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UNIT I

UNIT I. Reaction Mechanisms. Reaction Mechanisms. Definition: the series (sequence) of steps by which a reaction takes place Ex. ) 5C 2 O 4 2- + 2MnO 4 - + 16H +  involves 23 reacting particles chance of this taking place in one step is almost “0”. Reaction Mechanisms.

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UNIT I

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  1. UNIT I Reaction Mechanisms

  2. Reaction Mechanisms • Definition: the series (sequence) of steps by which a reaction takes place • Ex.) 5C2O42- + 2MnO4- + 16H+ • involves 23 reacting particles • chance of this taking place in one step is almost “0”

  3. Reaction Mechanisms • Even a 3 particle collision… 2H2(g) + O2(g)  probably doesn’t take place in a single step • 1,000 times less probable than a 2 particle collision • Most reactions (other than simple 2 particle collisions eg. Ag+ + Cl-AgCl(s) ) take place in a series of simple steps

  4. Provincial Exam Questions

  5. Reaction Mechanisms •  the series (sequence) of steps by which a reaction takes place • cannot be determined by just looking at overall reaction • deduced through much study and research (up to years) • you will not be asked to come up with a mechanism from scratch • some mechanisms are known, many are yet to be discovered

  6. Reaction Mechanisms Example • For the overall reaction: 4HBr + O2 2H2O + 2Br2 • 5 reactant particles • Doesn’t take place in a single step!

  7. Reaction Mechanisms • Step 1:HBr + O2HOOBr (found to be slow) • Step 2:HBr + HOOBr  2HOBr (fast) • Step 3:HOBr + HBr  H2O + Br2 (very fast)

  8. Reaction Mechanisms • Each step is called an Elementary Process. • The Rate Determining Step is the slowest step in the mechanism. •  The overall reaction can never be faster than the RDS. • The only way to speed up an overall reaction is to speed up the RDS (eg. by increasing the concentration of a reactant in the RDS).

  9. Reaction Mechanisms • In this case, increasing [HBr] or [O2] would speed up Step 1 (the RDS) and hence the overall rate. •  Speeding up a fast step (not RDS) will have no effect on the overall rate. • For example, adding HOOBr or HOBr has no effect.

  10. Determining Overall Reaction Cancel stuff which is identical on both sides and add up what’s left. Example 1 • HBr + O2HOOBr • HBr + HOOBr 2HOBr • HOBr + HBr H2O + Br2 _______________________________

  11. Determining Overall Reaction Example 2 1.) A + 2X  AX2 2.) AX2 + X  AX + X2 3.) AX + A  A2 + X _______________________

  12. Determining Overall Reaction Example 3 The following reaction occurs in a 3 step mechanism. This is the overall reaction: 2A4+ + B+ 2A3+ + B3+ step 1: A4+ + C2+ C3+ + A3+ step 2: A4+ + C3+ C4+ + A3+ step 3: ? Find step 3.

  13. Determining Overall Reaction Example 4 Consider the following reaction for the formation of HCl in the presence of light. Cl2 + CHCl3 HCl + CCl4 The following is the proposed reaction mechanism: Step 1: Cl2 Cl + Cl Step 2: Step 3: Cl + CCl3 CCl4 Determine Step 2 of the reaction mechanism.

  14. Provincial Exam Questions

  15. Reaction Intermediate • A species (atom, molecule or ion) which is produced in one step and used up in a later step. • The reaction intermediate appears on the right first and then on the lower left. • For the mechanism: • HBr + O2HOOBr • HBr + HOOBr 2HOBr • HOBr + HBr H2O + Br2 • intermediates are ___________ & ___________

  16. Provincial Exam Questions

  17. Provincial Exam Questions

  18. Provincial Exam Questions

  19. Reaction Intermediate Notes: • An intermediate doesn’t accumulate (like a product) because as soon as it is formed, it gets used up again. • Intermediates are not necessarily unstable (in other circumstances, they may last a while). • An activated complex is very unstable and short-lived. It doesn’t usually obey bonding “rules.” • Hebden Textbook Page 28 Questions #46-53

  20. PE Diagram for a Reaction Mechanism AC (Step 1) AC (Step 2) AC (Step 3) HOOBr PEHBr + O2 HOBr H2O + Br2 Reaction Proceeds Label this diagram: RDS, Ea (Overall Rxn), Ea (Step 1), ∆H.

  21. PE Diagram for a Reaction Mechanism Notes: • each “bump” is a step • the higher the bump (greater Ea), the slower the step • the highest bump (from the reactants level) is for the RDS • AC’s are found at top of bumps, intermediates in middle “valleys”, products in the final “valley” • the Ea for the forward overall rxn is vertical distance from reactants to top of highest bump

  22. PE Diagram for a Reaction Mechanism • The Ea for the overall forward reaction is the difference in energy between the reactants and the top of the highest peak.

  23. PE Diagram for a Reaction Mechanism Example: Given the following Potential Energy Diagram for a reaction mechanism: 80 70 PE 60 50 Reaction Proceeds

  24. PE Diagram for a Reaction Mechanism 1. This mechanism has steps 2. Ea for overall rxn = ______kJ 3. Step is the RDS 4. Step is the fastest step. 5. The overall rxn is thermic 6. H = kJ 7. H for reverserxn = kJ 8. Ea (reverse rxn) = kJ 9. RDS for reverserxn is step ________

  25. PE Diagram for a Reaction Mechanism Example: Draw a Potential Energy Diagram for a reaction mechanism with 2 steps. The first step is fast and the second step is slow. The overall reaction is exothermic. With labeled arrows show the overall Activation Energy (Ea) and the H for the forward reaction. • Hebden Textbook Page 30 Questions #54-55

  26. Catalysts Catalyst: an introduced substance which produces an alternate mechanism with a lower activation energy.

  27. Provincial Exam Questions

  28. Catalysts Notes: • energy required (Ea) is lesswith the catalyst, so at the same temperature, more molecules can make it over the “barrier” and reaction rate speeds up • catalyzed reactions usually involve more steps but it’s highest Ea (highest bump) is never as high as the uncatalyzed reaction • a catalyst NEVERchanges the PE of reactants of products - only the route between them (no change in H! ) • uncatalyzed reaction still continues at its own slow rate when a catalyst is added (usually insignificant compared to catalyzed rate) • if catalyst speeds up forward reaction, it also speeds up (reduces Ea for) the reverse reaction

  29. Catalysts

  30. Catalysts • Catalysts sometimes work by: • providing a surface whose spacing of atoms is just right to break a reactant molecule and hold it for an attack from another reactant • helping to form an intermediate which can react more easily to form products • enzymeAnimation • http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/animations/Catalyst2NOO2N28.html

  31. Catalysts Example 2H2O2(l)  2H2O(l) + O2(g) (very slow uncatalyzed) • add some KI (I-) Demonstration Catalyzed Mechanism: step 1) H2O2 + I- ---> H2O + OI- step 2) H2O2 + OI- ---> H2O + O2 + I- overall 2H2O2 ---> 2H2O + O2

  32. Provincial Exam Questions

  33. Provincial Exam Questions

  34. Provincial Exam Questions

  35. Provincial Exam Questions

  36. Catalysts • Read pages 30-36 in Hebden Textbook • Complete Questions #56-61 on page 34 • Review examples of real catalysts on pages 34-36 • Complete Questions 62-63 on page 36 • Do Provincial Exam Questions on Unit 1 • THIS IS THE END OF UNIT 1

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