 2 (g) 1) H 2 (g) + ICl(g)  HCl(g) + HI(g) Rate = k 1 [H 2 ][ICl] 2)HI(g) + ICl(g)  HCl(g) + I 2 (g) Rate = k 2 [HI][ICl] 7 Tro: Chemistry: A Molecular Approach, 2/e

8. Rate Laws of Elementary Steps 8 Tro: Chemistry: A Molecular Approach, 2/e

9. Rate Determining Step In most mechanisms, one step occurs slower than the other steps The result is that product production cannot occur any faster than the slowest step – the step determines the rate of the overall reaction We call the slowest step in the mechanism the rate determining step – the slowest step has the largest activation energy The rate law of the rate determining step determines the rate law of the overall reaction 9 Tro: Chemistry: A Molecular Approach, 2/e

10. Another Reaction Mechanism The first step in this mechanism is the rate determining step. The first step is slower than the second step because its activation energy is larger. The rate law of the first step is the same as the rate law of the overall reaction. 10 NO 2(g) + CO (g)  NO (g) + CO 2(g) Rate obs = k[NO 2 ] 2 1. NO 2(g) + NO 2(g)  NO 3(g) + NO (g) Rate = k 1 [NO 2 ] 2 Slow 2. NO 3(g) + CO (g)  NO 2(g) + CO 2(g) Rate = k 2 [NO 3 ][CO] Fast Tro: Chemistry: A Molecular Approach, 2/e

11. Validating a Mechanism To validate (not prove) a mechanism, two conditions must be met: 1.The elementary steps must sum to the overall reaction 2.The rate law predicted by the mechanism must be consistent with the experimentally observed rate law 11 Tro: Chemistry: A Molecular Approach, 2/e

12. Mechanisms with a Fast Initial Step When a mechanism contains a fast initial step, the rate limiting step may contain intermediates When a previous step is rapid and reaches equilibrium, the forward and reverse reaction rates are equal – so the concentrations of reactants and products of the step are related – and the product is an intermediate Substituting into the rate law of the RDS will produce a rate law in terms of just reactants 12 Tro: Chemistry: A Molecular Approach, 2/e

13. An Example 1.2 NO (g)  N 2 O 2(g) Fast 2.H 2(g) + N 2 O 2(g)  H 2 O (g) + N 2 O (g) SlowRate = k 2 [H 2 ][N 2 O 2 ] 3.H 2(g) + N 2 O (g)  H 2 O (g) + N 2(g) Fast k1k1 k−1k−1 2 H 2(g) + 2 NO (g)  2 H 2 O (g) + N 2(g) Rate obs = k [H 2 ][NO] 2 13 Tro: Chemistry: A Molecular Approach, 2/e

14. Example 13.9: Show that the proposed mechanism for the reaction 2 O 3(g)  3 O 2(g) matches the observed rate law Rate = k[O 3 ] 2 [O 2 ] −1 1.O 3(g)  O 2(g) + O (g) Fast 2.O 3(g) + O (g)  2 O 2(g) Slow Rate = k 2 [O 3 ][O] k1k1 k−1k−1 14 Tro: Chemistry: A Molecular Approach, 2/e

15. Practice – Mechanism Determine the overall reaction, the rate determining step, the rate law, and identify all intermediates of the following mechanism 1.A + B 2  AB + BSlow 2.A + B  ABFast 15 Tro: Chemistry: A Molecular Approach, 2/e

16. Practice – Mechanism Determine the overall reaction, the rate determining step, the rate law, and identify all intermediates of the following mechanism 1.A + B 2  AB + BSlow 2.A + B  ABFast reaction2 A + B 2  2 AB B is an intermediate The first step is the rate determining step Rate = k[A][B 2 ], same as RDS 16 Tro: Chemistry: A Molecular Approach, 2/e

17. Catalysts Catalysts are substances that affect the rate of a reaction without being consumed Catalysts work by providing an alternative mechanism for the reaction – with a lower activation energy Catalysts are consumed in an early mechanism step, then made in a later step mechanism without catalyst O 3(g) + O (g)  2 O 2(g) V. Slow mechanism with catalyst Cl (g) + O 3(g)  O 2(g) + ClO (g) Fast ClO (g) + O (g)  O 2(g) + Cl (g) Slow 17 Tro: Chemistry: A Molecular Approach, 2/e

18. Ozone Depletion over the Antarctic 18 Tro: Chemistry: A Molecular Approach, 2/e

19. Molecular Interpretation of Factors Affecting Rate – Catalysts Catalysts generally speed a reaction They give the reactant molecules a different path to follow with a lower activation energy – heterogeneous catalysts hold one reactant molecule in proper orientation for reaction to occur when the collision takes place and sometimes also help to start breaking bonds – homogeneous catalysts react with one of the reactant molecules to form a more stable activated complex with a lower activation energy 19 Tro: Chemistry: A Molecular Approach, 2/e

20. Energy Profile of a Catalyzed Reaction polar stratospheric clouds contain ice crystals that catalyze reactions that release Cl from atmospheric chemicals 20 Tro: Chemistry: A Molecular Approach, 2/e

21. Practice – Mechanism Determine the overall reaction, the rate determining step, the rate law, and identify all catalysts and intermediates of the following mechanism 1.HQ 2 R 2 + R −  Q 2 R 2 − + HRFast 2.Q 2 R 2 −  Q 2 R + R − Slow 21 Tro: Chemistry: A Molecular Approach, 2/e

22. Practice – Mechanism reactionHQ 2 R 2  Q 2 R + HR R − is a catalyst Q 2 R 2 − is an intermediate The second step is the rate determining step Rate = k[Q 2 R 2 − ] = k[HQ 2 R 2 ][R − ] Determine the overall reaction, the rate determining step, the rate law, and identify all catalysts and intermediates of the following mechanism 1.HQ 2 R 2 + R −  Q 2 R 2 − + HRFast 2.Q 2 R 2 −  Q 2 R + R − Slow 22 Tro: Chemistry: A Molecular Approach, 2/e

23. Catalysts Homogeneous catalysts are in the same phase as the reactant particles – Cl (g) in the destruction of O 3(g) Heterogeneous catalysts are in a different phase than the reactant particles – solid catalytic converter in a car’s exhaust system 23 Tro: Chemistry: A Molecular Approach, 2/e

24. Types of Catalysts 24 Tro: Chemistry: A Molecular Approach, 2/e

25. Catalytic Hydrogenation H 2 C=CH 2 + H 2 → CH 3 CH 3 25 Tro: Chemistry: A Molecular Approach, 2/e

26. Enzymes Because many of the molecules are large and complex, most biological reactions require a catalyst to proceed at a reasonable rate Protein molecules that catalyze biological reactions are called enzymes Enzymes work by adsorbing the substrate reactant onto an active site that orients the substrate for reaction 26 1)Enzyme + Substrate  Enzyme─SubstrateFast 2)Enzyme─Substrate → Enzyme + ProductSlow Tro: Chemistry: A Molecular Approach, 2/e

27. Enzyme–Substrate Binding: the Lock and Key Mechanism 27 Tro: Chemistry: A Molecular Approach, 2/e

28. Enzymatic Hydrolysis of Sucrose 28 Tro: Chemistry: A Molecular Approach, 2/e