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Reaction Kinetics (5)

Physical Chemistry. Reaction Kinetics (5). Xuan Cheng Xiamen University. Pyrolysis Acetaldehyde Methane Polymerization Monomer Initiator Relaxation. 高温分解 乙醛 甲烷 聚合 单体 引发剂 迟豫. Physical Chemistry. Reaction Kinetics. Key Words. Physical Chemistry. Reaction Kinetics.

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Reaction Kinetics (5)

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  1. Physical Chemistry Reaction Kinetics (5) Xuan Cheng Xiamen University

  2. Pyrolysis Acetaldehyde Methane Polymerization Monomer Initiator Relaxation 高温分解 乙醛 甲烷 聚合 单体 引发剂 迟豫 Physical Chemistry Reaction Kinetics Key Words

  3. Physical Chemistry Reaction Kinetics Rice-Herzfeld Mechanisms A chain reaction can lead to a simple rate law. Pyrolysis of acetaldehyde Simple rate laws can follow from quite complex chain mechanisms. The Rice-Herzfeld mechanism for the pyrolysis of acetaldehyde is (a) Initiation: (b) Propagation: (c) Retardation: (d) Termination:

  4. Physical Chemistry Reaction Kinetics Rice-Herzfeld Mechanisms The net rates of the formation of the two intermediates are The sum of the two equation is

  5. Physical Chemistry Reaction Kinetics Rice-Herzfeld Mechanisms The rate of formation of CH4 is in agreement with the three-halves order observed experimentally. However, the true mechanism is more complicated than R-H mechanism. Other products (acetone, CH3COCH3, and propanaldehyde, CH3CH2CHO) can be formed. Prob. 17.81

  6. Physical Chemistry Reaction Kinetics Free-Radical Polymerizations Chain polymerization Results in the rapid growth of an individual polymer chain for each activated monomer, and often occurs by a radical chain process. Let I and M stand for the initiator and monomer (a) Initiation (b) Propagation (c) Termination

  7. Physical Chemistry (17.99) Reaction Kinetics Free-Radical Polymerizations (a) Initiation (fast) The rate-determining step is the formation of the radicals R. (b) Propagation The chain of reactions propagates quickly, fis the yield of the initiation step, the fraction of radicals that R successfully initiate a chain.

  8. Physical Chemistry (17.101) Reaction Kinetics Free-Radical Polymerizations (c) Termination Assume that the rate of termination is independent of the length of the chain, the rate of change of radical concentration by this process is The total radical concentration is approximately constant throughout the main part of the polymerization. (the rate at which radicals are formed by initiation  the rate at which they are removed by termination)

  9. Physical Chemistry (17.102) (17.103) Reaction Kinetics Free-Radical Polymerizations Applying the steady-state approximation The steady-state concentration of radical chains The rate of propagation of the chains (the monomer is consumed) Central feature The rate of polymerization is proportional to the square root of the initiator concentration.

  10. Physical Chemistry (17.104) (17.105) (17.103) for termination by combination (17.104) Reaction Kinetics Free-Radical Polymerizations The degree of polymerization (DP) The number of monomers in the polymer

  11. Physical Chemistry Reaction Kinetics Fast Reactions Experimental methods for fast reactions Rapid-flow method Movable spectrometer Mixing chamber Pistons Disadvantage: A large volume of reactant solution The reactants are mixed as they flow together in a chamber. The reaction continues as the thoroughly mixed solutions flow through the outlet tube, and observation of the composition at different positions along the tube is equivalent to the observation of reactant mixture at different times after mixing.

  12. Physical Chemistry Reaction Kinetics Fast Reactions Experimental methods for fast reactions Stopped-flow method Movable spectrometer Mixing chamber Stopping syringe Pistons Disadvantage: A large volume of reactant solution Small samples The two solutions are mixed very rapidly by injecting them into a tangential mixing chamber. Beyond the mixing chamber there is an observation cell fitted with a stopping syringe, when a required volume (1 mL) has been injected. The reaction continues in the thoroughly mixed solution and is monitored.

  13. Physical Chemistry Reaction Kinetics Fast Reactions Experimental methods for fast reactions Flash photolysis method The gaseous or liquid sample is exposed to a brief photolytic flash of light and then the contents of the reaction chamber are monitored. Both emission and adsorption spectroscopy may be used to monitor the reaction, and the spectra are observed electrochemically or photographically at a series of times following the flash.

  14. Physical Chemistry [A] T2 Exponential relaxation T1 Time, t kb (17.107) Reaction Kinetics Fast Reactions Temperature-jump relaxation methods Relaxation The return of a system to equilibrium Temperature jump Consider the reversible reaction For all times after the T jump Equilibrium concentrations at T2 let

  15. Physical Chemistry [A] T2 Exponential relaxation T1 Time, t (17.109) (17.107) (17.110) Reaction Kinetics Fast Reactions Temperature-jump relaxation methods (17.108) At equilibrium The perturbation is small

  16. Physical Chemistry (17.110) Reaction Kinetics Fast Reactions Temperature-jump relaxation methods Where x is the departure from equilibrium at the new temperature and x0 is the departure from equilibrium immediately after the temperature jump. The concentration of A (and of B) relaxes into the new equilibrium at a rate determined by the sum of the two new rate constants.

  17. Physical Chemistry Reaction Kinetics Fast Reactions Analyzing a temperature-jump experiment The H2O(l) H+(aq) + OH-(aq) equilibrium relaxes in 37 s at 298K and pH7, pKw=14.01. Calculate the rate constants for the forward and reverse reactions. The equilibrium condition is

  18. Physical Chemistry Reaction Kinetics Fast Reactions Analyzing a temperature-jump experiment The H2O(l) H+(aq) + OH-(aq) equilibrium relaxes in 37 s at 298K and pH7, pKw=14.01. Calculate the rate constants for the forward and reverse reactions. K and Kw are dimensionless kf and kb are expressed in different units

  19. Physical Chemistry Reaction Kinetics Reactions in Liquid Solutions Solvent Effects on Rate Constants gas-phase reaction solvent liquid-phase reaction Ionic Reactions solvation gas-phase reaction solvent liquid-phase reaction ions Encounters, Collisions, and the Cage Effect gas-phase reaction Molecules are far apart and move freely between collisions liquid-phase reaction Little empty space between molecules and can’t move freely

  20. Physical Chemistry C C B B Cage effect for B and C Reaction Kinetics Reactions in Liquid Solutions Encounters, Collisions, and the Cage Effect Encounters A process in which B and C diffuse together to become neighbors Collisions Each encounter in solution involves many collisions between B and C

  21. Physical Chemistry C C B B Cage effect for B and C Reaction Kinetics Reactions in Liquid Solutions Diffusion-controlled Reactions Encounter pair Gas-phase More encounters, shorter time together Liquid-phase Less encounters but stay near each other for much longer than in a gas

  22. Physical Chemistry Reaction Kinetics Reactions in Liquid Solutions Diffusion-controlled Reactions Suppose the rate of formation of an encounter pair BC is The steady-state concentration of BC The overall rate law for the formation of products

  23. Physical Chemistry Reaction Kinetics Reactions in Liquid Solutions Diffusion-controlled Reactions The overall rate law for the formation of products (1) diffusion-controlled reaction If the rate of separation of the unreacted encounter pair is much slower than the rate at which it forms products The rate of reaction is governed by the rate at which the reactant particles diffuse through the medium.

  24. Physical Chemistry Reaction Kinetics Reactions in Liquid Solutions Diffusion-controlled Reactions The overall rate law for the formation of products (2) activation-controlled reaction If the rate of separation of the unreacted encounter pair is much faster than the rate at which it forms products The reaction proceeds at the rate at which energy accumulates in the encounter pair from the surrounding solvent.

  25. Physical Chemistry Reaction Kinetics Reactions in Liquid Solutions Diffusion-controlled Reactions The rate of a diffusion-controlled reaction is calculated by considering the rate at which the reactants diffuse together. where B C, nonionic (17.111) where B= C, nonionic (17.112) where B C, ionic (17.113)

  26. Physical Chemistry Reaction Kinetics Reactions in Liquid Solutions Diffusion-controlled Reactions When apply the Stokes-Einstein equation (16.37)  Is the solvent’s viscosity where B C, nonionic (17.111) where B C, nonionic if rB= rC where B C, nonionic (17.114) where B= C, nonionic (17.115)

  27. Physical Chemistry Reaction Kinetics Reactions in Liquid Solutions Activation Energies Gas-phase reactions: high temperature (up to 1500K) Gas-phase reactions: activation energy range -3100 kcal/mol Liquid-phase reactions: relatively lower temperature (up to 500K) Liquid-phase reactions: activation energy range 235 kcal/mol Home Work 17.67 17.70 17.77 17.83 17.87 17.89

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