2020高中化學化學反應機理和催化劑

1、第4節(jié)反應機理、基本概念所謂基元反應是指反應物分子一步直接轉化為產(chǎn)物的反應。如：N2+co=NO+co2反應物NO2分子和CO分子經(jīng)過一次碰撞就轉變成為產(chǎn)物NO分子和CO20基元反應是動力學研究中的最簡單的反應，反應過程中沒有任何中間產(chǎn)物。Elementaryreactionsarestepsofmoleculareventsshowinghowreactionsproceed.Thistypeofdescriptionisamechanism.ThemechanismforthereactionbetweenCOandNO2isproposedtobeStep1NO2+NO2tNO3+NO(

2、anelementaryreaction)Step2NO3+COtNO2+CO2(anelementaryreaction)AddthesetwoequationsledtotheoverallreactionNO2+CO=NO+CO2(overallreaction)Amechanismisaproposaltoexplaintheratelaw,andithastosatisfytheratelaw.Asatisfactoryexplanationisnotaproof.例如：H2(g)+l2(g)=2HI(g)實驗上或理論上都證明，它并不是一步完成的基元反應，它的反應歷程可能是如下兩步基

3、元反應：I2=I+I(快)H2+21=2HI(慢)化學反應的速率由反應速率慢的基元反應決定?；磻驈碗s反應的基元步驟中發(fā)生反應所需要的微粒(分子、原子、離子)的數(shù)目一般稱為反應的分子數(shù)。分子數(shù)MolecularityofElementaryReactionsThetotalorderofratelawinanelementaryreactionismolecularity.Theratelawofelementaryreactionisderivedfromtheequation.Theorderisthenumberofreactingmoleculesbecausetheymustco

4、llidetoreact.Amoleculedecomposesbyitselfisaunimolecularreaction(step);twomoleculescollideandreactisabimolecularreaction(step);&threemoleculescollideandreactisatermolecularreaction(step).3tO2+0rate=kO3NO2+NO2tNO3+NOrate=kNOJ2B+B+AtBr2+Ar*rate=kBr2ArCaution:Deriveratelawsthiswayonlyforelementayreactio

5、ns.單分子反應SO2C12的分解反應SO2C12=SO2+Cl2雙分子反應NO2的分解反應2NO2=2NO+O2三分子反應HI的生成反應H2+21=2HI分子或更多分子碰撞而發(fā)生的反應尚未發(fā)現(xiàn)。ElementaryReactionsareMolecularEventsN2O5二N02+N03NO+O2+NO2JNO2+NO3Amechanismisacollectionofelementarystepsdevisetoexplainthethereactioninviewoftheobservedratelaw.Youneedtheskilltoderivearatelawfromamech

6、anism,butproposingamechanismistaskafteryouhavelearnedmorechemistryForthereaction,2NO2(g)+F2(g)t2NO2F(g),theratelawis,rate=kNOF.Cantheelementaryreactionbethesameastheoverallreaction?Iftheywerethesametheratelawwouldhavebeenrate=kNOJ2FJ,Therefore,theytheoverallreactionisnotanelementaryreaction.Itsmecha

7、nismisproposednext.反應機理中的慢反應步驟決定總反應的速率!Theratedeterminingstepistheslowestelementarystepinamechanism,andtheratelawforthisstepistheratelawfortheoverallreaction.The(determined)ratelawis,rate=kNOF，forthereaction,2NO2(g)+F2(g)-2NO2F(g),andatwo-stepmechanismisproposed:NO2(g)+F2(g)-NO2F(g)+F(g)NO2(g)+F(g)t

8、NO2F(g)Whichistheratedeterminingstep?AnswerTherateforstepiisrate=kNO2F,whichistheratelaw,thissuggeststhatstepiistherate-determiningorthes-l-o-wstep.二、如何由給出的反應機理推導出速率方程例1、ThedecompositionofH2O2inthepresenceofkfollowthismechanism,H2O2+卜一時一H20+IO-slowH2O2+IO一k2-H2O+O2+|-fastWhatistheratelaw?SolutionThe

9、slowstepdeterminestherate,andtheratelawis:rate二H2O2I-SincebothH2O2andI一aremeasurableinthesystem,thisistheratelaw.快速平衡假設法！例2、Derivetheratelawforthereaction,H2+Br2=2HBr,fromtheproposedmechanism:Bq二2BrH2+Br一k2-HBr+HH+Brk3HBrfastequilibrium（k“k.JslowexplainfastSolution:Thefastequilibriumconditionsimplys

10、aysthatBr2=心BrandBr=（可心回）Theslowstepdeterminestheratelaw,rate=k2HJBr=k2H2=kHJBr2滄totalorder1.5Brisanintermediatek=k2lkM-s-1例3、ThedecompositionofN2O5followsthemechanism:N2O5二NO2+NO3fastequilibriumNO2+NO3您NO+O2+NO2slowNO3+NOfNO2+NO2fast/Derivetheratelaw./Solution:NO2&NO3areintermediateK,equilibriumcon

11、stantKdifferfromkTheslowstepdeterminestherate,rate=k2NO2NO3From1,wehaveNO2NO3/=K/N2O5Thus,rate=Kk2N2O5穩(wěn)態(tài)近似法！Intermediate以假設中間產(chǎn)物的濃度恒定不變?yōu)榛A！即、中間產(chǎn)物的生成速率與其消耗速率相等。RRzprod八consR二R/Aprod一八consRVR八prod八consRateofproducingtheintermediate,Rprod,isthesameasitsrateofconsumption,Rcons.Beabletoapplythesteadystate

12、approximationtoderiveratelaws假設H?+I2t2HI的反應機理如下：Step(1)I22IStep(1)2Ik、I?Step(2)H2+21S2HIDerivetheratelaw.Derivation:rate=k2H2I2(causethisstepgivesproducts!)butIisanintermediate,thisisnotaratelawyet.SincekxI2(=rateofproducing=kI2+k2HJI2(=rateofconsumingSteadyStateThus,k、I9I2=+*2H?/.rate=kk2H2I2/kA+k2H2Fromthepreviousresult:&k2H2l2rate=仮1+k2H2Discussion.If心k2H2then旳+k2H2=k2