A simple method for studying the kinetics of a chemical reaction is described and the significance of reaction orders in deducing reaction mechanisms is demonstrated. In this student laboratory experiment, oxidation of iodide by iron(III) ions in an acidic medium is transformed into a clock reaction. By means of the initial rates method, it is found that the reaction is second order with respect to iodide and first order with respect to iron(III) ions. Having established the rate law, the students are encouraged to propose a possible mechanism for the reaction investigated.

A laboratory exercise has recently been reported in which the students use the initial rates method based on the clock reaction approach to deduce the rate law and propose a reaction mechanism for the oxidation of iodide by iron(III) ions. The same approach is used in the exercise proposed herein; the students determine the dependence of the reaction rate on ionic strength and temperature in a straightforward manner. The previously proposed mechanism is further supported by the results, the rate-determining step is identified, and the activation parameters E a , Δ ⧧ H, and Δ ⧧ S are determined for the overall reaction of the first two steps of the mechanism. The significance of such kinetic studies in elaborating reaction mechanisms is clearly demonstrated to the students.