Kinetics
Reaction Mechanism
Chapter 14 part V
Reaction Mechanisms
• The steps that add up
to the overall
balanced reaction
equation.
• May be simple with 2
or 3 steps or very
complicated in
reactions that may
take many steps.
Consider the reaction A + B  AB. The rate
law for this reaction is k[A]2. A mechanism for
this reaction that meets the criteria given
might look like this:
A + A  A2
SLOW
A2 + B  AB + A
These two steps reveal that when added they
yield the balanced equation. The species A2,
which does not appear in the balanced
equation, is considered an "unstable"
intermediate. See Transition State.
Elementary Steps
Elementary Steps
• Each of the two steps
is an elementary step.
• Each elementary step
is a reaction whose
rate law may be
written from is
molecularity.
• Molecularity is
defined as the
number of species
that must collide to
produce that reaction.
Unimolecular, bimolecular &
termolecular reactions
• Unimolecular: reaction
involving one molecule.
• Bimolecular: reaction
involving two molecules.
• Termolecular: is very rare
and involves three
molecules. These are
rare as it is difficult to
have effective collisions
with three molecules,
Collisions: To be effective they
need sufficient Kinetic energy and
correct orientation
Defining Reaction Mechanism
•
A Series of elementary steps that satisfy
two requirements.
1. The sum of the elementary steps must
give the overall balanced equation for the
reaction.
2. The mechanism must agree with the
experimentally determined rate law.
Elementary steps and Rate Law
Elementary step
Aproduct
A+Aproduct
(2A product)
A+B product
A+A+B product
(2A+B product)
A+B+C product
Molecularity Rate Law
Uni.
rate=k[A]
Bi.
rate=k[A]2
Bi.
Ter.
rate=k[A][B]
rate=k[A]2[B]
Ter.
rate=k[A][B][C]
The Slow Step:
or the Rate Determining Step
• The label SLOW on the first step in the
mechanism produces the "agreement"
with the rate law. Each of the steps in a
mechanism has its own rate but there is
thought to be one which is much slower
than all the others. This step is known as
the rate determining step because the
reaction cannot proceed any faster than
the slowest step in the mechanism.
The Slow Step
• The general rule for assigning the
SLOW step in a mechanism is
that it must contain the same
items (generally reactants) as the
rate law (in the same amounts).
• The slow step has the same rate
law as the experimentally proven
rate law.
•
Rate Determining step
The rate law as
defined by each
elementary step:
Step 1.
Rate = k[NO2]2
Step 2.
Rate=k[NO3][CO]
Which is the rate
determining step?
Does this mechanism
adhere to the
requirements?
Consider the reaction:
• 2 NO2 + F2→2 NO2F; the rate law for this
reaction is k[NO2][F2].
• A proposed mechanism has two steps.
The last step is shown below:
• NO2 + F → NO2F
To write the first step all
we need to do is
compare the balanced
equation with the step
we already have and fill
in the difference:
Mechanism
• NO2 + F2 → NO2F + F
NO2 + F → NO2F
--------------------------2 NO2 + F2→ 2 NO2F
In order for this mechanism to
"agree" with the given rate law the
first step must be SLOW since it
contains the two reactants found
in the rate law. The species F
does not appear in the balanced
equation (and we know F is not a
stable form of fluorine) so it must
be an intermediate.
The reaction energy profile
should show two "hills" with the
first one being higher than the
second. The activation energy of
the overall reaction is thus
determined by the SLOW step.
Because the reaction is
exothermic the diagram ends
lower than it began
Energy Profile
Reaction Mechanisms
• The SLOW steps are not necessarily
the first steps in mechanisms.
• There are many other possible
scenarios for multi-step mechanisms.
• And because proposing mechanisms
for real reactions requires a lot of
experience and background
knowledge.
• You should be able to do the sorts of
things we have done here.
Another proposed mechanism
•
•
•
•
•
NO2 + F2NOF2 +O
NO2 + O NO3
NOF2 +NO2  NO2F + NOF
NO3 + NOF  NO2F + NO2
2NO2 + F2  2NO2F
Slow
Fast
Fast
Fast
• Does this follow the two rules? Is it a
possible mechanism?