15.4 Mechanisms(SLN)

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Kinetics 15.4

Reaction MechanismsCatalysis

Reaction MechanismsMost reactions do not happen in one step.

e.g.2 NO(g) + O2(g) 2 NO2(g)

Occurs in two elementary steps

2 NO(g) N2O2(g)N2O2(g) + O2(g) 2 NO2(g)

2 NO(g) + O2(g) 2 NO2(g)

{Reaction Mechanism

Reaction Mechanisms are the series of steps that sum to the overall reaction.

Reaction MechanismsReaction Intermediate• A substance that is produced and then

consumed during the overall reaction.

2 NO(g) N2O2(g)

N2O2(g) + O2(g) 2 NO2(g)

2 NO(g) + O2(g) 2 NO2(g)

Produced

Consumed

N2O2 is a reaction intermediate

Determining a Rate Law

• The rate law for an overall reaction can only be found experimentally.

e.g. Consider this overall reaction.NO2(g) + CO(g) NO(g) + CO2(g)

Rate = k [NO2]2 [CO]o

Rate = k [NO2]2

Determining a Rate LawOverall Reaction: NO2(g) + CO(g) NO(g) + CO2(g)

But, rate laws for elementary steps are predictable!

Step 1. 2 NO2(g) NO(g) + NO3(g)

Rate = k1 [NO2]2

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

Rate = k2 [NO3][CO]

For any elementary step…aA + bB dD + eE Rate = k[A]a[B]b

Same rate law as the overall reaction!

Determining a Rate LawOverall Reaction: NO2(g) + CO(g) NO(g) + CO2(g)

Step 1. 2 NO2(g) NO(g) + NO3(g) [Slow]Rate = k1 [NO2]2

Step 2. NO3(g) + CO(g) NO2(g) + CO2(g) [Fast]Rate = k2 [NO3][CO]

NO3 is consumed as soon as it is produced.

Adding CO does not increase the rate, making it zero order in the overall reaction.

The rate of the overall reaction is always equal to the rate of the slowest elementary step.

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Determining a Rate Law

1) Chemists determine the rate law through experimentation.

2) They use the rate law to figure out what the elementary steps are.

– The rate law tells them what the slowest step is.– Then they try to figure out what the fast steps

are.

Catalysis

• A catalyst increases the rate of a chemical reactions by providing a mechanism with a lower activation energy.

• Catalysts are not produced or consumed in the reaction.

• They are there before the reaction starts, and they return when the reaction is complete.

CatalysisThe destruction of ozone.Cl(g) + O3(g) O2(g) + ClO(g)

ClO(g) + O(g) O2(g) + Cl(g)

O3(g) + O(g) 2 O2(g)

Cl is back to react

with another O3

Each Cl(g) atom will destroy about 100 ozone molecules before it reacts with something else and

leaves with stratosphere.

Cl reacts with O3

O3(g) + O(g) 2 O2(g)

Cl

Catalysis and Activation Energy

Ea Ea

O3(g) + O(g) 2 O2(g)

no catalyst O3(g) + Cl(g) ClO(g) + O2(g)

with a catalyst

A catalyst provides a mechanism with a lower activation energy, thereby increasing the reaction rate.

Catalysis and Equilibrium

Ea Ea

no catalystwith a catalyst

A catalyst lowers the activation energy for the forward and the reverse reactions.

Keq for a reaction is the same at the same temperature, with of without the catalyst.

CatalysisHomogeneous Catalyst• The catalyst is in the same phase as the

reactants. e.g. The destruction of ozone. (Cl, O3, and O

are all gases)

Heterogeneous Catalyst• The catalyst is in a different phase than the

reactants.e.g. Synthesis of ammonia. (solid and gas)



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Heterogeneous CatalysisN2(g) + 3 H2(g) 2 NH3(g)

Solid Transitional Metals

NN

HH

HH

H

HReactants are in the gas phase.

Catalyst is a solid.



NN

HH

H H

HH

Reactants slam into the surface.

Bonds within each gas molecule break.

Bonds form between the metal and the individual atoms.



N

NH

H

H H

HH Migration

• Atoms move around on the surface of the metal.



N

N

H

H

H

H

H

H Reaction

• Nitrogen atoms form bonds with hydrogen atoms.



N

N

H

H

H

H

H

H

Desorption

• Products enter the gas phase.



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15.4 Mechanisms(SLN)

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