Chemical KineticsChemical KineticsChapter 15Chapter 15

H2O2 decomposition in an insect

H2O2 decomposition catalyzed by MnO2

REACTION RATES

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

P =products R = reactants

Relative Rates

Reactant

2A 4B + C

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

Rate Calculations

Collision Theory

Collisions

Collisions

EnergyCollisions

Energy

Orientation

NO

NO

YES

Factors Affecting RXN Rates

• *Nature of Reactants

• Temperature• Concentration• Surface Area/ Physical

state• Catalysts

Simulation: RATE

8MECHANISMSA Microscopic View of Reactions

Sections 15.5 and 15.6

MECHANISMSA Microscopic View of Reactions

Sections 15.5 and 15.6

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

experiment ----> theory

REACTION ORDERREACTION ORDERIn general, forIn general, for

a a AA + b + b BB --> x --> x X X with a catalyst “C”with a catalyst “C”

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

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

• can be 0, 1, 2 or fractions

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

10More on MechanismsMore on MechanismsMore on MechanismsMore on Mechanisms

Reaction of cis-butene --> trans-

butene is UNIMOLECULAR - only one reactant is

involved.

BIMOLECULAR — two different

molecules must collide

--> products

A bimolecular reaction

Exo- or endothermic?

11Collision 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

12MechanismsMechanisms

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

sequence of elementary steps.

Adding elementary steps gives NET reaction.

13MechanismsMechanisms

Most rxns. involve a sequence of elementary steps.

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

Rate = k [I-] [H2O2]

NOTE

1. Rate law comes from experiment

2. Order and stoichiometric coefficients not necessarily the same!

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

reaction.

14MechanismsMechanisms

Proposed 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 of the reaction controlled by slow step —

RATE DETERMINING STEP, rds.

Rate can be no faster than rds!

Proposed 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 of the reaction controlled by slow step —

RATE DETERMINING STEP, rds.

Rate can be no faster than rds!

Most rxns. involve a sequence of elementary steps. 2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O

Rate = k [I-] [H2O2]

15MechanismsMechanisms

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

17Rate Laws Rate Laws and and MechanismsMechanisms

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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Sovled problems: pg 144

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