Average rate of a reaction is defined as the rate of change of concentration per unit time. It is calculated by dividing the total change in concentration of any one of the reactant or product by the total time taken to do so. Thus, mathematically

Average rate
                                          


Experimental determination of reaction rate

The rate of a reaction is determined by measuring the concentration of any of the reactants or products after a definite interval of time. For this purpose any measurable property such as change in volume, pressure, refractive index, electrical conductivity or thermal conductivity etc. which is related to the concentration of species participating in the reaction is selected. The change in this property is measured as a function of time. The common practice to find the change in concentration is to withdraw a small amount of reaction mixture (2cm³ to 5cm³) at difference intervals of time. The sample is then placed in the freezing mixture of ice and sodium chloride. It is called freezing of reaction. The concentration at which the given interval is then determined by a suitable method. Then a graph plotted between concentration and time.

Calculation of average rate of the reaction

The average rate of reaction at any instant of time can also be calculated from the data of concentrated and time interval. On the basis of kinetic data a graph is plotted between concentrations versus time;

One the two axis concentration and time, two equidistance points are taken with respect to the time at which the average rate is to be determined. The difference in the concentration corresponding to the points is calculated. The difference of concentration is then divided by the time interval between the selected equidistant points. This is illustrated by the example.

The average rate at time’t’ can be calculated by measuring the change in concentration in the time interval from t₁ to t₂ (where t₁ and t₂ are two equidistant points about t.

Thus, average rate at time‘t’



In a similar manner the average rate can also be determined by measuring the change in concentration or any other measurable property of product species as a function of time.

The above idea is illustrated by taking the following example:

Formation of ethyl acetate by reaction of acetic acid with methyl alcohol.
                          


The above reaction was performed by warming 500 cm³ of 1 M solution of acetic acid. The concentration of acetic acid (one of the reactant) and methyl acetate (one of the product) were determined separately at regular time intervals and were subsequently plotted as function of time.