Chemical KineticsChemical KineticsChapter 15Chapter 15

H2O2 decomposition in an insect

H2O2 decomposition catalyzed by MnO2

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• We can use thermodynamics to tell if a reaction is product or reactant favored.

• But this gives us no info on HOW FAST reaction goes from reactants to products.

•KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM.

• We can use thermodynamics to tell if a reaction is product or reactant favored.

• But this gives us no info on HOW FAST reaction goes from reactants to products.

•KINETICS — the study of REACTION RATES and their relation to the way the reaction proceeds, i.e., its MECHANISM.

Chemical KineticsChemical Kinetics

Energy Diagram

thermodynamics

KINETICS

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the study of the study of REACTION RATESREACTION RATES and their relation to the way and their relation to the way the reaction proceeds, i.e., the reaction proceeds, i.e.,

its its MECHANISMMECHANISM..

•The reaction mechanism is our goal!

the study of the study of REACTION RATESREACTION RATES and their relation to the way and their relation to the way the reaction proceeds, i.e., the reaction proceeds, i.e.,

its its MECHANISMMECHANISM..

•The reaction mechanism is our goal!

KINETICS

5Reaction Reaction MechanismsMechanisms

The sequence of events at the molecular level that The sequence of events at the molecular level that control the speed and outcome of a reaction.control the speed and outcome of a reaction.

Br from biomass burning destroys stratospheric ozone. Br from biomass burning destroys stratospheric ozone. (See R.J. Cicerone, (See R.J. Cicerone, ScienceScience, volume 263, page 1243, 1994.), volume 263, page 1243, 1994.)

Step 1:Step 1:Br + OBr + O33 ---> BrO + O ---> BrO + O22

Step 2:Step 2:Cl + OCl + O33 ---> ClO + O ---> ClO + O22

Step 3:Step 3:BrO + ClO + light ---> Br + Cl + OBrO + ClO + light ---> Br + Cl + O22

NET: NET: 2 O2 O33 ---> 3 O ---> 3 O22

REACTION RATES

RR = [P ] = - [R ] t t

P =products R = reactants

7Determining a Reaction Determining a Reaction RateRate

Rate = the change Rate = the change in [dye] divided in [dye] divided by timeby time

The rate is The rate is determined from determined from the plot.the plot.

Rate = the change Rate = the change in [dye] divided in [dye] divided by timeby time

The rate is The rate is determined from determined from the plot.the plot.

Dy

e C

on

ce

ntr

atio

nD

ye

Co

nc

en

trat

ion

Time

Relative Rates

Reactant

2A 4B + C

- [A ] = [B ] = [C ] 2 t 4 t t

Rate Calculations

Factors Affecting RXN Rates

• *Nature of Reactants

• Temperature• Concentration• Surface Area/ Physical

state• Catalysts

Collision Theory

Collisions

Collisions

EnergyCollisions

Energy

Orientation

NO

NO

YES

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Concentrations and Concentrations and Rates Rates

To postulate a reaction To postulate a reaction mechanism, we studymechanism, we study

•• reaction ratereaction rate and and

•• its its concentration concentration dependencedependence

To postulate a reaction To postulate a reaction mechanism, we studymechanism, we study

•• reaction ratereaction rate and and

•• its its concentration concentration dependencedependence

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Concentration and rateConcentration and rate

What is concentration of reactant as function of time?

The rate law is

Rate -[A]

time = k [A]

REACTION ORDERREACTION ORDERIn general, forIn general, for

a a AA + b + b BB --> x --> x X X

Rate = k [A]Rate = k [A]mm[B][B]nn

The exponents The exponents m,, and n m,, and n •• are the reaction order

• can be 0, 1, 2 or fractions

•• must be determined by must be determined by experiment!experiment!

Rate contantRate contant: : Arrhenius equation Arrhenius equation Rate contantRate contant: : Arrhenius equation Arrhenius equation

k Ae -Ea/RTk Ae -E

a/RTRate

constant

Temp (K)

8.31 x 10-3 kJ/K•molActivation energy

Frequency factor

Frequency factor = frequency of collisions with correct geometry.

Rate constant is dependent on only the activation energy and temperature

Simulation: RATE

17MECHANISMSA Microscopic View of Reactions

MECHANISMSA Microscopic View of Reactions

Mechanism: how reactants are converted to products at the molecular level.RATE LAW ----> MECHANISM

experiment ----> theory

18More on MechanismsMore on MechanismsMore on MechanismsMore on Mechanisms

Reaction is

UNIMOLECULAR if only one reactant is

involved.

BIMOLECULAR if two different molecules

must collide to form a products

A bimolecular reaction

19Collision TheoryCollision TheoryCollision TheoryCollision Theory

Reactions require

(a) activation energy and

(b) correct geometry.

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

2. Activation energy 2. Activation energy and geometryand geometry

1. Activation energy 1. Activation energy

20MechanismsMechanisms

O3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a

sequence of elementary steps.

Adding elementary steps gives NET reaction.

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

1. Rate law determined from experiment is:

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

Most rxns. have sequence of elementary steps.

NOTE

2. Order and stoichiometric coefficients NOT the same!

3. Rate law reflects all chemistry down to and including the slowest

step in multistep reaction.

22MechanismsMechanisms

Proposed MechanismProposed Mechanism

Step 1 — slow HOOH + I- --> HOI + OH-

Step 2 — fast HOI + I- --> I2 + OH-

Step 3 — fast 2 OH- + 2 H+ --> 2 H2O

Rate is controlled by slow step —

RATE DETERMINING SSTEP, RDS.

Rate can be no faster than RDS!

Proposed MechanismProposed Mechanism

Step 1 — slow HOOH + I- --> HOI + OH-

Step 2 — fast HOI + I- --> I2 + OH-

Step 3 — fast 2 OH- + 2 H+ --> 2 H2O

Rate is controlled by slow step —

RATE DETERMINING SSTEP, RDS.

Rate can be no faster than RDS!

2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O Rate = k [I-] [H2O2]

23MechanismsMechanisms

Elementary 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.

Elementary 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.

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

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Simulation:” Mechanisms

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Sovled problems: pg 144

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NONO22 + CO NO + CO + CO NO + CO22

NO2 + CO reaction:

Rate = k[NO2]2

Single step

Two possible mechanisms

Two steps: step 1 Two steps: step 2

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Ozone Decomposition Ozone Decomposition MechanismMechanism

Proposed mechanismProposed mechanism

Step 1: fast, equilibriumStep 1: fast, equilibrium

O3 (g) <--> O2 (g) + O (g)

Step 2: slowO3 (g) + O (g) ---> 2 O2 (g)

2 O2 O33 (g) ---> 3 O (g) ---> 3 O22 (g) (g)

Rate = k [O3]2

[O2]

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