物理化學(xué)英文課件：Reaction-Kinetics5

1、Reaction Kinetics (5) Xuan ChengXiamen UniversityPhysical Chemistry1Key WordsPyrolysisAcetaldehydeMethane PolymerizationMonomerInitiatorRelaxation高溫分解乙醛甲烷聚合單體引發(fā)劑遲豫Physical ChemistryReaction Kinetics2Physical ChemistryRice-Herzfeld MechanismsSimple rate laws can follow from quite complex chain mechan

2、isms.(a) Initiation:(b) Propagation:(c) Retardation:(d) Termination:The Rice-Herzfeld mechanism for the pyrolysis of acetaldehyde isA chain reaction can lead to a simple rate law.Pyrolysis of acetaldehydeReaction Kinetics3Physical ChemistryRice-Herzfeld MechanismsThe net rates of the formation of th

3、e two intermediates areThe sum of the two equation isReaction Kinetics4Physical ChemistryThe rate of formation of CH4 isRice-Herzfeld Mechanismsin agreement with the three-halves order observed experimentally.However, the true mechanism is more complicated than R-H mechanism.Other products (acetone,

4、 CH3COCH3, and propanaldehyde, CH3CH2CHO) can be formed.Prob. 17.81Reaction Kinetics5Free-Radical PolymerizationsPhysical ChemistryLet I and M stand for the initiator and monomerChain polymerizationResults in the rapid growth of an individual polymer chain for each activated monomer, and often occur

5、s by a radical chain process.(a) Initiation (b) Propagation (c) Termination Reaction Kinetics6Free-Radical PolymerizationsPhysical Chemistry(a) Initiation (b) Propagation (fast)The rate-determining step is the formation of the radicals R.The chain of reactions propagates quickly,f is the yield of th

6、e initiation step, the fraction of radicals that R successfully initiate a chain. (17.99)Reaction Kinetics7Free-Radical PolymerizationsPhysical Chemistry(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 p

7、rocess isThe 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)(17.101)Reaction Kinetics8Free-Radical PolymerizationsPhysical ChemistryApplying th

8、e steady-state approximationThe steady-state concentration of radical chainsThe rate of propagation of the chains (the monomer is consumed)(17.102)(17.103)The rate of polymerization is proportional to the square root of the initiator concentration.Central featureReaction Kinetics9Physical ChemistryF

9、ree-Radical PolymerizationsThe degree of polymerization (DP)The number of monomers in the polymer(17.104)(17.105)(17.103)for termination by combination(17.104)Reaction Kinetics10Physical ChemistryFast ReactionsExperimental methods for fast reactionsRapid-flow methodPistons Mixing chamber Movable spe

10、ctrometerThe 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

11、 after mixing.Disadvantage:A large volume of reactant solutionReaction Kinetics11Physical ChemistryFast ReactionsExperimental methods for fast reactionsStopped-flow methodPistons Mixing chamber Movable spectrometerThe two solutions are mixed very rapidly by injecting them into a tangential mixing ch

12、amber. 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.Reaction KineticsStopping syringeDisadvantage:A large volume of reactant solutionSma

13、ll samples12Physical ChemistryFast ReactionsExperimental methods for fast reactionsFlash photolysis methodThe 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

14、 monitor the reaction, and the spectra are observed electrochemically or photographically at a series of times following the flash.Reaction Kinetics13Physical ChemistryFast ReactionsTemperature-jump relaxation methodsRelaxation The return of a system to equilibriumTime, tT2T1Exponential relaxationAT

15、emperature jumpConsider the reversible reactionkbFor all times after the T jump(17.107)Equilibrium concentrations at T2letReaction Kinetics14Physical ChemistryFast ReactionsTemperature-jump relaxation methodsTime, tT2T1Exponential relaxationAAt equilibrium(17.107)The perturbation is small(17.108)(17

16、.109)(17.110)Reaction Kinetics15Physical ChemistryFast ReactionsTemperature-jump relaxation methods(17.110)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 t

17、he new equilibrium at a rate determined by the sum of the two new rate constants.Reaction Kinetics16Physical ChemistryFast ReactionsAnalyzing a temperature-jump experimentThe H2O(l) H+(aq) + OH-(aq) equilibrium relaxes in 37 s at 298K and pH7, pKw=14.01. Calculate the rate constants for the forward

18、and reverse reactions.The equilibrium condition isReaction Kinetics17Physical ChemistryFast ReactionsAnalyzing a temperature-jump experimentThe H2O(l) H+(aq) + OH-(aq) equilibrium relaxes in 37 s at 298K and pH7, pKw=14.01. Calculate the rate constants for the forward and reverse reactions.K and Kw

19、are dimensionlesskf and kb are expressed in different unitsReaction Kinetics18Physical ChemistryReactions in Liquid SolutionsSolvent Effects on Rate Constantsgas-phase reactionliquid-phase reactionsolventIonic Reactionsgas-phase reactionliquid-phase reactionsolventionssolvationEncounters, Collisions

20、, and the Cage Effectgas-phase reactionliquid-phase reactionMolecules are far apart and move freely between collisionsLittle empty space between molecules and cant move freelyReaction Kinetics19Physical ChemistryReactions in Liquid SolutionsEncounters, Collisions, and the Cage EffectEncountersCollis

21、ions A process in which B and C diffuse together to become neighborsEach encounter in solution involves many collisions between B and CBCBCCage effect for B and CReaction Kinetics20Physical ChemistryReactions in Liquid SolutionsDiffusion-controlled ReactionsGas-phaseLiquid-phaseMore encounters, shor

22、ter time togetherLess encounters but stay near each other for much longer than in a gasBCBCCage effect for B and CEncounter pairReaction Kinetics21Physical ChemistryReactions in Liquid SolutionsDiffusion-controlled ReactionsSuppose the rate of formation of an encounter pair BC isThe steady-state con

23、centration of BCThe overall rate law for the formation of products Reaction Kinetics22Physical ChemistryReactions in Liquid SolutionsDiffusion-controlled ReactionsThe overall rate law for the formation of products If the rate of separation of the unreacted encounter pair is much slower than the rate

24、 at which it forms products(1) diffusion-controlled reactionThe rate of reaction is governed by the rate at which the reactant particles diffuse through the medium.Reaction Kinetics23Physical ChemistryReactions in Liquid SolutionsDiffusion-controlled ReactionsThe overall rate law for the formation o

25、f products If the rate of separation of the unreacted encounter pair is much faster than the rate at which it forms products(2) activation-controlled reactionThe reaction proceeds at the rate at which energy accumulates in the encounter pair from the surrounding solvent.Reaction Kinetics24Physical ChemistryReactions in Liquid SolutionsDiffusion-controlled ReactionsThe rate of a diffu