Ch15_Kinetics_2

Ch15_Kinetics_2 - MECHANISMS A Microscopic View of...

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Unformatted text preview: MECHANISMS A Microscopic View of Reactions H3C How are reactants converted to products at How are reactants converted to products at the molecular level? the molecular level? Want to connect the Want to connect the 4 trans 27 kJ/mol CH3 CC H H cis-2-butene Mechanisms • Reaction passes thru a • Reaction passes thru a TRANSITION STATE TRANSITION STATE where there is an where there is an activated complex Activated Complex -262 kJ 4 kJ/mol 3 Rate = k [ trans-2-butene ] Conversion requires twisting around the C=C bond. Energy involved in conversion of trans to cis Energy involved in conversion of trans to cis butene butene 266 kJ MECHANISMS Conversion of trans to cis butene Conversion of trans to cis H3C H CC H CH3 trans-2-butene MECHANISM MECHANISM theory theory MECHANISMS energy 2 For example For example Sections 15.5 and 15.6 RATE LAW ----> RATE LAW ----> experiment ----> experiment ----> MECHANISMS 1 that has sufficient that has sufficient energy to become a energy to become a product. product. energy 5 Activated Complex -262 kJ 266 kJ 4 kJ/mol trans 27 kJ/mol cis 31 kJ/mol MECHANISMS Also note that trans-butene is MORE Also note that trans-butene is STABLE than cis-butene by about 4 kJ/ mol. STABLE than cis-butene by kJ/mol. Therefore, trans ---> cis is ENDOTHERMIC . ENDOTHERMIC Therefore, trans ---> cis is ENDOTHERMIC. This is the connection between thermo This is the connection between thermodynamics and kinetics. dynamics and kinetics. energy Activated Complex ACTIVATION ENERGY, Ea cis 31 kJ/mol = energy req’d to form activated complex. Here Ea = 266 kJ/ mol See Figure 15.15 Page 1 -262 kJ 266 kJ 4 kJ/mol trans 27 kJ/mol cis 31 kJ/mol 6 Activation Energy A flask full of trans -butene iis stable because A flask full of trans-butene s stable because only a tiny fraction of trans molecules have only a tiny fraction of trans molecules enough energy to convert to cis. enough energy to convert to cis. differences in differences in activation activation energy cause energy cause reactions to vary reactions to vary from fast to slow. from fast to slow. Reactions require Reactions require (a) activation energy and (a) activation energy and (b) correct geometry. (b) correct geometry. O3(g) + NO(g) ---> O 2(g) + NO2(g) O3(g) + NO(g) ---> O 2(g) + NO2(g) 8 10 More About Activation Energy Arrhenius equation — Arrhenius equation A bimolecular reaction BIMOLECULAR — two BIMOLECULAR — two different molecules different molecules must collide must collide --> products --> products Rate constant k = Ae 11 Temp (K) -E a / R T Activation energy 8.31 x 10-3 kJ/K•mol Frequency factor = frequency of collisions with correct geometry. Plot ln k vs. 1/T 2. Activation energy and geometry 9 Exo- or endothermic? or Frequency factor 1. Activation energy 1. Mechanisms Reaction of Reaction of trans --> cis trans --> cis iis UNIMOLECULAR s UNIMOLECULAR only one reactant is only one reactant is involved. involved. 1. Why is cis <--> trans reaction Why cis trans observed to be 1st order? As [trans] doubles, number of As [trans] molecules with enough E also doubles. 2. Why is the cis <--> trans reaction Why cis trans reaction faster at higher temperature? Fraction of molecules with sufficient Fraction activation energy increases with T. In general, In general, Collision Theory Mechanisms 7 E 1 l n k = - (a )( ) + l n A ---> straight line. RT slope = -Ea /R Page 2 Mechanisms O3 + NO reaction occurs in a single O3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a ELEMENTARY step. sequence of elementary steps. sequence of elementary steps. Adding elementary steps gives NET reaction. Adding elementary steps gives NET reaction. 12 Mechanisms Mechanisms 13 Proposed Mechanism Proposed Mechanism Rate = k Proposed mechanism Step 1: fast O3 (g) Step 2: slow [O 3 ]2 [O 2 ] ¸ Rate Rate O2 (g) + O (g) O3 (g) + O (g) ---> 2 O 2 (g) What is a catalyst? Catalysts and society Page 3 [II--] [H2O2 ] — as observed!! [ ] [H2O2 ] — as observed!! The species HOI and OH -- are reaction The species HOI and OH are reaction intermediates. Rate can be no faster than rds! Rate can be no faster than rds! Dr. James Cusumano, Catalytica Inc. + H2O2 + 2 ---> I2 + 2 H2O Rate Rate = k [I-] [H2O2] Step 3 — fast 2 OH- + 2 H+ --> 2 H2O RATE DETERMINING STEP,, rds. RATE rds. Catalysts speed up reactions by altering the mechanism to lower the activation energy barrier. 15 H+ Elementary Step 1 is bimolecular and involves II-Elementary Step 1 is bimolecular and involves and HOOH. Therefore, this predicts the rate and HOOH. Therefore, this predicts the rate law should be law should be Rate of the reaction controlled by slow step — Rate of the reaction controlled by slow step — CATALYSIS I- Step 1 — slow HOOH + I- --> HOI + OHStep 2 — fast HOI + I- --> I2 + OH- Step 1 — slow HOOH + II-- --> HOI + OH -Step 1 — slow HOOH + --> HOI + OH Step 2 — fast HOI + II-- --> II2 + OH-Step 2 — fast HOI + --> 2 + OH Step 3 — fast 2 OH-- + 2 H+ --> 2 H2O Step 3 — fast 2 OH + 2 H+ --> 2 H2O 16 2 O3 (g) ---> 3 O 2 (g) 2 Most rxns. involve a sequence of elementary steps. 2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O Rate = k [I-] [H2O2] Most rxns. involve a sequence of elementary Most rxns. steps. steps. 2 II-- + H2O2 + 2 H+ ---> II2 + 2 H2O 2 + H2O2 + 2 H+ ---> 2 + 2 H2O Rate = k [I --] [H2O2] Rate = k [I ] [H2O2] NOTE NOTE 1. Rate law comes from experiment 1. Rate law comes from experiment 2. Order and stoichiometric coefficients not Order and stoichiometric coefficients 2. necessarily the same! necessarily the same! 3. Rate law reflects all chemistry down to 3. Rate law reflects all chemistry down to and including the slowest step in multistep and including the slowest step in multistep multistep reaction. reaction. Ozone Decomposition Mechanism Mechanisms 14 17 CATALYSIS IIn auto exhaust systems — Pt, NiO n auto exhaust systems — Pt, NiO 2 CO + O2 ---> 2 CO2 CO 2 NO ---> N 2 + O2 18 CATALYSIS 19 CATALYSIS Catalysis and activation energy Catalysis and activation energy Catalysis MnO2 catalyzes decomposition of H 2O2 2 H2O2 ---> 2 H2O + O2 2. Polymers: Polymers: H2C=CH 2 ---> polyethylene 2 2 3. Acetic acid: Acetic CH 3OH + CO --> CH 3CO2H CH 3 3 2 4. Enzymes — biological catalysts 4. Enzymes 21 Iodine-Catalyzed Isomerization of cis-2-Butene Uncatalyzed Uncatalyzed reaction Catalyzed reaction Iodine-Catalyzed Isomerization of cis-2- Butene 20 22 Figure 15.19 Page 4 Figure 15.18 ...
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This note was uploaded on 10/19/2011 for the course CHM 2210 taught by Professor Mcquade during the Fall '08 term at FSU.

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