An Undergraduate Chemical Kinetics Experiment -Formation of the Chromium-EDTA Complex-

Abstract

The reaction on the title is experimentally examined for an undergraduate chemical kinetics experiment. This reaction has been used as the basis for an experiment for an undergraduate course of University of Pennsylvania, whose results of practices were already reported by C. E. Hedrick. It is necessary to re-examine the reaction condition and the method of caluculation of the reaction rate, since the reaction proceeds not so simply as Hedrick reported. In our experiment, the concentration of the Cr³⁺--EDT A complex formed is continuously measured by the spectrophotometrical techniques and is recorded on the chart paper. The reactions are run at the definite temperatures of 20-50℃, the definite concentrations of 0.75×10^-3-4.50×10^-3 of Cr³⁺ mole/litre and 8×10^-6-4×10^-5 of H⁺ mole/litre, and in the presence of the excess EDTA. Usually, with the proceeding of the reaction, there is a considerable change in the hydrogen ion concentration and this change dominates the reaction rate. The initial rate of the reaction is, therefore, tried to obtain prior to the determination of the rate equation. In the above procedure, the rate of complex formation, r is found to obey the following equation: r=k〔Cr³⁺〕〔H⁺〕^-1 The accurate observation of the reaction rate requires, first of all, the precise measurement of PH value of the reaction system. The reaction of Cr³⁺-- EDT A complex formation is the successive one and the rate equation obtained satisfactorily explains the reaction mechanism. Therefore, this kinetics experiment may be appropriate for the prelude to a kinetics experiment concerning the steady-state method.