1 / 27

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

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]

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about ' MECHANISMS A Microscopic View of Reactions Sections 15.5 and 15.6' - winifred-cohen

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
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

For example

Rate = k [trans-2-butene]

Conversion requires twisting around the C=C bond.

Conversion of trans to cis butene

Energy involved in conversion of trans to cis butene

See Figure 15.15

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

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.

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.

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.

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

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

BIMOLECULAR — two different molecules must collide --> products

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

BIMOLECULAR — two different molecules must collide --> products

A bimolecular reaction

Exo- or endothermic?

Reactions require

(a) activation energy and

(b) correct geometry.

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

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

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

Adding elementary steps gives NET reaction.

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

Adding elementary steps gives NET reaction.

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!

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.

• 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

• 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)

• Taking the natural log and rearranging:

• Straight line plot of ln k vs 1/T

• Slope of –Ea/R

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

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

In auto exhaust systems — Pt, NiO

2 CO + O2 ---> 2 CO2

2 NO ---> N2 + O2

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

3. Acetic acid:

CH3OH + CO --> CH3CO2H

4. Enzymes — biological catalysts

MnO2 catalyzes decomposition of H2O2

2 H2O2 ---> 2 H2O + O2

Catalysis and activation energy

Uncatalyzed reaction

Catalyzed reaction

Figure 15.18

Figure 15.19