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16.3 Reaction Mechanisms16.3 Reaction Mechanisms

Steps of a ReactionSteps of a Reaction

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The Ozone LayerThe Ozone LayerOzone is most important in the stratosphere, at this Ozone is most important in the stratosphere, at this level in the atmosphere, ozone absorbs UV radiationlevel in the atmosphere, ozone absorbs UV radiation

10 Km

50 Km

100 Km

Ozone Layer

Stratosphere

Mesosphere

TroposphereMt. Everest

TroposphereStratosphereMesosphere

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Mechanism of Ozone; Chapman Mechanism of Ozone; Chapman CycleCycle

Chapman cycle shows that OChapman cycle shows that O33 exist at steady exist at steady state. It is constant in the stratosphere.state. It is constant in the stratosphere.

O

O2

O3

O

O2

h

h

OO22

OO22

OO33

2 O2 O

OO

320 nm or less

242 nm or less

Slow ozone removal step

O3 lives for ~200 - 300 s before it dissociates.Ozone removal step:

O3 + O 2O2

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Reaction Coordinate for ozone Reaction Coordinate for ozone destructiondestruction

A Key reaction in the upper atmosphere is

O3 (g) + O (g) 2O2 (g) The Ea (fwd) is 19 kJ, and the Hrxn as written is -392 kJ.

A reaction energy diagram for this reaction with the calculate Ea(rev) is shown.

O3 +O

2O2

O kJO kJ

19 kJ19 kJ

-392 kJ-392 kJ

EEactact (rev) (rev)= 411 kJ= 411 kJ

Reaction ProgressReaction Progress

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Path to Destruction: OzonePath to Destruction: Ozone

1. Water Vapors: H2O OH• + H•

H• + O3 OH• + O2

OH• + O H• + O2

Net: O + O3 2O2

2.. N2 , Dinitrogen : N2 + O2 2NO

NO + O3 NO2 + O2

NO2 + O NO + O2

Net: O + O3 2O2

3. CFCs CCl2F2 CClF2• + Cl•Chlorofluorocarbons Cl• + O3 ClO• + O2

ClO• + O Cl• + O2

Net: O + O3 2O2

10,000 O3 will breakdown to O2 for every Cl•

10,000 O3 will breakdown to O2 for every Cl•

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Influence by CFC: OzoneInfluence by CFC: OzoneComparison of activation energies in the uncatalyzed decompositions of ozone. The destruction of ozone can be catalyzed by Cl atoms which leads to an alternative pathway with lower activation energy, and therefore a faster reaction.

Progress of

reaction

En

erg

y (

kJ)

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Reaction MechanismReaction Mechanism

The mechanism of a reaction is the The mechanism of a reaction is the sequence of steps (at a molecular level) sequence of steps (at a molecular level) that leads from reactant to products.that leads from reactant to products.Elementary Steps

Sequence of steps which describes an actual molecular event.

StoichiometryThe overall stoichiometric reaction is

the sum of the elementary steps.

Scientist want to learn about mechanism because an Scientist want to learn about mechanism because an understanding of the mechanism (how bonds break and form) understanding of the mechanism (how bonds break and form) may lead to conditions to improve reaction product yield, (or may lead to conditions to improve reaction product yield, (or prevent side products formation. i.e, depletion of ozone.)prevent side products formation. i.e, depletion of ozone.)

Scientist want to learn about mechanism because an Scientist want to learn about mechanism because an understanding of the mechanism (how bonds break and form) understanding of the mechanism (how bonds break and form) may lead to conditions to improve reaction product yield, (or may lead to conditions to improve reaction product yield, (or prevent side products formation. i.e, depletion of ozone.)prevent side products formation. i.e, depletion of ozone.)

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Ozone: RevisitedOzone: RevisitedChapman’s Cycle

O3 O2 + O

O + O3 2O2

2O3 3O2

Elementary Steps give rise to Rate LawSince elementary steps describes a molecular collision, the rate law for an elementary step (unlike the overall reaction) can be written from the Stoichiometry.

Consider an elementary step

iA + jB Product (slow step) rate = k [A]i •[B]j

The rate of the reaction is directly proportional to concentrations of the colliding species.

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Elementary Step: Rate LawElementary Step: Rate LawConsider the following proposed mechanism for the conversion of Consider the following proposed mechanism for the conversion of NONO22 to N to N22OO55. What is the rate law.. What is the rate law.

Step1 Step1 NONO22 + O + O33 NO NO33 + O + O22 (slow)(slow)

Step2 Step2 NONO33 + NO + NO2 2 N N22OO55 (fast)(fast)

rate = krate = k11[NO[NO22]]11 [O [O33]]11

In a series of steps, the slowest step determines the overall rate. In a series of steps, the slowest step determines the overall rate.

In the mechanism for a chemical reaction, the slowest step is the In the mechanism for a chemical reaction, the slowest step is the rate-determining step.rate-determining step.

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Elementary Steps: Order of Elementary Steps: Order of reactionreaction

Elem. StepElem. Step Rate LawRate Law OrderOrder MolecularityMolecularity11 A A Product Product Rate = k[A]Rate = k[A] 1st order1st order unimolecularunimolecular

22 2A 2A Product Product Rate = k[A]Rate = k[A]22 2nd order2nd order bimolecularbimolecular

A + B A + B Prod. Prod. Rate = k[A][B]Rate = k[A][B]

33 3A 3A Product Product Rate = k[A]Rate = k[A]33 3rd order3rd order TermolecularTermolecular

2A + B 2A + B Product Product Rate = k[A]Rate = k[A]22[B][B]

A + B + C A + B + C Prod Prod Rate = k[A][B][C]Rate = k[A][B][C]

* Termolecular mechanism (elementary step) is very rare.* Termolecular mechanism (elementary step) is very rare.

Scientist who propose such a mechanism must make careful Scientist who propose such a mechanism must make careful measurements.measurements.

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Multiple Elementary Steps Multiple Elementary Steps

Most reactions involve more than one elementary Most reactions involve more than one elementary step.step.

Rate-Limiting - When one step is much slower than Rate-Limiting - When one step is much slower than any other, the overall rate is determined by the any other, the overall rate is determined by the slowest “Rate-determining” step.slowest “Rate-determining” step.

Reaction is only as fast as the slowest elementary Reaction is only as fast as the slowest elementary stepstepAnalogy: Analogy: Leaving class after an exam.Leaving class after an exam.

On way skiing, speed only as fast as creepy crawler 12-cars On way skiing, speed only as fast as creepy crawler 12-cars ahead.ahead.

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Rate Determining Step from Rate Determining Step from Rate LawRate Law

Consider: NO2 + CO NO + CO2

Mechanism: (1) NO2 (g) + NO2 (g) NO3 g) + NO (g)

(2) NO3 (g) + CO (g) NO2 + CO2 (g)

Net: NO2 (g) + CO (g) CO2 (g ) + NO (g)

Rate = k[NO2] 2

Which is Rate determining step (1) or (2) ?Which is Rate determining step (1) or (2) ?• • RDS is the step that determines the rate law.

When scientists propose a mechanism, they can only say that it is When scientists propose a mechanism, they can only say that it is consistent with the experimental data.consistent with the experimental data.

There may be other mechanism that is consistent with experimental There may be other mechanism that is consistent with experimental data.data.

If experiments are done in the future to disprove the mechanism, then If experiments are done in the future to disprove the mechanism, then his proposed mechanism must be revised.his proposed mechanism must be revised.

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RDS and Rate Law: ExampleRDS and Rate Law: ExampleConsider the reaction : NO (g) + O3 NO2 + O2

Two mechanisms (elementary steps) are proposed:

Mechanism 1 NO + O3 NO2 + O2

Proposed rate law: Rate = k [NO] [ORate = k [NO] [O33]]

Mechanism 2 O3 O2 + O (slow)

NO + O NO2 (fast)

Proposed rate law: Rate = k [ORate = k [O3 3 ]]

What are the Rate Laws ?When a potential mechanism is proposed, 2 factors must be considered -

•Rate determining step must be consistent with observed rate law.•Sum of all the steps must yield the observed stoichiometry.

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Complicated Reaction Complicated Reaction MechanismMechanism

Reaction mechanism in which slow step (rate determining step) involves an intermediate.

Consider:Consider: A A B B

Mechanism:Mechanism: A A int (fast) int (fast)

int int B (slow B (slow))

NET:NET: A A B B

RATE = k[int]RATE = k[int]-but the rate law cannot be written in terms of an -but the rate law cannot be written in terms of an intermediateintermediate

(catalyst okay, but not intermediate).(catalyst okay, but not intermediate).

-It must be expressed in terms of stable species-It must be expressed in terms of stable species

How is the Rate Law modified?How is the Rate Law modified?

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Modification of Rate LawModification of Rate LawRATE = k [int]

Written in terms of reactants-

The rate law is now expressed in terms of the reactant.

Rate k [int]

keq [int ]

[A] [int] keq[A]

Rate k • keq[A] K' [A]

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Rate Laws from Mult. Steps Rate Laws from Mult. Steps Mechanism.Mechanism.

Consider the reaction: what would be the rate law based on the two proposed mechanism:2 NO2 (g) + O3 N2O5 + O2

Mechanism (1) Mechanism (2)NO2 + NO2 N2O4 (fast) NO2 + O3 NO3 + O2 (slow)

N2O4 + O3 N2O5 + O2 (slow) NO3 + NO2 N2O5 (fast)

Rate k [N2O4 ] [O3]

keq [N2O4]

[NO2] [NO2]

[N2O4] keq [NO2]2

Rate k • keq [NO2]2[O3]

Rate k[NO2 ]2[O3]

Termolecular

Rate k[NO2 ] [O3]bimolecular

Rate is based onslowest elem. step

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Rate Laws from Mult. Steps Rate Laws from Mult. Steps Mechanism.Mechanism.

The decomposition of hydrogen peroxide is catalyzed by iodide ion. The catalyzed reaction is thought to proceed via a two-step mechanism:

H2O2 (aq) + I- (aq) H2O(l) + IO- (aq) (slow)

H2O2 (aq) + IO- (aq) H2O(l) + I- (aq) + O2 (g) (fast)

a) Rate Law:

Rate Law = k [H2O2] [I- ]

b) Overall reaction:

2 H2O2 (aq) H2O(l) + O2 (g)

c) Intermediate: IO- (aq)

Catalyst: I- (aq)

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Enzyme Catalysis ReactionEnzyme Catalysis ReactionConsider oxidation of ethanol to aldehyde:

CH3CH2OH (l) ADH CH3CHO + R-H2

ADH - Alcohol dehydrogenase Mechanism

E + S ES ES E + P (slow)E + S E + P

Rate = k [ES]Keq = [ES] Keq [E] [S] = [ES] [E] [S]

Rate = K Keq [E] [S] = K’ [E] [S]

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SummarySummaryThe dynamics of the series of The dynamics of the series of steps of a chemical change is steps of a chemical change is what kinetics tries to explain. what kinetics tries to explain. Variation in reaction rate are Variation in reaction rate are observed through concentration observed through concentration and temperature changes, which and temperature changes, which operate on the molecular level operate on the molecular level through the energy of particle through the energy of particle collision. Kinetics allows us to collision. Kinetics allows us to speculate about the molecular speculate about the molecular pathway of a reaction. Modern pathway of a reaction. Modern industry and biochemistry industry and biochemistry depend on its principles. depend on its principles. However, speed and yield are However, speed and yield are very different aspects of a very different aspects of a reaction. Speed is in the kinetic reaction. Speed is in the kinetic domain, likelihood (spontaneity) domain, likelihood (spontaneity) is in the thermodynamic domain.is in the thermodynamic domain.