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Chemical KineticsConcernsreactionrates:Speedwithwhichreactantsareconvertedtoproductscollege.hmco/chemistry/shared/media/animations/oscillatingreaction.html 5/4/20241Fireworks explode/give bright colors due to very fast chemical reactionsThe rusting of a bridge is also a chemical reaction,but it is very slow.The eroding of a mountain takes even longer5/4/20242Rate:the speed at which something happensAverage rate or speed=quantity t Ratechemreaction=reactantorproduct=xmolunittimeLsAlwayspositive=molarconcentrationForward Rate:reactantsproductsReverse Rate:productsrecombinetobecomereactantsNet Rate:forwardrate-reverserateAverage Rate:speedofentirereactionfromstarttofinishInstantaneous Rate:speedofreactionatonemomentintime5/4/20243The Collision Theory5/4/20244Atoms,ions,and molecules(reacting substances)must collide in order to react:Speed of moleculesDo they touch?Chemical reaction occurs?TooslowlyNoNoJustrightYes,withcorrectorientationNewbondsformedYesToofastYes,butbounceoffeachotherNo5/4/20245Activated complex:temporary,unstable arrangement of atoms that may form products or may break apart to reform reactantsReacting substances must collide with sufficient energy to form the activated complexcollege.hmco/chemistry/shared/media/animations/uctransitionstates.html 5/4/20246Whenreactantmoleculescollidew/enoughenergy,orbitalsoverlapTransition stateReactantsbrokendownintoparticlesNotreactantsorproductsAslikelytoformreactantsasproductsReactantmoleculesundergofurtherchangeRearrangementofmolecularstructureBreakingofexistingbondsMakingnewones5/4/20247Enough kinetic energy for reactants to leap over reaction barrier?5/4/20248Exothermic reactions release energy and form products at lower energy level.Endothermic reactions absorb energy and form products at lower energy level.H=(-)5/4/20249Covers period of time between mixing of reactants and point at which chemical reaction stops or reaches equilibriumHow fast chemicals react and factors that influence rate allows chemists to exercise precise control over chemical reactions5/4/202410Chemical KineticsUnderstandstepsbywhichreactiontakesplaceCalledreaction mechanism5/4/202411RateofchemicalreactiondictateswhetherreactioncanoccurKnowingratelaw(expressionrelatingratetoconcentrationsofreactants)helpschemistadjustreactionconditionstogetmoresuitablerateIf2reactionscompetingforsinglereagent,knowingratelawletsyoufavorexclusiveformationofsingleproduct5/4/2024125/4/2024135/4/202414Measurement of reaction rate based on rate of appearance of product or disappearance of reactantDeterminedbymeasuringconcentrationofoneormorechemicalsatdifferenttimesduringcourseofreactionUsekinetic curve(concentration vs.time curve)Ifreactantmeasured,decreaseinw/timeisrecordedSlopeofcurveatpointintimefromkineticcurvedeterminesrateofproductproduction(orreactantsdisappearance)5/4/202415ChangecanbeDisappearanceofreactantsDecreaseRatehasnegativesignAppearanceofproductIncreaseRatehaspositivesignAlwaysdefinerateaspositivequantityindicateconcentrationinmol/L5/4/202416ABRateaverage=-A=+B ttRateAdisappearing=RateBappearingRateofappearance(ordisappearance)ofsubstanceisdividedbyitsstoichiometriccoefficientaA+bBcC+dDRate=-A=-B=+C=+DatbtctdtGetsamereactionratenomatterwhichismeasured5/4/202417Rate is change in of some species w/respect to time,then measure of all species at multiple times to determine rate.InitialH2/I2equalatalltimesandinitialproductiszero5/4/202418RateofformationofHIistwicerateofdisappearanceofH2orI2atanygiventimeRateslowsintimeduetodecreasingofreactantsStatedmathematically,relationshipbetweenformationofproductsanddisappearanceofreactantsforreactionisH2+I22HI5/4/202419Instantaneous rateRatenotconstantDecreasesw/time(changewithtime)RatemeasuredforgivenpointintimeRatesintables:averagerateoverspecifictimeperiodDeterminedbyfindingslopeoflinetangenttopointrepresentingparticulartimeRate=-(slopeoftangentline)Slopeoftangentline=changeiny/changeinx=dA/dt(derivative):ratesmeasuredovertimeintervalsthatapproach0second5/4/202420DecompositionofNO22NO2(g)2NO(g)+O2(g)5/4/2024212NO2(g)2NO(g)+O2(g)RateofconsumptionofNO2=RateofproductionofNO=2(RateofproductionofO2)becauserateofproductionofNOistwicethatofO2NO2=NO=(O2)2t2t t(multiplyallby2)NO2=NO=2(O2)ttt5/4/2024225/4/202423www2.wwnorton/college/chemistry/gilbert/tutorials/interface.swf?chapter=chapter_14&folder=reaction_rate 5/4/202424Factors affecting rate of reaction5/4/202425Nature of reactantsSomenaturallyoccurfasterthanotherNumberofreactingspeciesStateofreactingspecies(particlesformingsolidsmoveslowerthangases/insolution)Complexityofreaction5/4/202426Change in concentration of reactantsreactantsincreasesrateForreactiontooccur,particlesmustcollideIfhigher,chanceofcollisiongreaterCaseswherethisdoesntaffectrateofreactionWherecatalystisalreadyworkingasfastasitcanStepsincertainmulti-stepreactionslikelytohavewidelydifferentrates(somefast/someslow)5/4/202427OverallrategovernedbyhowfastAsplitsuptomakeXandY(rate determining step)IncreasingAincreaseschancesofstephappeningIncreasingBspeedsup2ndstepwhichis“waitingaround”for1sttohappen,sohardlymakesdifferencetooverallrate5/4/202428TemperatureParticlesonlyreactwhentheycollideMostparticlesdonthaveenergyneededtoreactIncreasingthat#increasesrateofcollisions/probabilityoffavorablecollisionsIncreasingtemperaturealwaysincreasesreactionratesDecreasingtemperaturealwaysdecreasesreactionFormanyreactionsoccurringataroundroomT,rateofreactiondoublesforevery10C(9/11Criseintemperature#degreesneededtodoubleratechangesgraduallyasT5/4/202429Entropy&temperaturePlace3dropsofbluefoodcoloringin3flasks.Place3dropsofyellowfoodcoloringin3more.Keep1ofeachatroomtemperature,andwarm1,andcooltheother.Usingnotecards,inverttheblueflaskovertheyellowandremovethecard.Securetheflaskwithaclamp.Allowtheflaskstostandandrecordthetimenecessaryforbothflaskstobecomethesameshadeofgreen.1.Istheentropygreatestwhenthecolorsoftheflasksaredifferentorthesame?Explain.2.Explaintherelationshipbetweendiffusionandentropy.3.Whatinfluencedoesanincrease/decreaseintemperaturehaveonthediffusionobserved?5/4/202430Particles present have very wide range of energiesFor gases,shown on Maxwell-Boltzmann Distribution Plot#particles having each particular energyArea under curve=total#particles present5/4/202431For reaction to occur,particles must collide with energies activation energyMany dont have enough energy to react when they collideEither change shape of curve,or move activation energy to left5/4/202432PressureIncreasingpressureincreasesrateofreactionParticlesareclosertogetherHaveincreasedprobabilityoffavorablecollisions5/4/202433Use a CatalystSubstancewhichspeedsupreactionChemicallyunchangedatendofreactionUsuallyincreasesrate,butsomecandecreaseitProvidealternativepathwayw/loweractivationenergycollege.hmco/chemistry/shared/media/animations/homogeneouscatalysis.html 5/4/2024345/4/202435Read17.1-.2,pp.529-541,17.4,pp.546-547Qpp.554-555,#34,37,48-49,62,66,71Homework:5/4/202436Rate LawsTherateofachemicalreactiondictateswhetherareactioncanoccur5/4/202437Rate Laws:An introductionChemicalreactionsarereversibleWithtime,enoughproductsaccumulateReversereactionbecomesimportantConcentrationofreactantsdependsondifferenceinratesofforwardandreversereactionsFocusonreactionsoonafterreactantsaremixed,beforeproductsbuilduptosignificantlevelsReactionratedependsonlyonofreactants5/4/202438Rate=kAnRatelawdependsonofreactantsMust be determined by experimentProportionalityconstantk-rate constantRelatesandorderstorateofreactionConstantvalueatfixedTforgivenreactionUnitsofrateconstantdependonorderofreactionn-order of reactantCanbeinteger(includingzero)orfraction5/4/202439Important Rate law points1.productsdonotappearReactionratestudiedunderconditionswherereversereactiondoesnotcontributetooverallrate2.ValueofexponentnmustbedeterminedbyexperimentCannotbewrittenfrombalancedequationDifferential Rate Law(Rate law)-expresseshowratedependsonconcentrationIntegrated rate law-expresseshowconcentrationdependsontime5/4/202440Possibletowriterate lawthatrelatesreactionratetoconcentrationofreactantsRate=kAxByCzx/y/z:reaction orders(reactantwithwhichitisassociated)Maybezero,positive,negative,integer,orfractionMustbedeterminedbylaboratoryexperimentsNorelationshiptostoichiometriccoefficientsofbalancedchemicalequation5/4/202441Ifreactioniszero orderwithrespecttoreactantRatedoesnotdependonconcentrationofthatreactantDoublingconcentrationwillnotincreaseordecreaserateconcentration-time graphrate-concentration graph5/4/202442Iffirst orderwithrespecttoreactantRatedirectlyproportionaltoconcentrationofthatreactantDoublingconcentrationdoublesrateconcentration-time graphrate-concentration graph5/4/202443Ifsecond orderwithrespecttoareactantRateisdirectlyproportionaltosquareofconcentrationofthatreactantDoublingconcentrationquadruplesraterate-concentration graphconcentration-time graph5/4/202444Overall orderofareaction:sumofindividualreactionorders(abovereaction-overallorderisx+y+z).Concentration of Reactant against TimeRate of Reaction against Concentration of ReactantEasier to use rate-concentration graph rather than concentration-time graph to decide if reaction is first or second-order5/4/2024455/4/202446Units used:Ratemeasuredinchangeinconcentrationpersecond-unitsaremoldm-3s-1UnitsforconcentrationsofA/Baremoldm-3,butifordersaremoreorless,theychangeIforderis0,justignorethemIforderis1,itsmoldm-3Iforderis2,unitsaremoldm-32=mol2dm-65/4/2024475/4/202448www2.wwnorton/college/chemistry/gilbert/tutorials/interface.swf?chapter=chapter_14&folder=reaction_order 5/4/2024495/4/2024505/4/2024515/4/202452NH4+NO2-N2+2 H2OExperimentNH4+oNO2-oinitialrateN2/s10.1000.0051.35x10-720.1000.0102.70 x10-730.2000.0105.40 x10-7rate=kNH4+1NO2-1orsimplykNH4+NO2-1.35x10-7M/s=k(0.100M)(0.005M)solving,k=2.7x10-4/Msor2.7x10-4L/mols5/4/202453NO2+CO NO+CO2ExperimentNO2oCOoinitialrateCO2/s10.100.00100.1820.100.00200.1830.200.00200.73rate=kNO22CO0orsimplykNO220.18M/s=k(0.10M)2(0.0010M)0k=18/Msor18L/mols5/4/2024542 NO2+2 H2 N2+2 H2O ExperimentNO2oH2oinitialrateN2/s10.00500.00201.3x10-520.0100.00205.1x10-530.0100.00401.0 x10-4rate=kNO22H21orsimplykNO22H21.3x10-5M/s=k(0.0050M)2(0.0020M)1k=2.6x102/M2s5/4/202455ExampleTheinitialrateofthereactionBrO3-(aq)+5Br-(aq)+8H+(aq)3Br2(l)+H2O(l)hasbeenmeasuredatthereactantconcentrationsshown(inmol/L):ExperimentBrO3-Br-H+Initialrate(mol/Ls)10.100.100.108.0 x10-420.200.100.101.6x10-330.100.200.101.6x10-340.100.100.203.2x10-3Rate=kAxBysoRate=kBrO3-1Br-1H+2Rate=1+1+2=45/4/202456ExampleThereactionofiodideionwithhypochloriteion,OCl-(whichisfoundinliquidbleach),followstheequationOCl-+I-OI-+Cl-Itisarapidreactionthatgivesthefollowingratedata:InitialConcentrationsRateofFormation(mol/Ls)OCl-I-Cl-1.7X10-31.7X10-31.75X1043.4X10-31.7X10-33.50X1041.7X10-33.4X10-33.50X104Determinevalueofrateconstant.Rate=kAxBy1.75x104=k(1.7x10-3)1(1.7x10-3)1rate=6.06x109L/molsecOCl-I-Rate=1+1=25/4/202457Homework:Read17.3,pp.542-545Qpp.554-556,#56,69-70,745/4/202458Reaction Mechanisms-how atoms or molecules interact to make productsComplex reaction-twoormoreelementarystepsMechanismmayneverbeprovenCannotseechemicalreactionBasedonscientificevidenceavailableforreactionGuessatidentityofmanyintermediatesIntermediatesusuallysoreactive,cantbeisolatedProducedinoneelementarystepandconsumedinanotherDoesnotappearinoverallreactionMechanismfordecompositionofozone,O35/4/202459Combine to produce path molecules take from reactant(s)to product(s)in chemical reaction-sum is overall reactionRepresents single collision or vibration that leads to chemical changeDetermines overall rate of reactionReaction can only go as fast as its slowest step-highest activation energy5/4/2024605/4/202461UseratelawstodeterminemechanismforchemicalreactionA+BY+ZNotsurehowprocessactuallyoccurs,butoverallreactionrateexpressedbyRate=kBPredictiffollowingmechanismisvalidorinvalidStep1:2AY+DSlowprocessStep2:B+DA+Z Fastprocess5/4/202462Reactionmechanismsmustconformtorules1.Overallequationforreactionneedstoequalsumofelementarysteps2AY+DB+DA+Z2A+B+DY+D+A+ZA+BY+Z(Sofar,looksgood)2.Ratelawforrate-determiningstepshouldmatchratelawforoverallreaction2AY+DRate=kA2(doesntmatchrate=kB)-invalid5/4/202463Rate determining stepOverall reaction2 NO2+F2-2 NO2FMechanism(two elementary steps)NO2+F2 NO2F+F(slow)F+NO2 NO2F(fast)Rate determining step(RDS)determines overall reaction rate law Rate overall rate for rds step=k1 NO2 F25/4/2024645/4/2024655/4/2024665/4/2024675/4/2024685/4/202469Homework:Read17.4,pp.546-549Qpp.554-556-#57,76Dotest,pg.557Uselinkforquizandsubmitasbefore.glencoe/qe/science.php?qi=10015/4/202470
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