# MECHANISMS A Microscopic View of Reactions Sections 15.5 and 15.6 - PowerPoint PPT Presentation

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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

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### 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

For example

Rate = k [trans-2-butene]

Conversion requires twisting around the C=C bond.

### MECHANISMS

Conversion of trans to cis butene

### MECHANISMS

Energy involved in conversion of trans to cis butene

See Figure 15.15

### 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

### 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

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.

### 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.

### MECHANISMS

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

### MECHANISMS

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

BIMOLECULAR — two different molecules must collide--> products

### 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

Reactions require

(a) activation energy and

(b) correct geometry.

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

### 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

### MECHANISMS

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

Adding elementary steps gives NET reaction.

### MECHANISMS

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

Adding elementary steps gives NET reaction.

### MECHANISMS

Most rxns. involve a sequence of elementary steps.

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

Rate = k [I-] [H2O2]

Step 1 — slowHOOH + I- --> HOI + OH-

Step 2 — fastHOI + I- --> I2 + OH-

Step 3 — fast2 OH- + 2 H+ --> 2 H2O

Rate of the reaction controlled by slow step —

RATE DETERMINING STEP, rds.

Rate can be no faster than rds!

### MECHANISMS

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

Rate = k [I-] [H2O2]

Step 1 — slowHOOH + I- --> HOI + OH-Step 2 — fastHOI + I- --> I2 + OH-

Step 3 — fast2 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

• 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 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 Equation

• Taking the natural log and rearranging:

• Straight line plot of ln k vs 1/T

• Slope of –Ea/R

### CATALYSIS

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

### 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

### CATALYSIS

In auto exhaust systems — Pt, NiO

2 CO + O2 ---> 2 CO2

2 NO ---> N2 + O2

### CATALYSIS

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

3.Acetic acid:

CH3OH + CO --> CH3CO2H

4. Enzymes — biological catalysts

### CATALYSIS

MnO2 catalyzes decomposition of H2O2

2 H2O2 ---> 2 H2O + O2

Catalysis and activation energy

Uncatalyzed reaction

Catalyzed reaction

Figure 15.18

Figure 15.19