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Reaction Mechanisms

Reaction Mechanisms. Steps of Reactions using Rate Laws. What is a mechanism?. A reaction mechanism is a description of the path, or sequence of steps , by which a reaction occurs. . START. END. REACTION MECHANISM. START. END. Example: Decomposition of O 3.

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Reaction Mechanisms

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  1. Reaction Mechanisms Steps of Reactions using Rate Laws

  2. What is a mechanism? • A reaction mechanism is a description of the path, or sequence of steps, by which a reaction occurs. START END

  3. REACTION MECHANISM START END

  4. Example: Decomposition of O3 • ALL reactions = series of elementary steps whose sum is the overall balanced reaction. • The oxygen atom, O, is called an intermediate. • Intermediate = is both created and destroyed in the mechanism, does not appear in the net equation.

  5. Elementary Steps UNIMOLECULAR • Can use molecularity to write rate law • Unimolecular, Bimolecular, Termolecular BIMOLECULAR

  6. Reaction Mechanisms Must satisfy two requirements: 1. sum of elementary = balanced eqn 2. mechanism = experimentally determined rate law To calculate rate law from a mechanism you need to first know the rate limiting step. rate limiting step

  7. Rate Limiting Step • The rate limiting step determines the rate of the reaction because it is the slowest step. • You can rationalize that a reaction can only go so fast as its slowest step

  8. Calculating Rate Law for Elementary Steps • Rate of an elementary step = k [A] m • wherem is the stoichiometric power from eqn • The rate law for the first elementary step in the is ….. rate = k [O3].

  9. Calculating Rate Law for Elementary Steps • The rate law for the second elementary (bimolecular) step in the is ….. rate = k [O][O3]

  10. What is the overall rate law? • THE SLOWEST step determines the rate law. rate = k [O3]

  11. How do you determine the rate determining step? • 1. Determine the rate law. • 2. Using chemical intuition and requirements, devise potential mechanisms. • Mechanisms can never be proven absolutely but could be possibly correct!

  12. Sample Exercise 14.14 p. 586 This example has a slow elementary step first! YAY! Easy! Sample Exercise 14.15 p. 588 This example has a fast elementary step first! YAY! Substitute!

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