Concentration Dependence

The rate of a reaction often depends on reactant concentrations, sometimes on product concentrations, and occasionally on the concentration of a species, X, which is not involved in the stoichiometric reaction. The dependence on concentrations can often be expressed by an equation, called the empirical rate law:

     (1.1)

In Eq. (16-4), k is called the rate constant, the exponents a, b, c, and x are called orders. The orders are usually integers, but may be fractions such as 1/2 or 2/3. A positive order a means that the rate increases with [A], a negative order c means that the rate decreases with increasing [C]. If the rate depends on [X] and x > 0, we say that X is a catalyst which increases the rate; if x < 0, we say that X is an inhibitor which decreases the rate. The sum of all the individual orders is called the overall order of the reaction.

Despite the apparent similarity of an empirical rate law to an equilibrium constant expression, the orders are not necessarily equal to stoichiometric coefficients.

Suppose in a reaction whose rate depends only on [A] and [B] that a = 1 and b = 2; the overall order then is 3. We say that the reaction rate is first-order in A, second-order in B, and third-order overall.

The units of Rate are mol L^-1s^-1, and [A] [B]² has units (mol/L)³, the rate constant k has units (mol/L)^-2s^-1. The units of k depend on the overall order of the reaction.