Passage V (Questions 24-28)
Two theories for predicting the rate of a chemical reaction are given below. As you read them, look for both their similarities and their differences. Then answer the questions that follow.
The rate of a chemical reaction is defined to be the number of moles of a specified reactant that is consumed per unit of time. Because reactants must collide in order for a reaction to occur, it might seem that reaction rates would depend upon the concentration of reactants--since the more reactants are present, the greater the likelihood of a reaction. This is, in fact, the case, as the following example shows.
For the reaction
N2O5 --> 2NO2 + 1/2 O2
the rate is proportional to the amount of N2O5 present. We may express this fact as a "rate law":
rate = k [N2O5]
where k is the rate constant.
A more complicated reaction will have a more complicated rate law. For the reaction
2NO + O2 --> 2NO2
the rate law will be
rate = k [NO]2[O2]1
where the powers in the rate law reflect the coefficients of the reactants in the chemical equation. This relationship between numbers of reactant molecules and exponents in the rate law is a general one.
Theory I is very often true, for it expresses the reasonable insight that the greater the concentration of reactants, the greater the likelihood of a reaction. It has a great shortcoming, however, in its assumption that all reactions proceed in one fell swoop rather than in several steps. Let us take an example, using the letters A, B, and C, to represent molecules. In the reaction
A + 2B --> C
Theory I predicts a rate law of
rate = k[A][B]2
But suppose the reaction actually proceeds in two stages, with the first one being
A + B --> AB
AB +B --> C
Let us also suppose that the first stage is much slower than the second, perhaps taking thousands of times as long. Then the rate of the overall process will essentially be the same as the rate of the first stage, which is given by
rate = k [a][B]
in contrast to Theory 1's prediction. Theory 2 implies, then, that we must understand the details of the reaction, including the relative speed of the various subreactions, in order to predict a rate law. Theory 1 is not totally wrong, just incomplete.