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Chapter 13

Chapter 13. Rates of Reaction. Reaction Rates. average rate the rate over a specific time interval instantaneous rate the rate for an infinitely small interval. Integrated Rate Laws. A ---> products rate = - ( D [A]/ D t) = k[A] m average rate rate = - (d[A]/dt) = k[A] m

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Chapter 13

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  1. Chapter 13 Rates of Reaction Dr. S. M. Condren

  2. Reaction Rates average rate • the rate over a specific time interval instantaneous rate • the rate for an infinitely small interval Dr. S. M. Condren

  3. Integrated Rate Laws A ---> products rate = - (D[A]/Dt) = k[A]m average rate rate = - (d[A]/dt) = k[A]m instantaneous rate Dr. S. M. Condren

  4. Average vs. Instantaneous Rate Dr. S. M. Condren

  5. For the reaction: The dependence of the concentration of H2 on time is shown below. Is the reaction rate faster at point A or point B? A, B, both rates are the same Dr. S. M. Condren

  6. Rate Determining Step • slowest step in a multistep mechanism • the step which determines the overall rate of the reaction Dr. S. M. Condren

  7. Rate Law • an expression which relates the rate to the concentrations and a specfic rate constant Dr. S. M. Condren

  8. Rate Law Reaction rate = k [A]m [B]n where m => order with regard to A n => order with regard to B overall order = m + n Dr. S. M. Condren

  9. The empirical rate law for the reaction is Rate = k[NO2][F2]. Which of the following mechanisms is consistent with this rate law? A) NO2 (g) + F2 (g) <=> NO2F(g) + F(g) fast NO2(g) + F(g) --->NO2F(g) slow B) NO2 (g) + F2(g) <==> NO2F(g) + F(g) slow NO2(g) + F(g) ---> NO2F(g) fast C) F2(g) <==> F(g) + F(g) slow 2NO2(g) + 2F(g) ---> 2NO2F(g) fast A, B, C Dr. S. M. Condren

  10. Order of Reaction • exponent of the concentration for a reactant that implies the number of molecules of that species involved in the rate determining step • first order, exponent equals one • second order, exponent equals two Dr. S. M. Condren

  11. Order of Reaction Dr. S. M. Condren

  12. Dr. S. M. Condren

  13. Graphical Determination of Order of Reaction This plot of ln[H2O2] vs. time produces a straight line, suggesting that the reaction is first order. Dr. S. M. Condren

  14. A Nanoscale View:Elementary Reactions • unimolecular - rearrangement of a molecule • bimolecular - reaction involving the collision of two particles • termolecular - reaction involving the collision of three particles Dr. S. M. Condren

  15. Unimolecular Reaction Dr. S. M. Condren

  16. Bimolecular Reaction Dr. S. M. Condren

  17. Collision Rate Model Three conditions must be met at the nanoscale level if a reaction is to occur: • the molecules must collide; • they must be positioned so that the reacting groups are together in a transition state between reactants and products; • and the collision must have enough energy to form the transition state and convert it into products. Dr. S. M. Condren

  18. Transition State: Activated Complex or Reaction Intermediates • an unstable arrangement of atoms that has the highest energy reached during the rearrangement of the reactant atoms to give products of a reaction Dr. S. M. Condren

  19. Activation Energy the minimum energy required to start a reaction Dr. S. M. Condren

  20. Temperature & Reaction Rate Dr. S. M. Condren

  21. Catalyst • susbstance which speeds up the rate of a reaction while not being consumed Homogeneous Catalysis - a catalyst which is in the same phase as the reactants Heterogeneous Catalysis - a catalyst which is in the different phase as the reactants catalytic converter • solid catalyst working on gaseous materials Dr. S. M. Condren

  22. Which curve illustrates the effect of a catalyst on the reaction diagram, given that it speeds up the rate of a reaction? A, B, C, D Dr. S. M. Condren

  23. Reaction Mechanism • A set of elementary reactions which represent the overall reaction Dr. S. M. Condren

  24. Catalytic Converter catalyst H2O(g) + HCs -------> CO(g) + H 2(g) (unbalanced) catalyst 2 H2(g) + 2 NO(g) ------> N2(g) + 2 H2O(g) catalyst CO(g) + O2(g) --------> CO2(g) (unbalanced) catalyst = Pt-NiO HCs = unburned hydrocarbons Dr. S. M. Condren

  25. Chain Mechanisms chain initiating step - the step of a mechanism which starts the chain chain propagating step(s) - the step or steps which keeps the chain going chain terminating step(s) - the step or steps which break the chain Dr. S. M. Condren

  26. Chain Mechanisms combustion of gasoline in an internal combustion engine chain initiating step - additives which generate free radicals, particles with unpaired electrons chain propagating step(s) - steps which generate new free radicals chain terminating step(s) - steps which do not generate new free radicals Dr. S. M. Condren

  27. Enzymes any one of many specialized organic substances, composed of polymers of amino acids, that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms. Dr. S. M. Condren

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