K A E-Ea/RT

Arrhenius LawTodayThe rate constant k is a function of temperatureReview Transition State TheoryArrhenius Theorypre-factorKinetic MechanismsWhy does a reaciton follow a particular rate law?What is actually happening in the reaction?barrierk A e-Ea/RTtemperatureArrhenius LawThe higher the temperature the more molecules thathave enough energy to make it over the barrierPrinciples of Chemistry II Vanden BoutLet's make a new Equationk Ae-Ea/RTPrinciples of Chemistry II Vanden BoutLet's make a new Equationlnk lnA -Ea/RTk A e-Ea/RTlnk lnA -Ea/RTlet's look at two temperatureslet's look at two temperatureslnk1 ln A -Ea/RT1lnk2 lnA -Ea/RT2lnk1 ln A -Ea/RT1lnk2 lnA -Ea/RT2subtract to get a new equation that doesn't have A-Ea 1 1ln(k2/k1) R [ T - T1]2Principles of Chemistry II Vanden BoutPrinciples of Chemistry II Vanden Bout

The activation energyfor this reaction is?!A.! !-1.2 x 104 K!B.! !1 x 105 J mol-1!!!C.! !1.2 x 104 J mol-1D.! !1 x 105 K!!E.! !-1 x 102 kJ mol-1Slope -Ea/REa -R x slopeEa -8.314 J K-1 mol-1 x (-1.2 x 104 K) 1 x 105 J mol-1Principles of Chemistry II Vanden BoutPrinciples of Chemistry II Vanden BoutWhy are reactions faster athigher temperatures?More molecules have sufficient energy to getover the barrier. BIG EFFECTMore molecules have collisions(but this is a very small effect) that isignored in Arrhenius view of transition statetheoryPrinciples of Chemistry II Vanden BoutPrinciples of Chemistry II Vanden Bout

Nonsense "real world" exampleWhy is something first order?second order?happy student withquiz turned instudent quiz TABecause of the number of molecules involvedin the "key" (slowest) step in the reactionstudent TA quizstudent with quizWe need to think about what is actually happeningstudent with quiz TAstudent with completed quizstudent with completed quiz TAThis is the mechanism of the reactionhappy student withquiz turned inWhat controls the rate of this reaction?Principles of Chemistry II Vanden BoutNonsense "real world" examplestudent TA quizstudent with quiz Vanden BoutNonsense "real world" examplehappy student withquiz turned instudent quiz TAPrinciples of Chemistry IIstudent with quiz TAstudent with completed quizhappy student withquiz turned inimagine step one was very slow (we passed outpreprinted bubble sheets in alphabetical order checking IDbefore handing you the quiz)student with completed quiz TAhappy student withquiz turned instudent quiz TAstudent TA quizstudent with quizstudent with quiz TAstudent with completed quizstudent with completed quiz TAhappy student withquiz turned inWhat we want. All steps to be fast except step 2Then the rate is dominated by step 2However step 1 still mattersRate dominated by "handing" out the quizPrinciples of Chemistry II Vanden BoutPrinciples of Chemistry II Vanden Bout

Nonsense "real world" examplehappy student withquiz turned instudent quiz TAstudent TA quizstudent with quizH2(g) Br2(g)What actually happens?Does a H2 and a Br2 molecule collide and react?Does something else happen?student with quiz TAstudent with completed quizhappy student withquiz turned instudent with completed quiz TAdominated by step 3. You need to have your quiznotarized before turning it in.All three steps matterPrinciples of Chemistry II Vanden BoutThe rate is dominated by the slowest stepthis is called the "rate limiting step"intermediatesStep 1Br2Step 2Br H2HBr HStep3Step4H Br2HBr Overall2Br2BrBrBr2H2(g) Br2(g)2HBr(g)Which Step Matters?Principles of Chemistry II Vanden BoutWith this mechanism what is the predicted rate law?Br2Br22HBr(g)2Brslow2BrslowBr H2HBr HfastBr H2HBr HfastH Br2HBr fastH Br2HBr fast2Br2BrBr2BrfastBr2fastWhat molecules are reactants in the slow step?As soon as some Br is formed it reacts veryrapidly through the other step.The rate is dominated by "waiting" for step 1Principles of Chemistry IIBronly one molecule of Br2Therefore rate k[Br2] Vanden BoutPrinciples of Chemistry II Vanden Bout

What if this was the mechanism?Br2What if this was the mechanism?2BrfastBr2Br H2HBr HslowH Br2HBr 2BrBrBr22BrfastBr H2HBr HslowfastH Br2HBr fastfast2BrCan't have Br in the rate law it is an intermediatewhere did the Br come from?Principles of Chemistry IIWhat is the rate law expected for the given mechanism?NO2 NO2NO3 NOslowNO3 CONO2 CO2fastNO2 CONO CO2!B.! !rate k[NO3][CO]!C.! !rate k[NO2]2[CO]!D.! !rate k[NO2]2!E.! !rate k[NO2]2[NO3][CO]!Principles of Chemistry IIfastYou need 1/2 mole of Br2 to get one mole Brthereforerate k[Br2]1/2[H2] Vanden Boutrate k[NO2][CO]Br2rate k[Br][H2]rate k[Br][H2]A.! !BrPrinciples of Chemistry IIMechanisms that involve productsCl2overallCl ClfastCl H2SHCl HSfastCl HSHCl SslowCl2 H2Stwo molecules of NO2in rate limiting step Vanden Bout Vanden Boutrate k[Cl][HS] x2HCl S[Cl][H2S][HCl][HS]Principles of Chemistry IIx[Cl2][Cl]2[Cl2][H2S] [HCl] Vanden Bout

12345Principles of Chemistry II Vanden BoutPrinciples of Chemistry II Vanden Bout

Principles of Chemistry II Vanden BoutA.!!-1.2 x 104 KB.!!1 x 105 J mol-1C.!!1.2 x 104 J mol-1D.!!1 x 105 KE.!!-1 x 102 kJ mol-1 The activation energy