Untitled - Kelly Walsh High School

Kinetics 193
Don’t Forget!
the concentration of HI to drop from 0.200 M to 0.190 M at 25°C? Answer:
1.10 10 20 s
This is a simple plug-in problem, with k 2.40 10 21 /M s, [A] 0 0.200 M, and
[A] t 0.190 M. You may simply plug in the values and then solve for t, or you first
rearrange the equation to give t (1/[A] t 1/[A] 0)/k. You will get the same
answer in either case. If you get a negative answer, you interchanged [A] t and [A] 0.
The order of reaction can be determined graphically by using the integrated
rate law. If a plot of the ln[A] versus time yields a straight line, then the reaction
is first order with respect to reactant A. If a plot of 1/[A] versus time yields
a straight line, then the reaction is second order with respect to reactant A.
The reaction half-life, t 1/2, is the amount of time that it takes for a reactant concentration
to decrease to one-half its initial concentration. For a first-order
reaction, the half-life is a constant, independent of reactant concentration and
has the following relationship:
t 1/2
For second-order reactions, the half-life does depend on the reactant concentration.
We calculate it using the following formula:
t 1/2
0.693
k
Radioactive decay is a first-order process. See Chapter 20 for a discussion of
half-lives related to nuclear reactions and other information on radioactivity.
The rate constant for the radioactive decay of thorium-232 is 5.0 10 11 /year.
Determine the half-life of thorium-232.
This radioactive decay process follows first-order kinetics. Substitute the value
of k into the appropriate equation:
t 1/2 0.693/k 0.693/5.0 10 11 year 1 1.4 10 10 year
1
k[A] 0
The half-life must have units of time never time 1 .