Untitled - Kelly Walsh High School

200 CHEMISTRY FOR THE UTTERLY CONFUSED
The latter method is usually preferable, and we shall employ it in this example.
When we rearrange the equation to find the rate constant, k, and enter the appropriate
values:
k
We will now examine a problem that is not first order. Molecules of butadiene
(C 4H 6) will dimerize to C 8H 12. The rate law is second order in butadiene, and the
rate constant is 0.014 L/mols. How many seconds will it take for the concentration
of butadiene to drop from 0.010 M to 0.0010 M?
This is a second-order reaction. The following relationships apply to first order
reactions:
These are the only equations we have exclusive to second-order reactions. For
this reason, one or more of these equations will be necessary to work the problem.
(In this problem, A is butadiene.)
Using the list of terms ([A] 0, [A] t, k, t, and t1/2), and the given information, we
find that k 0.014 L/mols, [A] 0 0.010 M, [A] t 0.0010 M, and t ? s The
1
only second-order relationship with these four terms is: . We
1
kt
can rearrange this relationship to isolate the time and enter the given values:
t
ln [A] 0
[A] t
t
1
[A] t
Rate k[A] 2
1
[A] 0
k
ln [1.000]
[0.355] 1.035637
0.243679 0.244 min1
4.25 min 4.25 min
1 1
[0.010] 0
0.014 L/mol·s
[0.0010] t
(900)
0.014 s 1 64285.7 6.4 10 4 s
1
[A] t
1
kt t1/2
[A] 0
1
k[A] 0
(1000 100) L/mol
0.014 L/mol·s
Kinetics is the study of the speed of reactions. The speed of reaction is affected
by the nature of the reactants, the temperature, the concentration of reactants,
the physical state of the reactants, and catalysts. A rate law relates the speed of
reaction to the reactant concentrations and the orders of reaction. Integrated
rate laws relate the rate of reaction to a change in reactant or product concentration
over time. We may use the Arrhenius equation to calculate the activation
[A] t
[A] 0