H. F. Shurvell Queen's University Kingston, Canada
I
The Activation Energy
I
of an Ionic Reaction A physical chemistry experiment
Recently in THIS JOURNAL, an experiment involving the oxidation of iodide ion by persulfate ion was described.' The experiment, in various forms suitable for student laboratory exercises, has also been described previously in laboratory manuals2 and a review of previous work on the reaction has been published.' The experiment can be arranged to furnish rate constants, as well as to st,udy the effect of temperature, ionic strength, and catalysts on the reaction rate. We should like to describe a simple modification of the reaction which can be used to give a value of an activation energy directly. The experiment in the form described here has been used successfully for many years a t Cambridge University and at Queen's University for elementary courses in physical chemistry. The stoiehiometric reaction between persulfate and iodide ions is: The reaction is himolecular4 and follows a secondorder rate equation:
If the reaction is performed urider conditions such that the concentration of iodide ion remains essentially constant, then the reaction is pseudo first order and the rate equation may be written:
where kr = kz[I-1. If the initial concent,ration of persulfate ion is, a, and the concentration at time t , is (a - x), then the integrated rate equation is:
After l/nth of the reaction is complete x
=
a/n and:
The time for a fixed fraction of reaction can be observed by adding a calculated amount of thiosulfate ion and some starch solution to the reaction mixture. The thio-
' MOEWS,P. C., JR., AND PETRVCCI, R. H., J. CHEM.EDUC., 41, 549 (1964). R. J., DENIRO,A. I
