Reaction Mechanisms
Consider a case where reactants have sufficient energy to
cross the activation barrier to form products.....
How does the reaction occur?
How many bonds are formed/broken?
How many steps are involved?
This is the reaction mechanism
Consider the reaction
NO(g) + O3(g)  NO2(g) + O2(g)
(important in ozone destruction!)
This reaction is thought to occur as the result of a
single collision between an NO and an O3
molecule
What are the necessary conditions for this to
occur?
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Similarly, the gas phase rearrangement reaction
CH3NC(g)  CH3CN(g)
Occurs as the result of a single collision between H3CNC
and another molecule
Processes which occur in a single step (i.e., one collision)
are called elementary steps (or elementary processes)
Note: elementary processes are very rare!! Most reactions
take place in a number of steps!
Definition:
Mechanism = series of elementary steps which add to give
the observed stoichiometric reaction
Steps in a mechanism are always elementary processes!
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E.g., The mechanism for the stoichiometric reaction
NO2(g) + CO(g)  NO(g) + CO2(g)
Appears to involve two elementary steps:
(1) NO2(g) + NO2(g)  NO3(g) + NO(g)
(2) NO3(g) + CO(g)  NO2(g) + CO2(g)
Add these steps together: what happens?
NO3 is an intermediate in this reaction- it is formed in one
step, then consumed in a subsequent step
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Molecularity of elementary steps
Molecularity = number of reactant molecules participating
in an elementary step
1 molecule: unimolecular
E.g., CH3CN  CH3NC
2 molecules: bimolecular
E.g., NO2(g) + NO2(g)  NO3(g) + NO(g), or
NO3(g) + CO(g)  NO2(g) + CO2(g)
3 molecules: termolecular (rare! why?)
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E.g., consider the following mechanism for the conversion
of ozone into O2:
(1)
O3(g)  O2(g) + O(g)
(2)
O3(g) + O(g)  2O2(g)
Write the overall equation for the reaction
Are there any intermediates?
What is the molecularity of each step in the mechanism?
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Rate laws of elementary steps
Rate laws for stoichiometric chemical reactions MUST be
determined experimentally (E.g. method of initial rates)
However, the rate law of any elementary step is based
directly on its molecularity, e.g.,
Unimolecular:
A products
First order: Rate = k[A]
Bimolecular:
A + B  products
Second order: Rate=k[A][B]
A + A  products
Second order: Rate = k[A]2
Termolecular: rate law for A+B+C prod ?
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Remember: reaction order for each reactant in an
elementary step = number of reactant molecules in that
step
E.g., write the rate laws for the elementary steps in the
ozone - O2 example above
Rate laws of multistep mechanisms
What if one elementary step in a multistep mechanism is
slower than the other(s)?
E.g. suppose that one step has a larger Ea (and
hence a smaller k)
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The slow step is called the rate-determining step
The overall reaction rate cannot exceed the rate of the
slowest (rate-determining) step
Therefore: the rate law for the rate-determining step
determines the rate law for the entire mechanism!
Examples:
The following mechanism has been proposed for the gasphase reaction of H2 with ICl:
H2(g) + ICl(g)
HI(g) + HCl(g)
k1
k2
HI(g) + HCl(g)
I2(g) + HCl(g)
Write the balanced equation for the overall reaction.
Identify any intermediates in the mechanism.
If the first step is slow and the second step fast, what rate
law is predicted by this mechanism?
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Note that, when k2>>k1, intermediate is consumed as fast
as it is produced....
What problem arises if the second (or subsequent) step is
slow?
A multistep mechanism will always involve
intermediates
If the second step is slow, the rate law will contain an
intermediate
Why is this not a good thing?
E.g., the stoichiometric reaction
2NO(g) + Cl2(g)  2NOCl(g)
Was found experimentally to obey the rate law
Rate = k[NO]2[Cl2]
The following mechanism has been proposed:
NO(g) + Cl2(g)
k1
NOCl2(g) + NO(g)
NOCl2(g)
k2
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2NOCl(g)
(a) What rate law would be predicted by this mechanism
if the first step were rate-limiting?
Does this rate law agree with experiment?
Note the appearance of intermediate in the rate law: is
there a way of dealing with this?
Assume equilibrium in the first step, i.e., it is reversible
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Problems du Jour
The following mechanism has been proposed for the gasphase reaction between CHCl3 and chlorine:
(1) Cl2(g)  2Cl(g) fast
(2) Cl(g) + CHCl3(g)  HCl(g) + CCl3(g) slow
(3) Cl(g) + CCl3(g)  CCl4(g) fast
(a) What is the overall reaction?
(b) What are the intermediates in the mechanism?
(c) What is the molecularity of each of the steps?
(d) What is the rate law predicted by this mechanism?
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