Chemical Kinetics in Real Time: Using the Differential Rate Law and Discovering the Reaction Orders

The reaction between 2,4-dinitrochlorobenzene and piperidine to form 2,4-dinitrophenylpiperidine in ethanol is a very suitable experiment for the undergraduate physical chemistry laboratory. The reaction, being first order with respect to each reactant, exhibits overall second-order kinetics. In this paper we describe the kinetic anlysis, in real time, of the spectrometrically-acquired rate data based on the differential rate law, where no assumptions in initial reactant concentrations are necessary, except for their values. The use of scientific spreadsheets for the numerical differentiation and regression analysis is discussed. A distinctive advantage in analyzing the data this way is that it allows the student to "discover" the order of the reaction with respect to each reactant, and hence the rate law.