ABSTRACT
The numbers −1, −3, and 2 are the stoichiometric coefficients of A, B, and P, respectively, in this reaction; by convention they are positive for products and negative for reactants. For a reaction of this kind the rate of reaction is the rate of consumption of reactants or formation of any product divided by the appropriate stoichiometric coefficient, i.e. (in this example),
An elementary reaction is one in which no reaction intermediates have been detected or need to be considered to describe the chemical reaction on a molecular scale. Such reactions are said to occur in a single step. The term molecularity, which applies only to an elementary reaction, refers to the number of molecular particles involved in the microscopic chemical process. With reactions in solution, solvent molecules are counted in the molecularity only if they enter into the overall process, but not if they merely provide an environment. For example,
has a molecularity of unity and is said to be unimolecular. For a simple reaction of the form
(7.1)
where CA and CP are concentration of reactant and product respectively. rA is the order of the reaction, and k is the rate constant and is a function of temperature. For Arrehenius type of reaction it is given as
(7.2)
The initial rate or initial velocity v0 of the reaction can be determined from
(7.3)
[CA]0 is the initial concentration of the substrate and [CA]1 at time=t1 The relationship between initial rate and initial substrate concentration for 0, 1, and 2
order reactions are shown in the Fig. 7.1. Hence this technique
could be followed in the laboratory to differentiate between various reaction orders. If the reaction is of the form A+B→P, the general form of the rate equation may be
. Once again the orders n and m can be estimated by determining the initial rates with respect to A and B independently, by keeping the initial concentration of the other substrate constant.
