Rates of Reaction

Introduction: The rate of a reaction has a special meaning for chemists.

Definition: The rate of a reaction is usually expressed in terms of a change in concentration of one of the participants per unit time.

rate = [ ]            t

The rate of a reaction is the speed at which reactants are converted into products. Experiments show that for most reactions, the []'s of all participants change the most rapidly at the beginning of the reaction. That is, the [products] shows the greatest rate of increase and the [reactants] shows the greatest rate of decrease at the beginning. This means that the rate of a reaction changes with time. Therefore a rate must be specified with a specific time unit.

A + B ---> AB

Methods of Measuring Reaction Rates

Techniques used to measure concentration vary with the reaction and available apparatus. Titrations can be done on reactions in solution. Changes in colour can be indications of concentration change and can be measured in a spectrometer. Density and electrical conductivity may vary with concentration. For gases a pressure change may tend to indicate a concentration change. The rate of a reaction is usually measured as a function of the concentration of a reactant or product over time. This does not have to true in all cases. The main feature of a rate is that it must be able to be measured physically. i.e. pressure, temperature, concentration, colour.