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Reaction Mechanisms

Reaction Mechanisms. Rxn Mechanism. The series of steps that chemical reactions occur by main points of kinetics is to learn these steps. NO 2(g) +CO (g) -->NO (g) +CO 2(g). Rate = k[NO 2 ] 2 Tells us about reactants and products and stoichiometry, but nothing about mechanism.

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Reaction Mechanisms

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  1. Reaction Mechanisms

  2. Rxn Mechanism • The series of steps that chemical reactions occur by • main points of kinetics is to learn these steps

  3. NO2(g)+CO(g)-->NO(g)+CO2(g) • Rate = k[NO2]2 • Tells us about reactants and products and stoichiometry, but nothing about mechanism

  4. NO2(g)+CO(g)-->NO(g)+CO2(g) • Mechanism thought to be: • NO2(g)+NO2(g)-->NO3(g)+ NO(g) • NO3(g)+CO(g)-->NO2(g)+CO2(g) • Two new rate constants k1, k2

  5. NO3(g) ??? • INTERMEDIATE- species that is neither a reactant or a product, but is formed and used during the rxn sequence

  6. Elementary Steps • Reactions whose rate law can be written from its molecularity

  7. Molecularity • Number of species that must collide to produce the reaction indicated by that step

  8. Unimolecular • Rxn involving one molecule • A-->prod • Rate = k[A]

  9. Bimolecular • Involving two molecules • A + A --> prod • A + B --> prod • Rate = k[A]2 • Rate = k[A][B]

  10. Termolecular • Involving 3 molecules • rare- hard for 3 to collide simultaneously • A + B + C --> prod • Rate = k[A][B][C]

  11. Rxn mech- precisely • 1. The sum of elementary steps must give overall balanced eqn for rxn.

  12. Rxn mech- precisely • 2. The mechanism must agree with the experimentally determined rate law

  13. Question • Does our mechanism balance? • YES

  14. Rate-determining step • The SLOW step in a rxn • Reactants can only become products as fast as their slow step will allow

  15. Back to example • Assume that the first rxn is the SLOW step • Formation of NO3 occurs more slowly than its reaction with CO

  16. Therefore... • The rate of CO2 production is controlled by the rate of formation of NO3

  17. Since... • It is an elementary step, we can write the rate law from the molecularity • Rate = k[NO2]2

  18. And look! • The overall rate can be no faster than the slowest step • Overall rate = k[NO2]2 !

  19. Finding mechanism • Always find rate law first • Then use chemical intuition and try to make mechanism

  20. But Remember • Mechanism can NEVER be proved absolutely

  21. 2NO2(g) + F2(g)  2NO2F(g) Rate Law = k[NO2][F2] Mechanism NO2 + F2 NO2F + F slow F + NO2  NO2F fast

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