1 / 29

14-8 Theoretical Models for Chemical Kinetics

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 .

asasia
Download Presentation

14-8 Theoretical Models for Chemical Kinetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 14-8 Theoretical Models for Chemical Kinetics • Kinetic-Molecular theory can be used to calculate the collision frequency. • In gases 1030 collisions per second. • If each collision produced a reaction, the rate would be about 106 M s-1. • Actual rates are on the order of 104 M s-1. • Still a very rapid rate. • Only a fraction of collisions yield a reaction. Collision Theory General Chemistry: Chapter 14

  2. 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. General Chemistry: Chapter 14

  3. Activation Energy General Chemistry: Chapter 14

  4. Kinetic Energy General Chemistry: Chapter 14

  5. 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. General Chemistry: Chapter 14

  6. Collision Theory General Chemistry: Chapter 14

  7. 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. General Chemistry: Chapter 14

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

  9. -Ea = -1.2104 K R Arrhenius Plot N2O5(CCl4)→ N2O4(CCl4) + ½ O2(g) -Ea = 1.0102 kJ mol-1 General Chemistry: Chapter 14

  10. -Ea 1 ln k = + ln A R T 1 -Ea 1 -Ea ln k2– ln k1 = + ln A - - ln A T2 R T1 R k1 1 1 -Ea ln = - k2 T1 T2 R Arrhenius Equation k = Ae-Ea/RT General Chemistry: Chapter 14

  11. 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. General Chemistry: Chapter 14

  12. 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. General Chemistry: Chapter 14

  13. d[HI] = k[H2][ICl] H2(g) + ICl(g) HI(g) + HCl(g) dt HI(g) + ICl(g) I2(g) + HCl(g) d[I2] = k[HI][ICl] dt d[P] = k[H2][ICl] dt Slow Step Followed by a Fast Step d[P] H2(g) + 2 ICl(g) → I2(g) + 2 HCl(g) = k[H2][ICl] dt Postulate a mechanism: slow fast H2(g) + 2 ICl(g) → I2(g) + 2 HCl(g) General Chemistry: Chapter 14

  14. Slow Step Followed by a Fast Step General Chemistry: Chapter 14

  15. d[P] = -kobs[NO2]2[O2] dt k1 2NO(g) N2O2(g) fast k-1 d[NO2] k2 k1 slow N2O2(g) + O2(g) 2NO2(g) = k2[N2O2][O2] [N2O2] [NO]2 = K[NO]2 = dt k1 [N2O2] k-1 K = = d[I2] k1 k-1 [NO] = k2 [NO]2[O2] dt k-1 Fast Reversible Step Followed by a Slow Step 2NO(g) + O2(g) → 2 NO2(g) Postulate a mechanism: 2NO(g) + O2(g) → 2 NO2(g) General Chemistry: Chapter 14

  16. Catalytic Converters • Dual catalyst system for oxidation of CO and reduction of NO. cat CO + NO CO2 + N2 General Chemistry: Chapter 14

  17. 14-5 Catalysis General Chemistry: Chapter 14

  18. Worked Examples Follow: General Chemistry: Chapter 14

  19. General Chemistry: Chapter 14

  20. General Chemistry: Chapter 14

  21. General Chemistry: Chapter 14

  22. CRS Questions Follow: General Chemistry: Chapter 14

  23. X+Y Energy A+B reaction coordinate Energy Energy A+B A+B X+Y X+Y reaction coordinate reaction coordinate 1. 2. A+B Energy 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? X+Y reaction coordinate 3. 4. General Chemistry: Chapter 14

  24. X+Y Energy A+B reaction coordinate Energy A+B X+Y reaction coordinate 1. 2. A+B Energy 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? X+Y reaction coordinate 3. 4. Energy A+B X+Y reaction coordinate General Chemistry: Chapter 14

  25. 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) R Energy 1. DH = 60 kJ mol-1 2. DH = -60 kJ mol-1 P 3. DH = 80 kJ mol-1 reaction coordinate 4. DH = -80 kJ mol-1 5. DH = 140 kJ mol-1 General Chemistry: Chapter 14

  26. 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) R Energy 1. DH = 60 kJ mol-1 2. DH = -60 kJ mol-1 P 3. DH = 80 kJ mol-1 reaction coordinate 4. DH = -80 kJ mol-1 5. DH = 140 kJ mol-1 General Chemistry: Chapter 14

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

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

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

More Related