Chemistry 1011
TOPIC
Rate of Reaction
TEXT REFERENCE
Masterton and Hurley Chapter 11
Chemistry 1011 Slot 5
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11.7 Reaction Mechanisms
YOU ARE EXPECTED TO BE ABLE TO:
• Define reaction mechanism and show how the
reaction order is dependent upon the mechanism
by which a reaction takes place.
• For a reaction taking place in more than one step,
identify the rate determining step and identify
reaction intermediates.
• Determine if a proposed reaction mechanism is
consistent with experimental rate data.
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Reaction Mechanism
• Description of path or sequence of steps by
which a reaction occurs
• Simplest case: single collision
• Frequently more than one step
• Rate expression and order of reaction
depend on mechanism
• This is why rate law and order must be
determined experimentally
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Reaction of CO(g) with NO2(g)
CO(g) + NO2(g)  NO(g) + CO2(g)
• At high temperatures, reaction is single step
• Rate = k[CO] x [NO2]
• At low temperatures, reaction is two step
process
NO2(g) + NO2(g)  NO3(g) + NO(g)
CO(g) + NO3(g)  CO2(g) + NO2(g)
CO(g) + NO2(g)  NO(g) + CO2(g)
• Rate = k [NO2]2
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Elementary Steps
• The individual steps that make up a reaction
pathway are called elementary steps
• Steps may be unimolecular, bimolecular, etc
• The molecularity is the number of
molecules that are involved in an
elementary process
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Rate Law for Elementary Steps
• For a complete reaction, the rate law and the
order must be determined experimentally.
• This is because the mechanism may be
simple or complex, and this determines the
rate law and order
• For elementary reactions, however, the rate
law can be determined from the equation
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Rate Law for Elementary Steps
Elementary Step
A  product
A + B  product
A + A  product
2A + B  product
Molecularity
unimolecular
bimolecular
bimolecular
termolecular
Rate Law
Rate = k[A]
Rate = k[A][B]
Rate = k[A]2
Rate = k[A]2[B]
• For the elementary step:
NO2(g) + NO2(g)  NO3(g) + NO(g)
• The rate law is:
Rate = k[NO2]2
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Slow and Fast Steps
• Frequently, one step in a mechanism will be
slower than the others
• This is the rate determining step
• Imagine a two step process:
Step 1: A + B  X + I
SLOW
Step 2: A + I  Y
FAST
Total: 2A + B  X + Y
• The first step is the rate determining step
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Overall Rate Law and Order
• Rate(First Step) = k1[A][B]
• This slow first step determines the overall
rate
• This is the rate law for the overall reaction:
2A + B  X + Y
which is therefore second order
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Mechanisms and Rate Laws
• The rate law expression is determined by
experiment
• The mechanism is a hypothesis about the
way the reaction occurs
• The proposed mechanism must yield a rate
law expression that is consistent with
experiment
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Mechanism with a Fast Initial
Step
• Sometimes the first step in a reaction mechanism,
which results in the creation of a reaction
intermediate, will be FAST
• The second step, where the reaction intermediate
is a reactant, may be SLOW
• When this happens, the rate determining step will
be the second step
• The rate law expression should then include the
concentration of the reaction intermediate
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Reaction Intermediates and Rate
Law
• The concentration of an intermediate cannot
be measured
• The concentration of an intermediate cannot
be included in a rate law expression if this is
to be compared to experiment
• The final rate law expression can only
include species occurring in the balanced
equation
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Reaction Intermediates and Rate
Law - an Example
Step 1: NO(g) + Cl2(g) NOCl2(g) FAST
Step 2: NOCl2(g) + NO(g)  2NOCl(g) SLOW
Overall: 2 NO(g) + Cl2(g)  2NOCl(g)
• Rate of overall reaction = rate of step 2
• Rate = k2[NOCl2][NO]
• This is unsatisfactory;
[NOCl2] cannot be measured
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Eliminating Intermediates from
the Rate Law Expression
• The first (fast) step in the reaction
NO(g) + Cl2(g) NOCl2(g) FAST
is reversible
• The reactants and products are in equilibrium
rate forward = rate reverse
k1[NO][Cl2] =k-1[NOCl2]
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Eliminating Intermediates from
the Rate Law Expression
[NOCl2] = k1[NO][Cl2]
k-1
Substitute in overall rate law expression
Rate of reaction = rate of step 2
Rate = k2k1 [NO]2[Cl2]
k-1
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Limitations of Mechanism
Studies
• Mechanisms are suggested in order to
explain observed rate laws and orders of
reaction
• Often more than one mechanism can
explain experimental results
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