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Catalyst

Catalyst. Diet Coke and Mentos. Justify – TPS. Why do smooth coated Mentos have a much slower reaction than rough coated Mentos?. Lecture 6.6 – Molecularity, Mechanisms, Rate Limiting Steps, and Catalysts. Today’s Learning Targets.

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Catalyst

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  1. Catalyst

  2. Diet Coke and Mentos

  3. Justify – TPS • Why do smooth coated Mentos have a much slower reaction than rough coated Mentos?

  4. Lecture 6.6 – Molecularity, Mechanisms, Rate Limiting Steps, and Catalysts

  5. Today’s Learning Targets • LT 6.16 – I can hypothesize about the moleculariry of a elementary reaction. • LT 6.17 – I can determine the rate law for a multistep reaction and discuss the relationship between the fast and slow steps for the reaction. • LT 6.18 – I can compare and contrast homogeneous and heterogeneous catalysts. Furthermore, I can identify a catalyst for a multi-step reaction.

  6. Reaction Mechanisms • The steps that a reaction takes in order to occur is the reaction mechanism • These are the steps of bonds breaking, atoms rearranging, and new bonds forming

  7. Elementary Reactions • Reactions that occur in a single step are elementary reactions • The number of molecules that participate in elementary reactions is known as the molecularityof the reaction. • Unimolecular – Single molecule involved • Bimolecular – Collision of two molecules required • Termolecular – Collision of three molecules required • Molecularity is never higher than 3 because the odds of 4 molecules colliding all at the same time and in the correct orientation is highly unlikely.

  8. Multistep Mechanisms • Most chemical reactions require multiple steps in order to occur • For example the reaction: • NO2 + CO  NO + CO2 • Requires 2 steps in order to occur: • NO2 + NO2 NO3 + NO • NO3 + CO  NO2 + CO2 • They must create an intermediate, those molecules that are produces and then consumed before reaction completion, before producing the final product • Insert Figure 14.20

  9. Rate Laws and elementary Reactions • Every reaction is made up of one or more elementary reaction • If the reaction is elementary, then its rate law is based on molecularity • For example, if the reaction is elementary and it is of the form: • A  Products • Then the rate law is simply: • Rate = k [A]

  10. The Big Football Game! Super Bowl This Way!

  11. Rate Limiting Steps • The majority of reactions require 2 or more elementary reactions. • Each step has its own rate constant and activation energy • The overall rate of the reaction can not exceed the rate of the slowest elementary reaction. • The slow step is the rate – determining step • The rate – limiting step greatly inhibits mechanism and rate law.

  12. Mechanisms with Slow Initial Step • For the multistep reaction previously discussed: • The first step is the rate limiting step, so k2 >> k1 • Therefore, we can only use the rate constant and reaction from the slow first step. Therefore, the rete law is:

  13. Table Talk • The decomposition of nitrous oxide, N2O, is believed to occur by a two step mechanism. What is the rate law using the mechanism below?

  14. Mechanisms with Fast Initial Step • Sometimes the first step is not the slow step. • Consider the following reaction: • NO + NO + Br2 2 NOBr • This occurs through two steps where the second step is the slow step k-1

  15. Table Talk • You run the following reaction: • NO + O2 NO2 • Based on the data the following mechanism is proposed. Determine the rate law using the mechanism below. k-1

  16. Catalysts • A catalyst is a substance that speeds up a chemical reaction by lowering the activation energy • It provides an alterative pathway by which the reaction can occur • ΔG is not impacted • A homogeneous catalyst is a catalyst whose phase is the same as the reactants • A heterogeneous catalyst is a catalyst whose phase is different from that of the reactants.

  17. White Board Problems

  18. White Board Problems • Complete the following table: • Determine the rate law for the following mechanism:

  19. White Board Problems • 3. What is the intermediate for this mechanism? • 4. For the Diet Coke and Mentos experiment, are the Mentos a heterogeneous or homogeneous catalyst? • 5. Based on the following diagram, how many intermediates are formed when a catalyst is used? Is it exothermic or endothermic?

  20. Rate Yourself • Using your learning target log, rate yourself 1 – 4 on 6.1 and 6.18

  21. Stations Review • Station 1 – • Station 2 – • Station 3 – • Station 4 – • Station 6 – • Station 7 – • Station 8 – • Station 9 – • Station 10 –

  22. Rate Yourself • Using your learning target log, rate yourself 1 – 4 on 6.1 and 6.18

  23. Closing Time • Read 14.6 and 14.7 • Homework: Reactions Mechanisms and Catalysts due Monday/Tuesday • Saturday School 11 – 2 this Saturday! • Unit 6 Exam next Monday/Tuesday!

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