燃烧学课件英文版(Combustion-03 Combustion Chemical Kinetic

COMBUSTION2COMBUSTIONKINETICS(4classhours)FirstandSecondLawsofthermodynamicsareusedtopredictthefinalequilibriumstateoftheproductsafterthereactioniscomplete.Chemicalkineticsdealswithhowfastthechemicalreactionproceeds.ChemicalReactionRatesandFactorsChemicalandPhysicalpropertiesofReactantsandActivationEnergyTemperatureChemicalReactionRatesClassifyingofChemicalReactionsChainReactionsConceptsandCharacteristicsexplosionlimitsofbranchedchainreactionSolutionsforReactionRatesCONTENTSOBJECTIVESChemicalMechanismsFactorsInfluencingtheChemicalReactionRatesDefinitionElementaryReactionsLawofMassActionArrheniusRateExpressionCollisionTheoryPressureLawofMassAction;ArrheniusRateExpression;CollisionTheory;ChemicalReactionRates;KineticClassifyingofChemicalReaction;FactorsInfluencingtheChemicalReactionRates;ChainReactions;ThreeLimitsforExplosionLawofMassAction;ArrheniusRateExpression;CollisionTheory;ChainReactions;ThreeLimitsforexplosionCollisionTheory;ThreeLimitsforExplosionKEYPOINTSOFKNOWLEDGEEMPHASESDIFFICULTIES2.1ChemicalReactionRates1DefinitionHowfastthefuelisconsumedisofinterest.Chemicalreactionratesmaybeexpressedastherateofdecreaseoftheconcentrationofareactantortherateofincreaseoftheconcentrationofaproduct.icdCwdtindnwdtixdXwdt(mol／m3·s)MoleConcentration（generallybeingused）(1／m3·s)MolecularConcentration(1／s)RelativeConcentrationThereactionratewNegativesignisduetothefactthatthefuelisconsumed.w——alwayspositive.CCXii()aA＋bBeE＋fFReactionRate:AAdCwdtBBdCwdtEEdCwdtFFdCwdtConstant-VolumeSystem:fwewbwawFEBAwwdenotesthechemicalreactionrateofthereactionsystem,whichistheonlyoneforthespecifiedsystem.Thus,wisdefinedassystemreactionrate.wherea,b,eandfarethestoichiometriccoefficients.wcanbecalculatedaccordingtotheconcentrationvariationofanarbitrarysubstanceeasilymeasuredinasystem.2ElementaryReactions2H2+O22H2OTheglobal(oroverall)reactiondoesnotreflecttheactualreactionprocess,itjustdescribestheinitialandfinalstates.Whenoxygenandhydrogenmoleculescollideandreact,theydonotdirectlyyieldwater,but,instead,formmanyintermediatespecies.Therearemanyintermediatereactionstepsintheoverallreaction.Thisoverallreactioniscalledglobalreaction.Theuseofglobalreactionstoexpressthechemistryinaspecificproblemfrequentlya“blackbox”approach.Globalreactioncannotprovideabasisforunderstandingwhatactuallyhappeningchemicallyinasystem.GLOBALREACTIONTohaveacompletepictureofthecombustionofH2andO2,morethan20elementaryreactionscanbeconsidered.Thecollectionofelementaryreactionsnecessarytodescribeanoverallreactioniscalledareactionmechanismorareactionpathway.Reactionmechanismsmayinvolveonlyafewsteps(i.e.,elementaryreactions)orasmanyasseveralhundred.Elementaryreactionisthatreactantsaretransformedtoproductsinmoleculecollisionwithasingle-stepandnotlikeglobalreactionwithmanysteps.ELEMENTARYREACTION3LawofMassActionThelawofmassactionstatesthatforanelementaryreactionthereactionrateisproportionaltotheproductoftheconcentrationsofthereactantspresentraisedtoapowerequaltothecorrespondingstoichiometriccoefficient.Thelawofmassactiondescribestherelationshipbetweenchemicalreactionratesandconcentrationsofthereactants.fFeEbBaAbaBAkw)()(k——thereactionrateconstant.Ingeneral,itisastrongfunctionoftemperature.CommentConcentrationofreactantsReactionRateprobabilityofmolecularcollisionThelawofmassactioncanonlybeusedfortheelementaryreactions.NoteMathematicalExpressionbaBAw)()(（A）——thefuelconcentration(kmol/m3orkg/m3)Example1Compoundreactionofhydrogenatomsisasfollows:HHHk23Find：thereactionrateofthecompoundreactionofhydrogenatom?1122122nnnnvMvMvMvMvMvMExample2Acomplicatedelementaryreactioncanberepresentedbytheequation:Find：thenetproductionrateofspeciesi?12121innniiwkMMMkMSolutioniniiiniiMvM1''1'Applyingthelawofmassactiontothereaction:iniiiiMkw1Productionrateofspeciesi:Netproductionrateofspeciesi:1iniiiiMkMConsumptionrateofspeciesi:iniiiiMkw1LawofMassAction：abwkABReactionRatesConcentrationofReactantsTheReactionRateConstantk4ArrheniusRateExpressionk=f(T,P,activator,solvent）StudyshowsthatreactionrateconstantkisaffectedheavilybytemperatureT.RelationshipbetweenReactionRateConstantkandtemperatureT{T}ⅠⅡⅢⅣⅤ{k}CommonReactionExplosionReactionEnzymeCatalysisReactionoxidationreactionofcarbon2NO+O22NO2Van’tHoffApproximateRule102~4ttkklnlnEkARTArrheniusRateExpressionlog2.303EkBRT2lndkEdTRTE——ActivationEnergyElementaryReaction（mostreactions）ln{k}{T-1}Theslopesoflinesontheseplotsareequalto(-E/R),thustheactivationenergyEmaybedetermined.lnlnEkARTSlope=-E/RRTEAekE↗：k↘;T↗：k↗①IfEislarger，islarger,T↗：k↗↗，Temperaturehasstrongeffectonk；dTkdlnIfEissmaller，issmaller,T↗：k↗，Temperaturehasweakeffectonk。dTkdln2lnRTEdTkdE0，RT200lndTkdT↗：k↗②WhenEisconstant,atahighertemperature,issmaller,T↗，k↗；dTkdlnWhenEisconstant,atalowertemperature,islarger,T↗：k↗↗.dTkdln2lnRTEdTkdExampleE=100kJ/mol，Aisconstant：64.3300310300310)300310(/100KKRKKKKmolKJekk96.6300310KKkk00.3400410KKkk08.2400410KKkkInasimilarway: