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Kinetics 191
Don’t Forget!
constant) and the rate doubled. Therefore, the reaction is first order with
respect to O 2 and the rate equation is:
Rate k[NO] 2 [O 2]
The rate constant can be determined by substituting the values of the concentrations
of NO and O 2 from any of the experiments into the rate equation above
and solving for k.
Using experiment 1: 0.05 M/s k[0.01 M] 2 [0.01 M]
k 0.05 M/s/[0.01 M] 2 [0.01 M]
k 5 104 /M2s However, sometimes because of the complexity of the numbers, you must manipulate
the equations mathematically. We use the ratio of the rate expressions of
two experiments to determine the reaction orders. We choose the equations so
that the concentration of only one reactant has changed while the others remain
constant. In the example above, we will use the ratio of experiments 1 and 2 to
determine the effect of a change of the concentration of NO on the rate. Then we
will use experiments 1 and 3 to determine the effect of O 2. We cannot use experiments
2 and 3 since both chemical species have changed concentration.
In choosing experiments to compare, choose two in which the concentration of
only one reactant changes while the others remain constant.
Comparing experiments 1 and 2:
Canceling the rate constants and the [0.01] n 0.05M/s k[0.01]
and simplifying gives:
m [0.01] n
0.20M/s k[0.02] m [0.01] n
1
Q1
4 2 Rm
Thus, m 2 (you can use logarithms to solve for m)
Comparing experiments 1 and 3: 0.05M/s k[0.01]m [0.01] n
0.10M/s k[0.01] m [0.02] n