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

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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

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

2MECHANISMSMECHANISMSMECHANISMSMECHANISMS

For exampleFor example

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

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

For exampleFor example

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

Conversion requires twisting around the C=C Conversion requires twisting around the C=C bond.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 Conversion of trans to cis butene Conversion of trans to cis butene

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

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

trans

cis

energy ActivatedComplex

26.9 kJ/mol

30.2 kJ/mol3.9 kJ/mol

233 kJ 229 kJ

MECHANISMSMECHANISMSMECHANISMSMECHANISMS

See Figure 15.15See Figure 15.15


Reaction passes thru a Reaction passes thru a TRANSITION STATE TRANSITION STATE where there is an where there is an

activated activated complexcomplex that has that has sufficient energy to sufficient energy to become a product. become a product.

ACTIVATION ACTIVATION ENERGY, EENERGY, Eaa = =

energy req’d to form energy req’d to form activated complex.activated complex.

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

Reaction passes thru a Reaction passes thru a TRANSITION STATE TRANSITION STATE where there is an where there is an

activated activated complexcomplex that has that has sufficient energy to sufficient energy to become a product. become a product.

ACTIVATION ACTIVATION ENERGY, EENERGY, Eaa = =

energy req’d to form energy req’d to form activated complex.activated complex.

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

trans

cis


26.9 kJ/mol


233 kJ 229 kJ

trans

cis


26.9 kJ/mol


233 kJ 229 kJ

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

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

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

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


trans

cis


26.9 kJ/mol


233 kJ 229 kJ

7Activation 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 molecules have enough energy to convert to cis.convert to cis.

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

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 molecules have enough energy to convert to cis.convert to cis.

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


1.1. Why is reaction observed to be 1st 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.

2.2. Why is the reaction faster at higher 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.

1.1. Why is reaction observed to be 1st 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.

2.2. Why is the reaction faster at higher 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.


Reaction of Reaction of trans --> cis is trans --> cis is UNIMOLECULARUNIMOLECULAR- - only one reactant is only one reactant is involved.involved.

Reaction of Reaction of trans --> cis is trans --> cis is UNIMOLECULARUNIMOLECULAR- - only one reactant is only one reactant is involved.involved.

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Reaction of Reaction of trans --> cistrans --> cisis is UNIMOLECULARUNIMOLECULAR- - only one reactant is only one reactant is involved.involved.

BIMOLECULARBIMOLECULAR — two — two different molecules different molecules must collidemust collide

--> products--> products

Reaction of Reaction of trans --> cistrans --> cisis is UNIMOLECULARUNIMOLECULAR- - only one reactant is only one reactant is involved.involved.

BIMOLECULARBIMOLECULAR — two — two different molecules different molecules must collidemust collide

--> products--> products


11MECHANISMSMECHANISMS

Reaction of Reaction of trans --> cis trans --> cis

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

BIMOLECULARBIMOLECULAR — two — two different molecules different molecules must collide must collide

--> --> productsproducts

Reaction of Reaction of trans --> cis trans --> cis

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

BIMOLECULARBIMOLECULAR — two — two different molecules different molecules must collide must collide

--> --> productsproducts

A bimolecular reactionA bimolecular reaction

Exo- or endothermic? Exo- or endothermic?

12Collision TheoryCollision TheoryCollision TheoryCollision Theory

Reactions require Reactions require

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

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

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





13Collision TheoryCollision TheoryCollision TheoryCollision Theory









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 rxns. involve a sequence of elementary Most rxns. involve a sequence of elementary steps.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 STEPRATE DETERMINING STEP, rds., rds.

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

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

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





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

RATE DETERMINING STEPRATE DETERMINING STEP, rds., rds.

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


Step 1Step 1 is is bimolecularbimolecular and involves I and involves I-- and HOOH. and HOOH. Therefore, this predicts the rate law should beTherefore, this predicts the rate law should be

Rate Rate [I [I--] [H] [H22OO22] — as observed!!] — as observed!!

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

Step 1Step 1 is is bimolecularbimolecular and involves I and involves I-- and HOOH. and HOOH. Therefore, this predicts the rate law should beTherefore, this predicts the rate law should be

Rate Rate [I [I--] [H] [H22OO22] — as observed!!] — as observed!!

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

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





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Arrhenius EquationArrhenius Equation

• Reaction rates depend on energy, frequency of collisions, temperature, and geometry of molecules given by:

• A = frequency of collisions with correct geometry at concentration of 1M (L/mol*s)

• R = gas constant (8.314 x 10-3 kJ/K*mol)• e-Ea/RT is fraction of molecules having the

minimum energy required for reaction

RTEaAek /

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• Calculate the value of the activation energy from the temp. dependence of the rate constant

• Calculate the rate constant for a given temp. (if activation energy and A are known)

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• Taking the natural log and rearranging:

• Straight line plot of ln k vs 1/T

• Slope of –Ea/R

mx b

1lnln

y

TR

EAk a

21CATALYSISCATALYSISCATALYSISCATALYSIS

Catalysts speed up reactions by Catalysts speed up reactions by altering the mechanism to lower altering the mechanism to lower the activation energy barrier.the activation energy barrier.





What is a catalyst? Catalysts and society

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


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

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


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

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


Catalysis and activation energyCatalysis and activation energy

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

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

Figure 15.18

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

Figure 15.19Figure 15.19

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

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