The amounts of reactants and products in a chemical reaction are related by stoichiometry. Therefore, the concentration of any reactant or product can be used to express the rate of a reaction. Take for e.g., in the reaction A + B C for every molecule of A consumed, a molecule of B is also consumed and thereby a molecule of C, the product, is formed. Hence, the rate of a reaction can be expressed as
For a reaction of the type A + 2B C + 2D, the rate expression changes. For every molecule of A consumed, two molecules of B are required to form one molecule of C and two molecules of D. In other words, the concentration of B changes at twice the rate of change of concentration of B changes at twice the rate of change of concentration of A, while the concentration of D increases at two times the rate of decrease of A. Therefore, the rate of reaction is given by,
From the above example, it should be clear that the rate of a reaction has no meaning unless it is measured with respect to a particular species in the reaction.
Example 1:Express the rate of the reaction given, in terms of reactants and product concentrations.
The rate of the reaction is expressed as