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ChE 452 Lecture 11

ChE 452 Lecture 11. Prediction of Mechanisms. Background: Last Time We Discussed General Scheme. Guess or predict all of the species that are likely to form during the reaction. Write down all of the possible reactions of those species.

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ChE 452 Lecture 11

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  1. ChE 452 Lecture 11 Prediction of Mechanisms

  2. Background: Last Time We Discussed General Scheme • Guess or predict all of the species that are likely to form during the reaction. • Write down all of the possible reactions of those species. • Use various rules to pare down the list to manageable number of steps.

  3. Rules • There must be at least one initiation reaction • Require a catalytic cycle • There should be at least one termination reaction

  4. Include Steps With Low Barriers Ea<(0.15 Kcal/Mole-°K)T initiation step Ea<(0.05 Kcal/Mole-°K)T catalytic cycle Ea<(0.07 Kcal/Mole-°K)T other side reactions

  5. Plan For Today • Predict a mechanism for H2 + Br22HBr • Consider C2H6C2H4+H2

  6. Example of Mechanism Prediction Consider H2 + Br2 2HBr (5.10) Assume HBr is the only product, H and Br are the only intermediates. Predict a mechanism for the reaction.

  7. Two Possible Initiation Steps: H2 + X  2H + X (5.11) Br2 + X  2Br + X (5.12)

  8. The Hydrogen Atoms Can React With Each of the Stable Species in the System: H + Br2 HBr + Br (5.13) H + HBr  H2 + Br (5.14) H + H2  H2 + H (5.15)

  9. The Bromine Atoms Can React With Each of the Stable Species in the System:

  10. The radicals can react with each other:

  11. Forbidden Reactions Transition States (5.22) Reactions without collision partners: (5.23) (5.24)

  12. Complete Mechanism of H2+Br22HBr

  13. Why Does the Standard Mechanism Have Fewer Steps? • Some of the steps have negligible rates - not included in standard mechanism! The mechanism represents the principal route from reactants to products, but other reactions can, and do occur.

  14. Next: Pare Down Mechanism Approach • First identify reactions that must be included • Estimate Ea • Eliminate any negligible rates (unless must be included)

  15. Reactions to Include • There must be an initiation reaction. • Must have a catalytic cycle • There should be a termination reaction.

  16. Which Are Initiation Reactions?

  17. Where Are We Now?  - Need one of these

  18. Find a Catalytic Cycle Producing Products

  19. Should Have Termination Reaction

  20. Next Rules to Exclude Reactions: Idea: if rates slow enough, we can ignore reactions. General method • Estimate Ea for all of the steps • If Ea large, eliminate the step • Do not eliminate steps that must be included (must have an initiation step, must have a catalytic cycle)

  21. Two Methods to Estimate Ea Polanyi relationship Blowers Masel Equation when when when (10.63) (10.65)

  22. Intrinsic Barriers and Transfer. Coefficients for Different Types of Reaction of Neutral Species. Return

  23. Use Polayni to Predict Whether equ 5.11 or 5.12 Will Be Preferred: Data from CRC (H2+X2H+X)=+104 kcal/mole Ea (equ 5.11) = 104cal / mole+1kcal/mole=105 kcal/mole (Br2+X 2Br+X)=+44 kcal/mole Ea (equ 5.12) = 44kcal/mole + 1kcal/mole=45=45 kcal/mole For initiation reactions Table 5.4 5.32

  24. General Rule for Initiation Reactions The most important initiation reaction will be the reaction which breaks the weakest bond in the reactants

  25. Where Are We Now? Ignore - Need - Needed - Possible - Possible - Possible - Needed

  26. Now Concentrate On (5.15) and (5.18) (5.15) (5.18) No net production of species Exchange reactions can be ignored when one is deriving a rate equation (5.34)

  27. Where We Are Now

  28. Now concentrate on (5.14) and (5.17) Both occur at high temperatures. Which is more important?

  29. Data From CRC Table 5.4

  30. Ea (equ 5.14)=12 kcal/mole + 0.3(-16 kcal/mole) =7 kcal/mole Ea (equ 5.17)=12 kcal/mole + 0.7(44 kcal/mole) =43 kcal/mole Clearly, 5.14 is going to be more important Will need to include 5.17 at high temperature where Ea<0.07 T

  31. Where Are We Now?

  32. Next Let us Consider 5.19, 5.20, 5.21

  33. Rules for Radical Recombination Reactions

  34. Where Are We Now? We derived the Bodenstein mechanism!!

  35. Discussion Problem 1) If you wanted a rate law for H2+Br22HBr. How would you get it?

  36. Discussion Problem

  37. Class Question • What did you learn new today?

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