Determining the Order of Reaction: Methods and Examples

Q#9.1: How order of reaction is experimentally determined.

Ans. Order of reaction is determined by the following two methods.

1. From Rate Law.

   In this method, the rate of reaction is measured using chemical and physical methods (volumetric, gravimetric, spectrophotometer, pH-meter, conductometric methods). The dependence of the rate of reaction on the concentration of reactants tells us about the order of reaction. For example, a reaction in which the rate of reaction doubles when the concentration of reactants is doubled will be of first order. If the rate increases four times by doubling the concentration, the reaction will be of second order.

2. Ostwald’s Isolation Or Large Excess Method.

   This method is used to determine the order of complicated reactions. For a reaction with two or more different reactants, the order can be found by considering the rate of reaction for one reactant while keeping the concentration of the remaining reactants constant. The same procedure is then applied to other reactants and so on.