Untitled - Kelly Walsh High School

190 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
concentration of reactant A is doubled, the rate will also double ([2] 1 2) and
if the concentration of reactant B is doubled, the rate will increase fourfold
([2] 2 4). This reaction is first order with respect to reactant A and second order
with respect to reactant B. In this example, the rate equation would then be:
Rate k[A][B] 2 (If the exponent is 1, it is generally not shown.)
If the concentration of a reactant changes and that has no effect on the rate of
reaction, then the reaction is zero-order with respect to that reactant ([2] 0 1).
Many times, we calculate the overall order of reaction: it is simply the sum of
the individual coefficients, third order in this example.
The rate law (the rate, the rate constant, and the orders of reaction) is determined
experimentally.
Once the rate has been determined, the orders of reaction can be determined
by conducting a series of reactions in which we change the concentrations of the
reactant species one at a time. We then mathematically determine the effect on
the reaction rate. Once the orders of reaction have been determined, we calculate
the rate constant.
Consider the reaction:
We collected the following kinetics data:
2 NO(g) O 2(g) l 2 NO 2(g)
Experiment Initial [NO] Initial [O 2] Rate of NO 2 formation (M/s)
1 0.01 0.01 0.05
2 0.02 0.01 0.20
3 0.01 0.02 0.10
There are a couple of ways that you might interpret the data above in order to
determine the rate equation. If the numbers involved are simple, then one can
reason out the orders of reaction. You can see that in going from experiment 1
to experiment 2, the [NO] doubles ([O 2] held constant) and the rate increased
fourfold. This means that the reaction is second order with respect to NO.
Comparing experiments 1 and 3, you see that the [O 2] doubles ([NO] was held