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UT CH 302 - Lecture notes
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Principles of Chemistry II © Vanden BoutTodayReview Transition State TheoryArrhenius TheoryKinetic MechanismsWhy does a reaciton follow a particular rate law?What is actually happening in the reaction? Principles of Chemistry II © Vanden BoutArrhenius LawThe rate constant k is a function of temperaturek = A e-Ea/RTArrhenius Lawpre-factorbarriertemperatureThe higher the temperature the more molecules that have enough energy to make it over the barrier Principles of Chemistry II © Vanden BoutLet's make a new Equationk = A e-Ea/RT lnk = lnA -Ea/RTlet's look at two temperatureslnk1 = ln A -Ea/RT1lnk2 = lnA -Ea/RT2 Principles of Chemistry II © Vanden BoutLet's make a new Equationk = A e-Ea/RT lnk = lnA -Ea/RTlet's look at two temperatureslnk1 = ln A -Ea/RT1lnk2 = lnA -Ea/RT2subtract to get a new equation that doesn't have A ln(k2/k1) = -EaR[]1T2T11-Principles of Chemistry II © Vanden Bout! A.!! -1.2 x 104 K! B.!! 1 x 105 J mol-1! C.!! 1.2 x 104 J mol-1!! D.!! 1 x 105 K! E.!! -1 x 102 kJ mol-1!The activation energy for this reaction is?Slope = -Ea/REa = -R x slopeEa = -8.314 J K-1 mol-1 x (-1.2 x 104 K) = 1 x 105 J mol-1 Principles of Chemistry II © Vanden Bout Principles of Chemistry II © Vanden Bout Principles of Chemistry II © Vanden BoutWhy are reactions faster at higher 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 state theoryPrinciples of Chemistry II © Vanden BoutWhy is something first order?second order?Because of the number of molecules involved in the "key" (slowest) step in the reactionWe need to think about what is actually happeningThis is the mechanism of the reaction Principles of Chemistry II © Vanden BoutNonsense "real world" examplestudent + quiz + TA happy student with quiz turned in student + TA + quiz student with quiz + TAstudent with quiz student with completed quizstudent with completed quiz + TAhappy student with quiz turned in What controls the rate of this reaction? Principles of Chemistry II © Vanden BoutNonsense "real world" examplestudent + quiz + TA happy student with quiz turned in student + TA + quiz student with quiz + TAstudent with quiz student with completed quizstudent with completed quiz + TAhappy student with quiz turned in imagine step one was very slow (we passed outpreprinted bubble sheets in alphabetical order checking IDbefore handing you the quiz)Rate dominated by "handing" out the quiz Principles of Chemistry II © Vanden BoutNonsense "real world" examplestudent + quiz + TA happy student with quiz turned in student + TA + quiz student with quiz + TAstudent with quiz student with completed quizstudent with completed quiz + TAhappy student with quiz turned in What we want. All steps to be fast except step 2Then the rate is dominated by step 2However step 1 still mattersPrinciples of Chemistry II © Vanden BoutNonsense "real world" examplestudent + quiz + TA happy student with quiz turned in student + TA + quiz student with quiz + TAstudent with quiz student with completed quizstudent with completed quiz + TAhappy student with quiz turned in dominated by step 3. You need to have your quiz notarized before turning it in.All three steps matter Principles of Chemistry II © Vanden BoutH2(g) + Br2(g) 2HBr(g)What actually happens?Does a H2 and a Br2 molecule collide and react?Does something else happen?Which Step Matters?H2(g) + Br2(g) 2HBr(g)Br2 2Br Step 1Br + H2 HBr + HStep 2H + Br2 HBr + BrStep32Br Br2Step4Overallintermediates Principles of Chemistry II © Vanden BoutThe rate is dominated by the slowest stepthis is called the "rate limiting step"Br2 2Br Br + H2 HBr + HH + Br2 HBr + Br2Br Br2slowfastfastfastAs soon as some Br is formed it reacts very rapidly through the other step. The rate is dominated by "waiting" for step 1 Principles of Chemistry II © Vanden BoutWith this mechanism what is the predicted rate law?Br2 2Br Br + H2 HBr + HH + Br2 HBr + Br2Br Br2slowfastfastfastWhat molecules are reactants in the slow step? only one molecule of Br2Therefore rate = k[Br2]Principles of Chemistry II © Vanden BoutWhat if this was the mechanism?Br2 2Br Br + H2 HBr + HH + Br2 HBr + Br2Br Br2slowfastfastfastrate = k[Br][H2]Can't have Br in the rate law it is an intermediatewhere did the Br come from? Principles of Chemistry II © Vanden BoutWhat if this was the mechanism?Br2 2Br Br + H2 HBr + HH + Br2 HBr + Br2Br Br2slowfastfastfastrate = k[Br][H2]You need 1/2 mole of Br2 to get one mole Brtherefore rate = k[Br2]1/2[H2] Principles of Chemistry II © Vanden BoutWhat is the rate law expected for the given mechanism?NO2 + NO2 NO3 + NO slowNO3 + CO NO2 + CO2 fastNO2 + CO NO + CO2 overall A.!! rate = k[NO2][CO]! B.!! rate = k[NO3][CO]! C.!! rate = k[NO2]2[CO]! D.!! rate = k[NO2]2! E.!! rate = k[NO2]2[NO3][CO]!two molecules of NO2 in rate limiting step Principles of Chemistry II © Vanden BoutMechanisms that involve productsCl2 Cl


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UT CH 302 - Lecture notes

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