Abstract
For a first-order reaction, the integrated rate expression is ln [Rt]/[Ro] = –kt, where [Ro] and [Rt] 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 At as the product absorbance at any time, and Af as the final product absorbance. By experimentally determining Af in advance and measuring At at any time during the reaction, the ratio At /Af reveals the extent of the reaction. The rate constant, k, can ultimately be determined from At /Af at various temperatures with a hand calculator. The thermodynamic parameter, Ea, 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 Ea and A in the second semester of general chemistry is very important because it provides a foundation for later discussions of catalysis.
