14.4 reaction mechanism -...

February 181 Reaction Mechanism

14.4 Reaction Mechanism

Steps of a Reaction

Fred Omega GarcesChemistry 201Miramar College


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

10 Km

50 Km

100 Km

Ozone Layer

Stratosphere

Mesosphere

TroposphereMt. Everest

TroposphereStratosphereMesosphere


Mechanism of Ozone; Chapman Cycle

Chapman cycle shows that O3 exist at steady state. It is constant in the stratosphere.

O

O2

O3

O

O2hν

hν

O2

O2

O3

2 O

O

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 g 2O2


Reaction Coordinate for ozone destruction

A Key reaction in the upper atmosphere is

O3 (g) + O (g)® 2O2 (g)

The Ea (fwd) is 19 kJ, and the DHrxn as written is -392 kJ.

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

O3 + O

2O2

O kJ

19 kJ

-392 kJ

Reaction Progress

Eact (rev)= 411 kJ


1. Water Vapors: H2O OH• + H•H• + O3 OH• + O2OH• + O H• + O2

Net: O + O3 2O2

2. N2 , Dinitrogen : N2 + O2 2NONO + O3 NO2 + O2NO2 + O NO + O2

Net: O + O3 2O2

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

ClO• + O Cl• + O2Net: O + O3 2O2

Path to Destruction: Ozone

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


Influence 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

Ener

gy (k

J)


Reaction MechanismThe mechanism of a reaction is the sequence of steps (at the molecular level) that shows how reactant chemicals combine to form the final products.

Elementary StepsSequence of steps which describes an actual molecular event.

StoichiometryThe overall stoichiometric reaction is the sum of the elementary steps.

Scientist want to learn about mechanisms because an understanding of the

mechanism (how bonds break and form) may lead to conditions to improve reaction

product yield, (or prevent side products formation. i.e, depletion of ozone.)


Ozone: Revisited

Chapman�s CycleO3 ® O2 + OO + O3 ® 2O22O3 ® 3O2

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.

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.


Elementary Step: Rate LawConsider the following proposed mechanism for the conversion of NO2 to N2O5. What is the rate law.Step1 NO2 + O3 ® NO3 + O2 (slow)Step2 NO3 + NO2 ® N2O5 (fast)

rate = k1[NO2]1 [O3]1

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

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


Elementary Steps: Order of reactionElem. Step Rate Law Order Molecularity

1 A ® Product Rate = k[A] 1st order unimolecular

2 2A ® Product Rate = k[A]2 2nd order bimolecularA + B ® Prod. Rate = k[A][B]

3 3A ® Product Rate = k[A]3 3rd order Termolecular2A + B ® Product Rate = k[A]2[B]A + B + C ® Prod Rate = k[A][B][C]

* Termolecular mechanism (3-elementary step) is very rare.Scientist who propose such a mechanism must make careful measurements.


Multiple Elementary Steps Most reactions involve more than one elementary step.

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

Reaction is only as fast as the slowest elementary step

Analogy: Leaving class after an exam.

On a single lane highway, speed of traffic is only as fast as creepy crawler 12-cars ahead.


Rate Determining Step from Rate Law

Consider: NO2 + CO g 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 limiting 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 consistent with the experimental data.

There may be other mechanisms that are consistent with experimental data as well.

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


RDS and Rate Law: ExampleConsider the reaction : NO (g) + O3 ® NO2 + O2Two mechanisms (elementary steps) are proposed:

Mechanism 1 NO + O3 ® NO2 + O2Proposed rate law: Rate = k [NO] [O3]

Mechanism 2 O3 ® O2 + O (slow)NO + O ® NO2 (fast)

Proposed rate law: Rate = K [O3]

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

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


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

Consider: A ® BMechanism: A D int (fast)

int ® B (slow)NET: A ® B

RATE = k[int]-but the rate law cannot be written in terms of an intermediate(catalyst and reactant okay, but not intermediate).

-It must be expressed in terms of stable speciesHow is the Rate Law modified?


Modification of Rate Law

RATE = 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]


Rate Laws from Mult. Steps Mechanism.Consider the reaction below, what is the rate law based on the two

proposed mechanism:2 NO2 (g) + O3 ® N2O5 + O2

Mechanism (1) Mechanism (2)NO2 + NO2 D N2O4 (fast) NO2 + O3 D NO3 + O2 (slow)N2O4 + O3 ® N2O5 + O2 (slow) NO3 + NO2 ® N2O5 (fast)

Rate = k [N2O

4] [O

3]

keq=

[N2O

4]

[NO2] [NO

2]

[N2O

4] = k

eq[NO

2]2

Rate = k ∗keq

[NO2]2[O

3]

Rate = k[NO2]2[O

3]

Termolecular

€

Rate = k[NO2] [O3]bimolecular

Rate is based onslowest elem. step


Rate Laws from Mult. Steps Mechanism.The decomposition of hydrogen peroxide is catalyzed by iodide ion. The

catalyzed reaction is thought to proceed b 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)


Enzyme Catalysis Reaction

Consider oxidation of ethanol to aldehyde:CH3CH2OH (l) ® ADH ® CH3CHO + R-H2

ADH - Alcohol dehydrogenaseMechanism

E + S D ESES ® 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]


In Class Exercise

Phosgene (Cl2CO) a poison gas used in WW1, is formed in the following reaction.Mechanism: i) H+ + Cl2 D Cl + HCl fast, reversible

ii) HCl + CO D ClCO + H+ fast, reversible

iii) ClCO + Cl® Cl2CO slow

i) What is the overall reaction?

ii) What is the intermediates and the catalyst for the reaction?

iii) What is the rate law from this mechanism?

iv)What is the order of the reaction with respect to each reactant?

v) What is the overall molecularity?

vi) How would doubling the concentration of CO affect the reaction?


Summary

The dynamics of the series of steps of a chemical change is what

kinetics tries to explain. Variation in reaction rate are observed

through concentration and temperature changes, which operate on

the molecular level through the energy of particle collision. Kinetics

allows us to speculate about the molecular pathway of a reaction.

Modern industry and biochemistry depend on its principles.

However, speed and yield are very different aspects of a reaction.

Speed is in the kinetic domain, yield is in the equilibrium domain.

14.4 reaction mechanism -...

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