The Law of Mass Action
229
where the quantities in brackets are the molar concentrations of the reactants;
a, b, c, m, n, . . . are the coefficients in the balanced chemical equation; and£ is
a proportionality constant (called the rate constant) that is characteristic of
each specific reaction.
One must immediately be aware of the limitations of the law of mass action.
Almost every chemical reaction is in actual fact an extremely complicated
process, and the familiar balanced chemical equation (which shows the molar
relationships between the original reactants and the final products) gives no
clue at all to the many intricate sequences of simple intermediate steps that are
followed in going from "reactants" to "products." Always bear in mind the
following points.
1. The law of mass action really applies only to each of the individual
intermediate steps in a chemical reaction.
2. The actual overall rate expression for a given chemical reaction can be
determined reliably only by experiment. It often is a very complicated
equation, appearing to bear no relation to the overall chemical equation
in the way that Equation 15-2 (the rate equation) is related to Equation
15-1 (the chemical equation).
3. The simple intermediate steps that make up a reaction mechanism
invariably involve (a) spontaneous decomposition of one molecule, (b)
most commonly a bimolecular collision between two molecules, or (c)
an unlikely termolecular collision between three molecules. From a
practical standpoint, nothing more complicated is ever observed.
It is important to know the overall rate expression for a reaction, because this
permits you to control the reaction and to predict the reaction times needed for
different conditions. This expression usually can be determined experimentally
without any knowledge of the reaction mechanism itself; in fact, it is a useful
aid to working out the mechanism. One objective of the material that follows is
an explanation of the manner in which the empirical (experimentally determined) rate expression is obtained for a great many reactions.
We shall not consider those reactions that have very complicated rate expressions. Instead, we shall consider those for which the empirical rate expression/.?
of the form given by Equation (15-2),
V = £[A]«[B]*[C]
C
•
(15-2)
but where a, b, and c are not necessarily the same as the coefficients in the
balanced chemical equation (Equation 15-1).
There are some common terms that are used in studies of reaction rates. One
speaks of the order of the reaction as being the sum of the exponents (a + b + c
+ • • •) in the empirical rate expression, and of the order of a reattant as being
229
where the quantities in brackets are the molar concentrations of the reactants;
a, b, c, m, n, . . . are the coefficients in the balanced chemical equation; and£ is
a proportionality constant (called the rate constant) that is characteristic of
each specific reaction.
One must immediately be aware of the limitations of the law of mass action.
Almost every chemical reaction is in actual fact an extremely complicated
process, and the familiar balanced chemical equation (which shows the molar
relationships between the original reactants and the final products) gives no
clue at all to the many intricate sequences of simple intermediate steps that are
followed in going from "reactants" to "products." Always bear in mind the
following points.
1. The law of mass action really applies only to each of the individual
intermediate steps in a chemical reaction.
2. The actual overall rate expression for a given chemical reaction can be
determined reliably only by experiment. It often is a very complicated
equation, appearing to bear no relation to the overall chemical equation
in the way that Equation 15-2 (the rate equation) is related to Equation
15-1 (the chemical equation).
3. The simple intermediate steps that make up a reaction mechanism
invariably involve (a) spontaneous decomposition of one molecule, (b)
most commonly a bimolecular collision between two molecules, or (c)
an unlikely termolecular collision between three molecules. From a
practical standpoint, nothing more complicated is ever observed.
It is important to know the overall rate expression for a reaction, because this
permits you to control the reaction and to predict the reaction times needed for
different conditions. This expression usually can be determined experimentally
without any knowledge of the reaction mechanism itself; in fact, it is a useful
aid to working out the mechanism. One objective of the material that follows is
an explanation of the manner in which the empirical (experimentally determined) rate expression is obtained for a great many reactions.
We shall not consider those reactions that have very complicated rate expressions. Instead, we shall consider those for which the empirical rate expression/.?
of the form given by Equation (15-2),
V = £[A]«[B]*[C]
C
•
(15-2)
but where a, b, and c are not necessarily the same as the coefficients in the
balanced chemical equation (Equation 15-1).
There are some common terms that are used in studies of reaction rates. One
speaks of the order of the reaction as being the sum of the exponents (a + b + c
+ • • •) in the empirical rate expression, and of the order of a reattant as being