Gary L. Bertrand
Professor Emeritus of Chemistry
Missouri University of Science and Technology
(formerly University of Missouri-Rolla)
a) The initial concentrations of components may be set in their stoichiometric ratio, so that this ratio of concentrations is maintained throughout the reaction.
b) The concentrations of all but one reactant may be set in large excess over that component. Under these conditions, the concentrations of the other components are essentially constant while that of the component of interest goes from a finite concentration to almost zero.
c) The temperature may be chosen so as to minimize the effects of other, competing reactions.
These conditions allow the study to be conducted by studying the changes in concentration of a single component with time. This allows a simpler mathematical description of the changes that are occurring. For a generic chemical reaction:
The rate of reaction is known to depend on the concentrations of some or all or the reactants, in some cases on the concentrations of some of the products, and possibly on the concentrations of other materials such as catalysts. Temperature usually has an effect (rates normally increase with temperature). The medium (gas phase, solvents, etc.) and even the shape of the container may have an effect. For simple reactions, the rate may often be described mathematically as a rate expression:
In order to simplify the mathematical relationships, experiments are often conducted under special conditions. One such situation is to have the intial concentrations of the reactants in their stoichiometric ratio:
so that the concentrations of A and B remain in that ratio as the reaction progresses:
The rate expression may then be simplified (?) to:
and may be treated as if there is only one reactant (A) with a pseudo-order of a + b ,
and an apparent rate constant k', with
Another simplifying technique is to have one reactant in large excess over the other, so that the concentration of the excess component does not change appreciably during the reaction:
and may also be treated as if there is a single reactant (A) with a pseudo-order of a and an apparent rate constant (k')
These simplified forms all have the basic relationship:
For n = 1 (first-order or pseudo-first-order):
The initial concentration of the component is not normally known with great accuracy because of uncertainties associated with starting the reaction. These equations are then considered to have two unknowns, the initial concentration ([A]t=0) and the apparent rate constant (k').
If the order or apparent order of the reaction is known, the concentration of component A may be measured as the reaction occurs, preferably covering the times in which the concentration changes from 2/3 of the initial value to 1/3 of the initial value. The apparent rate constant may then be determined graphically or by regression of the appropriate function of concentration
(ln[A]t or 1/[A]tn-1) vs time (t).