Kinetics

1
Kinetics

• Looking at rates of reactions

A B ? C t (min) A B C
(M) (M) (M) 0 100 100
0 1 72 72 28 2 52 52 48 3 37 37 63
4 27 27 73 5 19 19 81 Ave. rate
DC/Dt 81M-0 M 16.2 M/min 5 m-0
m Ave. rate -DA/Dt -(19M 100 M) 16.2
M/min 5 min 0 min
2
Kinetics

• Average Reaction Rates

A B ? C t (min) A B C
(M) (M) (M) 0 100 100
0 1 72 72 28 2 52 52 48 3 37 37 63
4 27 27 73 5 19 19 81 Ave. rate
-DA/Dt -(19M 100 M) 16.2 M/min
overall 5 min 0 min Ave. rate
-DA/Dt -(72 M 100 M) 28.0 M/min 1st
minute 1 min 0 min Ave rate -DA/Dt
-(19M 27 M) 8.0 M/min 5th minute 5
min 4 min
3
Kinetics

• Average Reaction Rates

A B ? C t (min) A B C
(M) (M) (M) 0 100 100
0 1 72 72 28 2 52 52 48 3 37 37 63
4 27 27 73 5 19 19 81 A big issue is
that the average rates are just
thataverages. Alsoyou can see that the rate of
the reaction varies quite a bit during the course
of the reaction. We need to be more precise in
our ability to look at reaction rates, to do this
we often use
4
Kinetics

• Instantaneous reaction rates

A B ? C Instantaneous rates
slope of a line tangent to the curve.
Which tangent (or instantaneous
rate) will have the largest value? The
one at t 0 The instantaneous rate at t 0 is
also referred to as the initial rate of the
reaction, because this is the rate at the very
beginning before there are any product present
(which causes the reverse reaction to become
important). The initial rate of the reaction can
be very useful in looking at kinetic data (and
will come up again) What do we do once we have
the instantaneous reaction rates?
5
Kinetics

• Instantaneous reaction rates

A B ? C t A instantaneous rate
rate/react (min) (M)
M/min
min-1 0 100 33.0 0.33 1 72 23.8 0.33 2
52 17.2 0.33 3 37 12.2 0.33 4 27 8.9
0.33 5 19 6.3 0.33 Two big things to take
away from this data! 1. The rate of a reaction
is proportional to the concentration of
reactant 2. The ratio of rate/react is
essentially constant We can summarize this with
the equation
6
Kinetics

• Instantaneous reaction rates

A B ? C t A instantaneous rate
rate/react (min) (M)
M/min
min-1 0 100 33.0 0.33 1 72 23.8 0.33 2
52 17.2 0.33 3 37 12.2 0.33 4 27 8.9
0.33 5 19 6.3 0.33 rate kreactantsn The
rate constant k, is independent of reactants,
but is dependent on temp. We call this a
differential rate law or a rate law. For the
above reaction the rate law would be Rate
-DA/Dt kAa Bb
7
Kinetics

• Instantaneous reaction rates

A B ? C t A instantaneous rate
rate/react (min) (M)
M/min
min-1 0 100 33.0 0.33 1 72 23.8 0.33 2
52 17.2 0.33 3 37 12.2 0.33 4 27 8.9
0.33 5 19 6.3 0.33 Rate -DA/Dt kAa
Bb The values of k, a, and b MUST BE
determined experimentally Sometimes the
coefficients of the reactions will match a and b,
but this is only by coincidence. These values
MUST be determined experimentally. How do we do
that? The isolation method
8
Kinetics

• Isolation Method

A 2B ? C We first assume that the rate law has
the form Rate k Aa Bb Experiments will be
carried out a number of times where the initial
concentration of one of the reactants will be
varied. Here is some typical data
Exp Initial Rate A B
(M/s) (M)
(M) 1 5.5 x 10-6 0.1 0.1 2 2.2 x
10-5 0.2 0.1 3 1.65 x 10-5 0.1 0.3