Mechanisms a microscopic view of reactions sections 15 5 and 15 6
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MECHANISMS A Microscopic View of Reactions Sections 15.5 and 15.6. How are reactants converted to products at the molecular level? Want to connect the RATE LAW ----> MECHANISM experiment ----> theory. MECHANISMS. For example Rate = k [trans-2-butene]

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Mechanisms a microscopic view of reactions sections 15 5 and 15 6
MECHANISMSA Microscopic View of ReactionsSections 15.5 and 15.6

How are reactants converted to products at the molecular level?

Want to connect the

RATE LAW ----> MECHANISM

experiment ----> theory


Mechanisms
MECHANISMS

For example

Rate = k [trans-2-butene]

Conversion requires twisting around the C=C bond.


Mechanisms1
MECHANISMS

Conversion of trans to cis butene


Mechanisms2
MECHANISMS

Energy involved in conversion of trans to cis butene

See Figure 15.15


Mechanisms3
MECHANISMS

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

ACTIVATION ENERGY, Ea = energy req’d to form activated complex.

Here Ea = 233 kJ/mol


Mechanisms4
MECHANISMS

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

Therefore, trans ---> cis is ENDOTHERMIC.

This is the connection between thermo-dynamics and kinetics.


Activation energy
Activation Energy

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

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


Mechanisms5
MECHANISMS

1. Why is reaction observed to be 1st order?

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

2. Why is the reaction faster at higher temperature?

Fraction of molecules with sufficient activation energy increases with T.


Mechanisms6
MECHANISMS

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


Mechanisms7
MECHANISMS

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

BIMOLECULAR — two different molecules must collide --> products


Mechanisms8
MECHANISMS

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

BIMOLECULAR — two different molecules must collide --> products

A bimolecular reaction

Exo- or endothermic?


Collision theory
Collision Theory

Reactions require

(a) activation energy and

(b) correct geometry.

O3(g) + NO(g) ---> O2(g) + NO2(g)


Collision theory1
Collision Theory

Reactions require

(a) activation energy and

(b) correct geometry.

O3(g) + NO(g) ---> O2(g) + NO2(g)

1. Activation energy

2. Activation energy and geometry


Mechanisms9
MECHANISMS

O3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a sequence of elementary steps.

Adding elementary steps gives NET reaction.


Mechanisms10
MECHANISMS

O3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a sequence of elementary steps.

Adding elementary steps gives NET reaction.


Mechanisms11
MECHANISMS

Most rxns. involve a sequence of elementary steps.

2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O

Rate = k [I-] [H2O2]

Step 1 — slow HOOH + I- --> HOI + OH-

Step 2 — fast HOI + I- --> I2 + OH-

Step 3 — fast 2 OH- + 2 H+ --> 2 H2O

Rate of the reaction controlled by slow step —

RATE DETERMINING STEP, rds.

Rate can be no faster than rds!


Mechanisms12
MECHANISMS

2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O

Rate = k [I-] [H2O2]

Step 1 — slow HOOH + I- --> HOI + OH-Step 2 — fast HOI + I- --> I2 + OH-

Step 3 — fast 2 OH- + 2 H+ --> 2 H2O

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

Rate  [I-] [H2O2] — as observed!!

The species HOI and OH- are reaction intermediates.


Arrhenius equation
Arrhenius 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


Arrhenius equation1
Arrhenius Equation

  • 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)


Arrhenius equation2
Arrhenius Equation

  • Taking the natural log and rearranging:

  • Straight line plot of ln k vs 1/T

  • Slope of –Ea/R


Catalysis
CATALYSIS

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


Catalysis1
CATALYSIS

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

Dr. James Cusumano, Catalytica Inc.

What is a catalyst?

Catalysts and society


Catalysis2
CATALYSIS

In auto exhaust systems — Pt, NiO

2 CO + O2 ---> 2 CO2

2 NO ---> N2 + O2


Catalysis3
CATALYSIS

2. Polymers: H2C=CH2 ---> polyethylene

3. Acetic acid:

CH3OH + CO --> CH3CO2H

4. Enzymes — biological catalysts


Catalysis4
CATALYSIS

MnO2 catalyzes decomposition of H2O2

2 H2O2 ---> 2 H2O + O2

Catalysis and activation energy

Uncatalyzed reaction

Catalyzed reaction




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