mechanisms a microscopic view of reactions sections 15.5 and 15.6 how are reactants converted to...

MECHANISMSMECHANISMSA Microscopic View of ReactionsA Microscopic View of Reactions

Sections 15.5 and 15.6Sections 15.5 and 15.6

MECHANISMSMECHANISMSA Microscopic View of ReactionsA Microscopic View of Reactions

Sections 15.5 and 15.6Sections 15.5 and 15.6

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 ----> MECHANISMMECHANISM

experiment ---->experiment ----> theorytheory

15.5 A 15.5 A MICROSCOPIC VIEW OF MICROSCOPIC VIEW OF REACTIONSREACTIONS

• The collision theory can be used to The collision theory can be used to explain reaction rates. explain reaction rates.

• In this theory, reactants must collide In this theory, reactants must collide in order to react. in order to react.

• One factor in determining if these One factor in determining if these collisions will be successful is the collisions will be successful is the energy of the collision. energy of the collision.

• The minimum energy required for The minimum energy required for reaction is called the activation reaction is called the activation energy. energy.

MICROSCOPIC VIEW OF REACTIONSMICROSCOPIC VIEW OF REACTIONS

• Figure 15.15 illustrates the activation energy illustrates the activation energy required for the conversion of cis to trans 2-required for the conversion of cis to trans 2-butene. butene.

–This reaction is exothermic since This reaction is exothermic since the product is at a lower energy the product is at a lower energy than the reactant.than the reactant.

• O.H. #63, #85, & O.H. #63, #85, & Figure 15.14 illustrate this concept illustrate this concept in a general way and show the relationship in a general way and show the relationship between the activation energy, Ebetween the activation energy, Eaa, and the energy , and the energy of the reaction, of the reaction, ΔΔE. E.

• The relative number of reactant molecules with The relative number of reactant molecules with energy equal to or greater than Eenergy equal to or greater than Eaa is illustrated in is illustrated in O.H. Figure 15.13, page 712.O.H. Figure 15.13, page 712.

Figure 15.15

Figure 15.14

MECHANISMSMECHANISMSMECHANISMSMECHANISMS

For exampleFor example

Rate = k [trans-2-butene]Rate = k [trans-2-butene]

Conversion requires twisting Conversion requires twisting around the C=C bond.around the C=C bond.

H3C

C C

CH3

H H

H3C

C C

H

H CH3trans-2-butene cis-2-butene


Conversion of trans to cis butene Conversion of trans to cis butene

Energy involved in conversion of trans to cis Energy involved in conversion of trans to cis butenebutene


See Figure 15.15See Figure 15.15

trans

cis

energy ActivatedComplex

26.9 kJ/mol

30.2 kJ/mol3.9 kJ/mol

233 kJ 229 kJ

ConcentrationConcentration• The effect of concentration is seen in the The effect of concentration is seen in the

rate equation. As might be expected, as rate equation. As might be expected, as the concentration increases, the rate the concentration increases, the rate increases. increases.

• If we assume the reaction between A and B If we assume the reaction between A and B occurs with the collision of A and B, then occurs with the collision of A and B, then the reaction would be first order in A and the reaction would be first order in A and first order in B. first order in B.

• The reaction rate would double if the The reaction rate would double if the concentration of A were doubled, and concentration of A were doubled, and increased by a factor of four if both the increased by a factor of four if both the concentration of A and B were doubled. concentration of A and B were doubled.

• Figure 15.12.Figure 15.12.

Figure 15.12

Concentration and collision frequency

Effects of Molecular OrientationEffects of Molecular Orientation

• Simple reactants have no particular Simple reactants have no particular orientation and are independent of this orientation and are independent of this consideration. consideration.

• More complex reactants may be very More complex reactants may be very sensitive to the orientation of the sensitive to the orientation of the molecules at the time of the collision. molecules at the time of the collision.

• Figure 15.16, page 715.Figure 15.16, page 715.

Figure 15.16


Reaction passes thru a Reaction passes thru a TRANSITION STATE where TRANSITION STATE where there is anthere is an activated activated complexcomplex that has sufficient that has sufficient energy to become a product.energy to become a product.

ACTIVATION ENERGY, EACTIVATION ENERGY, Eaa

= energy required to form = energy required to form activated complex.activated complex.

Here EHere Eaa = 233 kJ/mol = 233 kJ/mol

trans

cis

energyActivatedComplex

26.9 kJ/mol


233 kJ 229 kJ

Also note that trans-butene is MORE Also note that trans-butene is MORE STABLE than cis-butene by about 4 kJ/mol.STABLE than cis-butene by about 4 kJ/mol.

Therefore, trans ---> cis is ENDOTHERMIC.Therefore, trans ---> cis is ENDOTHERMIC.

This is the connection between thermo- This is the connection between thermo- dynamics and kinetics.dynamics and kinetics.


trans

cis

energy ActivatedComplex

26.9 kJ/mol


233 kJ 229 kJ

Activation EnergyActivation EnergyActivation EnergyActivation Energy

A flask full of trans-butene is stable A flask full of trans-butene is stable because only a tiny fraction of trans because only a tiny fraction of trans molecules have enough energy to convert molecules have enough energy to convert to cis.to cis.

In general, In general, differences in activation differences in activation energyenergy are the reason reactions vary from are the reason reactions vary from fast to slow.fast to slow.

MECHANISMSMECHANISMSMECHANISMSMECHANISMS1. Why is reaction observed to be 1st 1. Why is reaction observed to be 1st

order? order?

As [trans] doubles, number of molecules As [trans] doubles, number of molecules with enough E also doubles.with enough E also doubles.

2. Why is the reaction faster at higher 2. Why is the reaction faster at higher temperature?temperature?

Fraction of molecules with sufficient Fraction of molecules with sufficient activation energy increases with T.activation energy increases with T.

MECHANISMSMECHANISMSReaction of Reaction of

trans --> cis trans --> cis is is

UNIMOLECULAR - UNIMOLECULAR - only one reactant is only one reactant is involved.involved.

BIMOLECULAR — two BIMOLECULAR — two different molecules different molecules must collide must collide

--> --> productsproducts A bimolecular reactionA bimolecular reaction

Exo- or endothermic? Exo- or endothermic?

The Arrhenius EquationThe Arrhenius Equation• Many reactions have a very significant temperature Many reactions have a very significant temperature

dependence, even doubling or tripling with a ten dependence, even doubling or tripling with a ten degree change in temperature. degree change in temperature.

• The primary temperature dependence is given by the The primary temperature dependence is given by the Arrhenius equation Arrhenius equation

RT

Ea

Aek

RT

Ea

e

• “A” is the pre-exponential factor and is related to the collision frequency corrected for orientation considerations. • The fraction of collisions with energy E > Ea

is

Arrhenius EquationArrhenius Equation

• A plot of A plot of lnln k vs. 1/T has a slope, m, with k vs. 1/T has a slope, m, with – m = m = -E-Eaa/R , and a y-intercept of /R , and a y-intercept of lnln A. A. – (Our text uses b for slope and a for y-(Our text uses b for slope and a for y-

intercept.) intercept.) –We will calculate the activation energy in the We will calculate the activation energy in the

lab using this method. lab using this method.

• The two-point form of the equation is: The two-point form of the equation is:

2

1 2 1

1 1ln aEk

k R T T

15.6 Reaction Mechanisms15.6 Reaction Mechanisms• The actual sequence of steps by which the The actual sequence of steps by which the

reaction proceeds in a multi-step process is reaction proceeds in a multi-step process is called the reaction mechanism. called the reaction mechanism.

• This sequence of steps is seldom known so This sequence of steps is seldom known so most mechanisms are simply guesses that fit most mechanisms are simply guesses that fit all we know about the reaction. all we know about the reaction.

• The individual steps are called elementary The individual steps are called elementary steps. steps.

• The number of molecules involved in a step is The number of molecules involved in a step is called the molecularity of that step and can be called the molecularity of that step and can be determined by counting based on the determined by counting based on the proposed mechanism. proposed mechanism.

Reaction MechanismsReaction Mechanisms

A ====> product, is unimolecular.A ====> product, is unimolecular.

A + B ===> product, is bimolecular. A + B ===> product, is bimolecular.

A + 2 B =====> product, is termolecular.A + 2 B =====> product, is termolecular.

• Unimolecular and bimolecular processes are Unimolecular and bimolecular processes are common, but termolecular processes are very common, but termolecular processes are very rare. rare.

• The rate equation for an elementary step can The rate equation for an elementary step can be written using the molecularity of that step. be written using the molecularity of that step.

• We see that for elementary steps the We see that for elementary steps the molecularity and the order are the same. molecularity and the order are the same.


• The value of the constant, k, however, cannot The value of the constant, k, however, cannot be determined and is left in symbol form. be determined and is left in symbol form.

• A mechanism is considered feasible if the sum A mechanism is considered feasible if the sum of the elementary steps is stoichiometrically of the elementary steps is stoichiometrically consistent and the order of the reaction is consistent and the order of the reaction is consistent with the order derived from the consistent with the order derived from the elementary steps. elementary steps.

• The slow step or rate-determining step is use The slow step or rate-determining step is use to determine the order of the reaction. to determine the order of the reaction.

Collision TheoryCollision TheoryCollision TheoryCollision TheoryReactions require Reactions require

(a) activation energy and (a) activation energy and

(b) correct geometry. (b) correct geometry.

OO33(g) + NO(g) (g) + NO(g) O O22(g) + NO(g) + NO22(g)(g)

2. Activation energy 2. Activation energy and geometryand geometry

1. Activation energy 1. Activation energy


OO33 + NO reaction occurs in a single + NO reaction occurs in a single ELEMENTARYELEMENTARY step. Most others involve a step. Most others involve a sequence of elementary steps.sequence of elementary steps.

Adding elementary steps gives NET reaction.Adding elementary steps gives NET reaction.


Most reactions involve a sequence of Most reactions involve a sequence of elementary steps.elementary steps.

2 I2 I-- + H + H22OO22 + 2 H + 2 H++ ---> I ---> I22 + 2 H + 2 H22OO

Rate = k [IRate = k [I--] [H] [H22OO22]]

Step 1 — slowStep 1 — slow HOOH + IHOOH + I-- --> HOI + OH --> HOI + OH--

Step 2 — fastStep 2 — fast HOI + IHOI + I-- --> I --> I22 + OH + OH--

Step 3 — fastStep 3 — fast 2 OH2 OH- - + 2 H + 2 H++ --> 2 H --> 2 H22OO

Rate of the reaction controlled by slow step —Rate of the reaction controlled by slow step —

RATE DETERMINING STEP, rds.RATE DETERMINING STEP, rds.

Rate can be no faster than rds!Rate can be no faster than rds!


Step 1 is bimolecular and involves IStep 1 is bimolecular and involves I-- and HOOH. and HOOH. Therefore, this predicts the rate law should Therefore, this predicts the rate law should bebe

Rate = k[IRate = k[I--] [H] [H22OO22] — as observed!!] — as observed!!

The species HOI and OHThe species HOI and OH-- are reaction are reaction intermediates.intermediates.

2 I2 I-- + H + H22OO22 + 2 H + 2 H++ ---> I ---> I22 + 2 H + 2 H22OO

Rate = k [IRate = k [I--] [H] [H22OO22]]

Step 1 — slowStep 1 — slow HOOH + IHOOH + I-- --> HOI + OH --> HOI + OH--

Step 2 — fastStep 2 — fast HOI + IHOI + I-- --> I --> I22 + OH + OH--

Step 3 — fastStep 3 — fast 2 OH2 OH- - + 2 H + 2 H++ --> 2 H --> 2 H22OO


2 NO + 2 H2 NO + 2 H22 ===> N ===> N22 + 2 H + 2 H22O , O ,

d(Nd(N22)/dt = k [NO])/dt = k [NO]22[H[H22]]

• Can this reaction occur in one step? Can this reaction occur in one step?

• Is the following mechanism feasible? If so, Is the following mechanism feasible? If so,

how?how?


KK

2 NO <===> N2 NO <===> N22OO22 , fast equilibrium step , fast equilibrium step

kk11

NN22OO22 + H + H2 2 ====> 2 NOH , slow step====> 2 NOH , slow step

kk22

2 NOH ====> N2 NOH ====> N22 + 2 OH , fast step + 2 OH , fast step

kk33

OH + HOH + H22 ====> H ====> H22O + H , fast stepO + H , fast step

kk44

H + OH ====> HH + OH ====> H22O , fast stepO , fast step

CATALYSTS AND REACTION RATECATALYSTS AND REACTION RATE

• Catalysts are substances that alter the Catalysts are substances that alter the reaction rate without being consumed. reaction rate without being consumed.

• They function by altering the reaction They function by altering the reaction mechanism. mechanism.

• The new mechanism has a different The new mechanism has a different activation energy. activation energy.

• Catalysts are of two varieties, Catalysts are of two varieties, homogeneous and heterogeneous. homogeneous and heterogeneous.

• Catalysts play a very significant role in Catalysts play a very significant role in most commercial chemical processes.most commercial chemical processes.

Figure 15.18

Iodine-Catalyzed Isomerization of cis-Iodine-Catalyzed Isomerization of cis-2-Butene2-Butene

Figure 15.19Figure 15.19

Figure 15.19Figure 15.19

CATALYSISCATALYSISCATALYSISCATALYSIS

Catalysts speed up reactions by Catalysts speed up reactions by altering the mechanism to lower the altering the mechanism to lower the

activation energy barrier.activation energy barrier.

What is a catalyst? Catalysts and society

Dr. James Cusumano, Catalytica Inc.Dr. James Cusumano, Catalytica Inc.

CATALYSISCATALYSIS

In auto exhaust systems — Pt, NiOIn auto exhaust systems — Pt, NiO

2 CO + O2 CO + O22 ---> 2 CO ---> 2 CO22

2 NO ---> N2 NO ---> N22 + O + O22

CATALYSISCATALYSIS

2.2. Polymers: Polymers: HH22C=CHC=CH22 ---> polyethylene ---> polyethylene

3.3. Acetic acid: Acetic acid:

CHCH33OH + CO --> CHOH + CO --> CH33COCO22HH

4. 4. Enzymes — biological catalystsEnzymes — biological catalysts

CATALYSISCATALYSIS

Catalysis and activation energyCatalysis and activation energy

Uncatalyzed reactionUncatalyzed reaction

Catalyzed reactionCatalyzed reaction

MnOMnO22 catalyzes catalyzes

decomposition of Hdecomposition of H22OO22

2 H2 H22OO22 ---> 2 H ---> 2 H22O + OO + O22

MnOMnO22 Catalyzed Decomposition of H Catalyzed Decomposition of H22OO22

mechanisms a microscopic view of reactions sections 15.5 and 15.6 how are reactants converted to...

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