級(jí)物理化學(xué)4、參考已雙語(yǔ)chapter2plex reactions

plex2.1Elementaryreactions2.2Reversiblereactions2.3Parallelreactions2.4ConsecutivereactionsMai?Bac plex2.5Steady-stateapproximation2.6Rate-determiningstepandPre-2.7Activationenergyfor 2.8SpeculationofreactionMai?BacElementary

ReactionorderReactionmechanismMai?BacAnAnelementaryreactionreferstoasingle-stepreactioninvolvingasmallnumberofmoleculesorions.Forexamples：Cl2M2ClClH2HClHHCl2HClCl2ClMCl2MAnAnoverallreactionisareactioncombinedwithaseriesofelementaryreactionsForexamples：H2 Mai?BacThemolecularityofanelementaryreactionisthenumberofmoleculescomingtogethertoreactinanelementaryreaction.Elementary UnimolecularAUnimolecularABP2ABP

BimolecularreactionTrimolecularreactionMai?BacLawLawofMassForanelementaryreaction,therateisproportionaltotheconcentrationofofthereactantmoleculesmeettogetherCl2M2ClClH2HClHCl2HCl Mai Mai ?Bac

ratek2[Cl][H2ReactionForanoverallreaction,thecombinationofaseriesofelementary-reactionstepsisthereactionmechanism.Mai?BacReversible(Opposing k-A

BConsecutive ABMai?BacMorecommonsituationisthattheforwardandthebackreactionsmaynotbeelementaryreactions,andmaybefirst-,second-orthird-orderreactionsFor A+ B+Mai?BacReversibleFirst-OrderReactions:DifferentialAt

k-

d[A]k1[A]k-t=tt=te

[A]0–[A]0–

[B]0+[B]0+

k1/k-1[B]=[A]e+[A]ek1/k-Differentiald[A](k1k-1)([A][A]e)Mai?BacReversibleFirst-OrderReactions:Integrated∫∫

=－(k1+k-0=(k+k)10→[A]0[A]e,[A]→[A][A]e,k→k1+k-Mai?Back1k-1。在平衡態(tài)產(chǎn)生一個(gè)微擾使反=1/(k1+k-間的方法求k1k-1，再通過(guò)平衡關(guān)Mai?BacMai?BacRelaxationTimefor1st-orderReversiblek krIntegratedrate

ln[A]0[A]e

k[A]e[A][A]e=([A]0[A]e)exp[(kf+kr)Assumee0，eexp{(kf+kr)t}=expR=1/(kf+Mai?BacRelaxationTimefor2nd-orderReversible A+B

C+Assume:eeCCe DDed[A]k[A][B]k dk([A])([B])k([C])([D] kfee=krCe={kf(ee)+kr(CeDe)}(kfkr)Mai?BacRelaxationTimefor2nd-orderReversibled{k([A][B])k([C][D]

RkfR

[D]eexp{[kf(ee)+kr(CeDe)]Mai?BacRelaxationTimeforSomeTypicalReversible fA f

+22A 2

4kf[A]e+krk B+k

k+k(BC rkA+ kr

kf(ee)+kA+kr

C+

kf(ee)+kr(CeMai?Bac率常數(shù)之比等于平衡常數(shù)K=[B]e/[A]e=kf/kbMai?BacTheparallelreactionistheoccurrenceofdifferentreactionsfromthesamereactant(s).Itisusuallyseeninorganicchemistry.Thedesiredproductisthemainproductandtheothersareby-products.TheTheoverallconversionrateisthesumofallwhichmaybethesame-orderreactionsormaybeMai?BacTwoParallelFirst-orderReactions:DifferentialC－－d[A]=(k+k)12d[B]=k d[C]=k Mai?BacTwoParallelFirst-orderReactions:IntegratedAssumeatt=0，[A]=[A]0,[B]0=[C]0=0,[A]=[A]0e-[B]

k1+

[B]/[C]=[B]/[C]=[C] (1–e-k1+Mai?BacA

—d[A]=(k+k+k)[A]n=

k3 k3D

（1）kapp=k1+k2+k3=E1-（3）[B]/[C]=k1/k2=exp(﹣ Mai?BacA+

k1,

G+G+

Bothreactionsarefirst-orderwithrespecttobothAandk2,If[C]0=[D]0=0，givetheexpressionfor[C]/[D]withTandk2,Answer：[C]/[D]=k1/k2=exp[(E2-TherearetwotypicalroutestoincreasetheselectivitytoToincreaseE2-E1byusingasuitableIfE2>E1，todecreasetheT,selectivitywillincrease,buttheratewilldecrease.IfE1>E2，toincreasetheT，boththeselectivityratewillMai?BacTheproductofareactionmayactasareactantofanotherreaction.Manyreactionsproceedasthis,i.e.,throughtheformationofanintermediate.Forexample:239UNpThetreatmentoftheconsecutivereactionsisrathercomplex.Hereweonlyconsidertwoconsecutiveirreversiblefirst-orderreactions.Mai?Bac A→B→—d[A]=k d[B]=k[A]﹣ d[C]=k Assumeatt=0，[A]=[A]0,[B]0=[C]0[A]0=[A]+[B]+ d[A]/dt+d[B]/dt+d[C]/dt=OnlytwoequationsareMai?Bac[A]=[A]0[A]=[A]0e-SubstitutethisintoEq.(2),wed[B]=k[A]e-

[B]k1[A]o(e[B]k1[A]o(ek1tek2tk2Using[C]=[A]0-[A]-[C][C][A]ok2ek1tk2ek2tMai?BacCharacteristicsofConsecutiveReactions：c～tThe[B]risestoa umatacertaint,while[A]isafirst-orderdecaywithtandthatofCrisesfrom0towardMai?Bac[B]k1[A]o(ek1tek2tk2 umshouldoccur d[B]d[B]

(kektkekt)12 122 k22 Mai ?B Mai ?Bac tlnk2lnk211

ke

kek2t

k1[A]o ek2tmax k lnk2ln

k2

[B]/[A]0onlydependsonk/kok2Foragivenvalueofk1,ask2increases,boththetimeatwhich[B]isa umandtheyieldofBdecrease.Mai?BacThereisaconsiderableincreaseinmathematicalcomplexityassoonasthereactionmechanismhasmorethanacoupleofsteps.Areactionschemeinvolvingmanystepsisnearlyalwaysunsolvable ytically.Awidelyusedalternativeapproachistomakeanapproximation.?Thesteady-stateapproximation穩(wěn)態(tài)近似)isoneofthemostimportantapproachesinchemicalkinetics.Mai?BacTheBasisoftheSteady-State A→B→Atk2>>k1，theconcentrationofBissmallandthechangeofitsconcentrationisnegligible.d[B]/dtd[B]=k[A]﹣k[B]≈0 —d[A]

=k1

Mai?BacWhentoUseSteady-StateThebasicrequirementfortheSSapproximationWhenWhenactiveintermediatessuchasatoms,radicalsandexcitedmoleculesappearinasequenceofreactions,theSSapproximationcanbeapplied.Mai?BacHowtoUseSteady-StateFirstidentifytheactiveintermediates,andwriteexpressionsfortheirnetratesofformation.Thensettheratesequaltozero.d[Xd[Xi=formationrateofX－consumingrateofX≈iiTheSSapproximationiswidelyusedinchainreactions,enzyme-catalyzedreactionsandphotochemicalMai?BacInasequenceofconsecutivereactions,ifonestepproceedssoslowtocontroltheoverallreactionrate,thisstepiscalledtherate-determiningsteprdsExample:2NO2+F2→Ratelawfrom1d[NO2

AssumedreactionNO+ NOF+

Slow 2 NO+ NO

r=Mai?Bac

A+

Ifk2<<k1,I→Pistherate-determiningFurtherifk-1>>k2,A+B→Iwillarriveataequilibrium.Wethuscanconsiderapre-equilibrium.K rd[P]k[I]kK[A][B] kappk2Kk2k1/k-Acomplicated esveryMai?BacHowHowtodevisetheratelawfromacomplexmechanismandtomakediscussionstosimplifythecomplicatedratelawExample:2O3→

Usingsteady-stateapproximationfor3 3

O2+

d[O]

O+O3→

[O]sskk

[O3r1d[O2

1k[O] [O][O]2k[O][O

12k[O][O]k[O][O

kk[O

k2[O][O3] Mai?BacDiscussionwithaComplicatedRater k1k2[O3(1)As[O3]issolowthatk-1[O2]>>k2[O3]isrk1k2[O3k[O[O]2[O322kapp=k1k2/k-1=(2)As[O3]issohighthatk-1[O2]<<k2[O3]isr=Mai?BacActivationEnergiesofElementaryandComposite芍芍Activationenergyofelementary芍Evaluationofactivationenergyfrombonding芍芍ActivationenergyofcompositeMai?BacTolman用統(tǒng)計(jì)平均的概念對(duì)基元反應(yīng)的活化能 aMai?BacA+ C+Ea﹣Ea’=Mai?BacEvaluationofActivationEnergyfromBondpositionofamoleculetotwoCl2+M→2Cl+Ea≈H=DCl-

H=DCl-Combinationofradicalstoamolecule，2Cl+M→Cl2+MEa≈ThereactionofaradicalwithaA+BC→A…B…C→AB+Forexothermicdirection：Ea=DBCForendothermicdirection：Ea’=EaHMai?BacEvaluationofActivationEnergyfromBondReactionsbetweentwomoleculesA-B+C-D→A-C+B-DForexothermicdirection：Ea=(DA-B+DC-D)×30%Forendothermicdirection：Ea’=EaH(H<0)Theaboverulesareonlyapplicabletosimplegas-phasereactionsandcannotbeusedforthereactionsinsolutions.However,theyareusefulforsimpleandrapidestimation.Mai?Bac22IH2HIH已知鍵能DHI297kJmol-1DH=435kJmol-1試估算該反應(yīng)正逆方向的活正方向的熱效應(yīng)： =- )=138kJmol-1>0,吸 其活化能Ea0.05DHI14.9kJmol-1Ea=Ea’+△rHm=152.9kJmol-Mai?BacForacompositereactionwhichisacombinationofseveralelementarysteps,theapparentratecoefficient(kapp)maybeamathematicalcombinationoftherateconstantsoftheelementaryreactions.Here,theactivationenergy(Ea)isalsotheapparentactivationenergy.Wecanevaluatetheapparentactivationenergyfromthoseoftheelementaryreactions,whichmaybesimplyestimatedfrombondenergies.Mai?Bacikobs=∏ki

Ea=RT2 =

ConsiderthefollowingmechanismforA+B→A+B k

AssumeaPre-equilibrium21 =k2[D]=(k2k1/k-21D2

kobs=kkk-Ea=E2+E1﹣E-Mai?Backobs=ERT

RT2(1

)RT

1(

dk2

k

1)k2 2)k3 3)

k1Ea1k2Ea2k3Ea3kikki

ForparallelA

d[A]CC

kk)[A]n

kobs=k1+k2+Mai?BacHowtoProposeaReaction Mai?BacHowtoProposeaReactionMai?BacFora ExperimentalRatelaw,rkJmolCH3andC2H5radicalshavebeenMai?BacToproposeaC2H62CH Ea351.5kJCH3C2H6CH4C2H

Ea33.5kJC2H5C2H4HC2H6C2H5H HC2H5C2H

Ea167kJmolEa29.3kJEa0kJMai?BacTodeviseratelawfromtheproposedd[CH kk

1/r 1 4

E1 )274kJmol- 7.Judge:BecausetheratelawandtheEaareconsistentwiththeexperimentalfacts,theproposedmechanismisareasonableone.Mai?Bac規(guī)則Ir=k∏例液相反應(yīng)Cr3+3Ce4+Cr6+由實(shí)驗(yàn)得到：rk[Ce4+]2[Cr3+[Ce3+]-推測(cè)機(jī)理：Ce4++Cr3+ Ce3++

Ce4++Cr4+→Ce3++

+

→ +由機(jī)理得到：rk2K[Ce4+]2[Cr3+[Ce3+]-1Mai?Bac規(guī)則II就總反應(yīng)計(jì)量數(shù)與反應(yīng)級(jí)數(shù)之間的關(guān)系而言II-1若反應(yīng)級(jí)數(shù)>3,由于4分子反應(yīng)不大可能，因而速控步前Mai?Bac 反應(yīng)A+B→P,如速率方程中,A出現(xiàn)分?jǐn)?shù)級(jí)有兩種:(1)A離解產(chǎn)生中間物直接參加速控步;(2)B規(guī)則IV若反應(yīng)無(wú)簡(jiǎn)單級(jí)數(shù)，速率方程的分母為幾項(xiàng)Mai?Bac Mai?BacForth