A Spectrophotometric Determination of the Energy of Activation (Ea) for a Complexation Reaction: The Kinetics of Formation of a Cr(III)/EDTA Complex

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 10 Issue 3 (2005) pp 196-199

A Spectrophotometric Determination of the Energy of Activation (E_a) for a Complexation Reaction: The Kinetics of Formation of a Cr(III)/EDTA Complex

Jose C. Barreto,* David Brown, Terry Dubetz, Joseph Kakareka, and Randall S. Alberte

Florida Gulf Coast University, Division of Mathematics and Sciences, the Biotechnology Program, and the Whitaker Center for Math and Science Education, Fort Meyers FL, 33965, jbarreto@fgcu.edu
Received January 28, 2005. Accepted March 7, 2005.

Published online: 23 May 2005

Abstract. For a first-order reaction, the integrated rate expression is ln [R_t]/[R_o] = –kt, where [R_o] and [R_t] represent reactant concentrations at time zero and any time, t, respectively. The formation of a purple complex by Cr(III) and EDTA closely adheres to first-order kinetics and, as the purple complex forms, the absorbance is easily measured over time. To study this reaction, we defined A_t as the product absorbance at any time, and A_f as the final product absorbance. By experimentally determining A_f in advance and measuring A_t at any time during the reaction, the ratio A_t/A_f reveals the extent of the reaction. The rate constant, k, can ultimately be determined from A_t/A_f at various temperatures with a hand calculator. The thermodynamic parameter, E_a, and the kinetic parameter, A, are then obtained from an Arrhenius plot of ln k versus 1/T. The complexation of Cr(III) by EDTA has several important advantages in this thermodynamic and kinetic study. The stoichiometry between reactants and products is 1:1, the reaction can be driven to completion and is irreversible, the final product has a stable color, and the reactants are almost colorless using the concentrations described. The reaction has a marked temperature dependence that allows students to observe the effect of temperature on the rate constant. An introduction to E_a and A in the second semester of general chemistry is very important because it provides a foundation for later discussions of catalysis.

Key Words: Laboratories and Demonstrations; analytical chemistry; drinking water; bottled water; ion chromatography; anion analysis; fluoride; chloride; nitrite; bromide; nitrate; phosphate; sulfate

(*) Corresponding author. (E-mail: jbarreto@fgcu.edu)

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