1. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 1 of 61 14-8 Theoretical Models for Chemical Kinetics  Kinetic-Molecular theory can be used to calculate the collision frequency.  In gases 10 30 collisions per second.  If each collision produced a reaction, the rate would be about 10 6 M s -1.  Actual rates are on the order of 10 4 M s -1. ◦Still a very rapid rate.  Only a fraction of collisions yield a reaction. Collision Theory

2. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 2 of 61 Activation Energy  For a reaction to occur there must be a redistribution of energy sufficient to break certain bonds in the reacting molecule(s).  Activation Energy:  The minimum energy above the average kinetic energy that molecules must bring to their collisions for a chemical reaction to occur.

3. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 3 of 61 Activation Energy

4. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 4 of 61 Kinetic Energy

5. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 5 of 61 Collision Theory  If activation barrier is high, only a few molecules have sufficient kinetic energy and the reaction is slower.  As temperature increases, reaction rate increases.  Orientation of molecules may be important.

6. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 6 of 61 Collision Theory

7. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 7 of 61 Transition State Theory  The activated complex is a hypothetical species lying between reactants and products at a point on the reaction profile called the transition state.

8. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 8 of 61 14-9 Effect of Temperature on Reaction Rates  Svante Arrhenius demonstrated that many rate constants vary with temperature according to the equation: k = Ae -E a /RT ln k = + lnA R -Ea T 1

9. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 9 of 61 Arrhenius Plot N 2 O 5 (CCl 4 ) → N 2 O 4 (CCl 4 ) + ½ O 2 (g) = -1.2  10 4 K R -Ea-Ea -E a = 1.0  10 2 kJ mol -1

10. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 10 of 61 Arrhenius Equation k = Ae -E a /RT ln k = + ln A R -Ea-Ea T 1 ln k 2 – ln k 1 = + ln A - - ln A R -Ea-Ea T2T2 1 R -Ea-Ea T1T1 1 ln = - R -Ea-Ea T2T2 1 k2k2 k1k1 T1T1 1

11. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 11 of 61 14-10 Reaction Mechanisms  A step-by-step description of a chemical reaction.  Each step is called an elementary process.  Any molecular event that significantly alters a molecules energy of geometry or produces a new molecule.  Reaction mechanism must be consistent with:  Stoichiometry for the overall reaction.  The experimentally determined rate law.

12. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 12 of 61 Elementary Processes  Unimolecular or bimolecular.  Exponents for concentration terms are the same as the stoichiometric factors for the elementary process.  Elementary processes are reversible.  Intermediates are produced in one elementary process and consumed in another.  One elementary step is usually slower than all the others and is known as the rate determining step.

13. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 13 of 61 Slow Step Followed by a Fast Step H 2 (g) + 2 ICl(g) → I 2 (g) + 2 HCl(g) dtdt = k[H 2 ][ICl] d[P] Postulate a mechanism: H 2 (g) + 2 ICl(g) → I 2 (g) + 2 HCl(g) slow H 2 (g) + ICl(g) HI(g) + HCl(g) fast HI(g) + ICl(g) I 2 (g) + HCl(g) dtdt = k[H 2 ][ICl] d[HI] dtdt = k[HI][ICl] d[I 2 ] dtdt = k[H 2 ][ICl] d[P]

14. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 14 of 61 Slow Step Followed by a Fast Step

15. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 15 of 61 Fast Reversible Step Followed by a Slow Step 2NO(g) + O 2 (g) → 2 NO 2 (g) dt = -k obs [NO 2 ] 2 [O 2 ] d[P] Postulate a mechanism: dtdt = k 2 [N 2 O 2 ][O 2 ] d[NO 2 ] fast 2NO(g) N 2 O 2 (g) k1k1 k -1 slow N 2 O 2 (g) + O 2 (g) 2NO 2 (g) k2k2 dtdt = k 2 [NO] 2 [O 2 ] d[I 2 ] k -1 k1k1 2NO(g) + O 2 (g) → 2 NO 2 (g) K = k -1 k1k1 = [NO] [N 2 O 2 ] = K[NO] 2 k -1 k1k1 = [NO] 2 [N 2 O 2 ]

16. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 16 of 61 Catalytic Converters  Dual catalyst system for oxidation of CO and reduction of NO. CO 2 + N 2 CO + NO cat

17. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 17 of 61 14-5 Catalysis

18. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 18 of 61  Worked Examples Follow:

19. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 19 of 61

20. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 20 of 61

21. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 21 of 61

22. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 22 of 61  CRS Questions Follow:

23. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 23 of 61 Energy reaction coordinate A+B X+Y Energy reaction coordinate A+B X+Y Energy reaction coordinate A+B X+Y Energy reaction coordinate A+B X+Y The reaction between A and B is determined to be a fairly fast reaction and slightly exothermic. Which of the following potential energy surfaces fit this description? 1. 4.3. 2.

24. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 24 of 61 Energy reaction coordinate A+B X+Y Energy reaction coordinate A+B X+Y Energy reaction coordinate A+B X+Y The reaction between A and B is determined to be a fairly fast reaction and only slightly exothermic. Which of the following potential energy surfaces fit this description? 4.3. 2. Energy reaction coordinate A+B X+Y 1.

25. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 25 of 61 Energy reaction coordinate 1.  H = 60 kJ mol -1 R P 2.  H = -60 kJ mol -1 5.  H = 140 kJ mol -1 4.  H = -80 kJ mol -1 3.  H = 80 kJ mol -1 A particular reaction was found to have forward and reverse activation energies of 60 and 140 kJ mol -1, respectively. The enthalpy change for the reaction is, (do not use a calculator)

26. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 26 of 61 Energy reaction coordinate 1.  H = 60 kJ mol -1 R P A particular reaction was found to have forward and reverse activation energies of 60 and 140 kJ mol -1, respectively. The enthalpy change for the reaction is, (do not use a calculator) 2.  H = -60 kJ mol -1 5.  H = 140 kJ mol -1 4.  H = -80 kJ mol -1 3.  H = 80 kJ mol -1

27. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 27 of 61 Energy reaction coordinate 4. 2. 3. A+B X+Y Energy reaction coordinate 1. A+B X+Y Energy reaction coordinate A+B X+Y Energy reaction coordinate A+B X+Y In which diagram to the right does the dashed line best represent the catalyzed version of the reaction’s potential energy profile?

28. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 28 of 61 In which diagram to the right does the dashed line best represent the catalyzed version of the reaction’s potential energy profile? Energy reaction coordinate 4. 2. 3. A+B X+Y Energy reaction coordinate 1. A+B X+Y Energy reaction coordinate A+B X+Y Energy reaction coordinate A+B X+Y

29. Prentice-Hall © 2007 General Chemistry: Chapter 14 Slide 29 of 61  Textbook End of Chapter ?’s:  P.611- #1, 3, 11, 13, 17, 19,  21, 33, 47, 51, 55, 100, 101,  102, 103, 104, 105