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