Lecture 22 polymerization reactions and photochemistry reactions

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1、Lecture22Thekineticsofcomplexreactions1Part1ChainreactionsandPolymerizationkinetics2TheratelawsofchainreactionsCH3CHO(g)CH4(g)+CO(g)overallreactionrateequationTheRice–Herzfeldmechanismforthisreactionisasfollows(thedotsignifiesanunpairedelectronandmarksaradical):

2、Initiation:CH3CHO·CH3+·CHOPropagation:CH3CHO+·CH3CH3CO·+CH4Propagation:CH3CO··CH3+COTermination:·CH3+·CH3CH3CH33Totesttheproposedmechanismweneedtoshowthatitleadstotheobservedratelaw.Accordingtothesteady-stateapproximation，thenetrateofchangeoftheintermediates(·CH

3、3andCH3CO·)maybesetequaltozero:Thesumofthetwoequationsis:Thesteady-stateconcentrationof·CH3radicalsis4ItfollowsthattherateofformationofCH4iswhichisinagreementwiththethree-halvesorderobservedexperimentallyInmanycases,achainreactionleadstoacomplicatedratelaw.Anexa

4、mpleisthehydrogen–brominereaction:5Thefollowingmechanismhasbeenproposedtoaccountforthisratelaw:Notehowthereactantsandproductsareshownasarmstothecircle,buttheintermediates(HandBr)occuronlyonthecircle.AschematicrepresentationThenetrateofformationoftheproductHBroft

5、hemechanismofthereactionbetweenhydrogenis:andbromine.6Example：Derivingtherateequationofachainreaction:Answer:Thenetratesofformationofthetwointermediatesare:7TheratelawshowsthatthereactionslowsdownasHBrforms,orasthe[HBr]/[Br2]ratioincreases.Figureshowsthatthenume

6、ricalintegrationoftheHBrratelawcanbeusedtoexplorehowtheconcentrationofHBrchangeswithtimeNumericalintegrationoftheratelawwithmathematicalsoftwareshowsthepredictedtimedependenceoftheconcentrationofHBrforthismechanism.8Self-testwhentheinitiationstepisphotolysisIabs

7、istheintensityofabsorbedradiation9Explosionsthermalexplosionchain-branchingexplosionAnexampleofbothtypesofexplosionisthereactionbetweenhydrogenandoxygen:Abranchingstepisanelementaryreactionthatproducesmorethanonechaincarrier.Forexample:Oatom:[He]2s22p4,hastwounp

8、airedelectrons.O2molecule::,with12valenceelectrons.10ThetemperatureandpressureofthesystemTheoccurrenceTheexplosionregionsforthereactionofanexplosionTheconditionsunder