Lecture Notes, Friday, September 28, 2001

In class on Wednesday we determined the rate law for a specific reaction. It is possible to write a general rate law for any balanced chemical equation. Consider the following general equation for a chemical reaction;

aA(g) + bB(g) ----> cC(g) + dD(g)

The rate law for the reaction is;

rate = k[A]m[B]n

Where k is the rate constant for the reaction and we refer to 'm' as the order of the reaction with respect to A. Similarly 'n' is the order of the reaction with respect to B. The sum of 'm + n' is the overall order of the reaction. For our purposes the sum of the exponents in the rate law will only have values of 0, 1, 2 or 3. Although half orders are possible most of the problems we will concern ourselves with will be whole number exponents. The implication of the exponents and their values will be covered when we discuss mechanism of chemical reactions. While it is possible to write the general rate law given the balanced chemical equation the exact values of 'm', 'n' and 'k' can only be determined from experimental data. It should be noted that the units on the rate constant depend on the order of the reaction.

As shown earlier the order of each reactant must be determined by experiment. When more then one reactant is involved the order is somewhat more difficult to determine experimentally. One straight forward method is to determine the rate by changing one of the reactants concentration while holding the other species concentration constant. From the rates measured we can find the order of the reaction with respect to the reactant whose initial concentration is changed. Let's look at a sample problem where this method is used.

Let's consider the following problem;

The following initial rate data were collected for the reaction

2NO2(g) + F2(g) ---> 2NO2F(g)

at 100 degrees C.

Experiment #

[NO2]

[F2]

Initial Rate (M/min)

1

0.0482 M

0.0318 M

1.9 x 10-3

2

0.0120 M

0.0315 M

4.69 x 10-4

3

0.0480 M

0.127 M

7.57 x 10-3

 

i) Determine the reaction order for NO2(g) and F2(g).

Show how the order with respect to NO2(g) is determined.

Show how the order with respect to F2(g) is determined.

 

ii) Determine the overall order of the reaction.

Show how the overall order for the reaction is determined.

iii) Write the specific rate law for the reaction.

Show the specific rate law for the reaction.

iv) Determine the magnitude and the units for the rate constant of this reaction.

Show how to determine the rate constant for the reaction.