1 / 24

Teacher Prep

Teacher Prep. 3 tennis balls. Reaction Mechanisms. Define “mechanism”. How would this word relate to chemistry?. Reaction Mechanisms. tennis ball activity What does this activity suggest?. Reaction Mechanisms.

keiji
Download Presentation

Teacher Prep

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. Teacher Prep • 3 tennis balls

  2. Reaction Mechanisms Define “mechanism”. How would this word relate to chemistry?

  3. Reaction Mechanisms tennis ball activity What does this activity suggest?

  4. Reaction Mechanisms Reactions that require a greater number of particles to collide at the same time will decrease the chances of a successful reaction to occur.

  5. Reaction Mechanisms Of all possible chemical reactions, how many would require three particles colliding? Many reactions take place through multiple two-collision steps at a time, especially when more than two reactants are involved.

  6. Reaction Mechanisms Overall: 4 HBr + O2 2 H2O + 2 Br2 Step 1: HBr + O2  HOOBr Step 2: HOOBr + HBr  2 HOBr Step 3: (HOBr + HBr  H2O + Br2) x2

  7. Reaction Mechanisms Each step of the mechanism is known as an elementary process. The complete series of elementary processes is known as the reaction mechanism of a chemical reaction. You are not expected to come up with elementary processes!!

  8. Reaction Mechanisms If each elementary process occurs at different rates, how fast is the overall reaction? Step 1: HBr + O2  HOOBr Step 2: HOOBr + HBr  2 HOBr Step 3: 2 HOBr+ 2 HBr 2 H2O + 2 Br2 slow fast fast

  9. Reaction Mechanisms 4 HBr + O2 2 H2O + 2 Br2 Potential energy diagram

  10. Rate Determining Step The slowest step of a mechanism is also known as the rate determining step (RDS).

  11. Catalysts Catalysts are known to lower EA. Sometimes the lower EA is also achieved because the catalyst uses a different reaction mechanism from the uncatalyzed reaction.

  12. Catalysts CH2O2(aq) CO(g) + H2O(l) uncatalyzed catalyzed

  13. RATE LAW FOR ELEMENTARY PROCESSES

  14. Rate Laws of Elementary Processes • ONLY FOR ELEMENTARY PROCESSES, the exponents of the rate law is the same as the coefficients of the balanced elementary process equation • if only given the overall equation, then rate experiments must be conducted

  15. Example #1 2 NO2Cl  2 NO2 + Cl2 • NO2Cl  NO2 + Cl • NO2Cl + Cl  NO2 + Cl2

  16. Example #2 2 NO2 2 NO + O2 • 2 NO2  NO3 + NO • NO3  NO + O2

  17. PREDICTING MECHANISMS

  18. Predicting Mechanisms • acceptable proposed mechanism must match experimental rate data • overall reaction rate is determined by the rate determining step

  19. Example #1b 2 NO2Cl  2 NO2 + Cl2 • NO2Cl  NO2 + Cl (slow) • NO2Cl + Cl  NO2 + Cl2 (fast)

  20. Example #2b 2 NO2 2 NO + O2 • 2 NO2  NO3 + NO (slow) • NO3  NO + O2 (fast)

  21. Predicting Mechanisms • proposed mechanisms that match experimental rate laws DO NOT PROVE the mechanism is correct • additional experimentation is required • other proposals may need to be examined

  22. Example #3 The reaction of ozone, O3, with nitric oxide, NO, forms nitrogen dioxide and oxygen gas. It is believed to be a one step mechanism to produce smog. Determine the rate law.

  23. Example #4 2 N2O5(g) 2 N2O4(g) + O2(g) • Determine the rate equation if this was a one step reaction. • Actual rate equation: rate = k[N2O5] How many steps are there in the reaction mechanism?

  24. Example #5 X + 2 Y + 2 Z  XY2Z2 • doubling [X] = no effect • doubling [Y] = 4 x rate • doubling [Z] = 2 x rate • What is the rate law? • Propose the reactants of the rate determining step?

More Related