Reaction Mechanisms

Elementary Reaction Steps - Molecularity

We have made a big deal out of the fact that you cannot predict the rate law for a chemical reaction from the balanced equation for that reaction. We now introduce a new class of simple reactions, called elementary reaction steps, for which the rate law IS determined by the balanced reaction equation. We also introduce a new term, molecularity, which tells us the number of molecules involved in an elementary reaction step.

We emphasize that the reaction velocity for each elementary reaction step IS determined by the reaction step and, later on, we will string a sequence of elementary reaction steps together to form what is called a mechanism. A reaction mechanism is a detailed (theoretical) description of how we think the chemical reaction proceeds. That is, it describes our thought about which molecule collides with which other molecule to form an intermediate product, which my go on to react with some other species, and so on, to produce the overall reaction. The elementary reaction steps must be balanced (as do all chemical reactions). We can usually tell the difference between an elementary reaction step and a balanced reaction by the fact that the elementary reaction step will have one or more rate constants associated with it, as we will see below.

It is probably easiest to describe the elementary reaction steps and their associated rate laws by just telling you what they are. As we proceed you will become aware of the fact that the rate law is completely determined by the elementary reaction step you write down.  (The converse is not true. We will see in the examples below that several different elementary steps may give rise to the same rate law.)

Unimolecular Reaction Steps

The elementary reaction step,

,               (1)
is unimolecular because there is only one molecule reacting, that is, molecule "A" is reacting. This unimolecular reaction step implies the rate law,
,               (2)
or, equivalently,
.               (3)
In words, these elementary reaction steps say that the molecule, A, spontaneously transforms into B at some rate k1. The algebraic sign in front of k1 tells whether you are gaining product or losing reactant depending on whether the concentration in the derivative is increasing or decreasing. For example, in Equation 2, [B] is increasing, and in Equation 3, [A] is decreasing. (All of the usual rules of stoichiometry still hold in elementary reaction steps. If you use up some reactant you must gain an equivalent amount of product.)

An elementary reaction step may be reversible or irreversible. Equation 1 is an irreversible unimolecular step. Equation 4 below is a reversible unimolecular step:

.               (4)
This reversible unimolecular step implies the following rate laws,
.               (6)
(Either one of these may be used, depending on whether we are trying to account for the disappearance of reactant, A, or the appearance of product, B, in our mechanism for a particular reaction.)

A unimolecular reaction step can have more than one product, for example,

.               (7)
The unmimolecular process given by Equation 7 implies the same rate law as the reaction in Equation 1, namely, either Equation 2 or Equation 3.

Bimolecular Reaction Steps

There are several varieties of bimolecular steps. For example,

implies the rate law,
,               (9)
,               (10)
and so on. In Equations 8, 9, and 10 we have given only one product, "C." We would get the same rate laws if there had been two or more products, for example as in,
.               (11)

Reversible Bimolecular Steps

The bimolecular reaction

implies the rate law
,              (13)
or it could be written as a rate of loss of A or B as we have seen above. The reversible bimolecular reaction,
,               (15)
and its variants.

Termolecular Reaction Steps

Termolecular reaction steps require three molecules coming together at the same time. They are rare because three-body collisions in the gas phase are rare, but there are cases of termolecular reactions in the literature. There are many varieties of termolecular reactions and they may be reversible. A typical termolecular process might be,

,               (16)
with its implied rate law
.               (17)
A reaction mechanism is a combination of one or more elementary reaction steps which start with the appropriate reactants and end with the appropriate product(s). We will illustrate the construction of a reaction mechanism with the famous Lindemann mechanism for a class of gas-phase reactions (on the next web page).


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Copyright 2004, W. R. Salzman
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Last updated 11 Oct 04