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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 15 (2010) pp 158-161

A Spectroscopic Analysis of the Kinetics of the Iodine Clock Reaction without Starch

Elizabeth Stock and Matt Morgan*

Department of Chemistry, Hamline University, St. Paul, MN 55104,
Received November 4, 2009. Accepted January 14, 2010.

Published: 10 March 2010

Abstract. The Landolt iodine clock reaction is commonly used to illustrate chemical kinetics in introductory chemistry courses. The ionic reactants in a typical iodine clock reaction are iodide and peroxydisulfate, with thiosulfate and starch as an indicator. The starch reacts with the iodine to form a dark blue starch complex. The rate of reaction can be deduced by equating the reaction rate to 1/time. This method, however, requires many assumptions that are difficult for students to understand. In this experiment we want to calculate the rate law of the iodine clock reaction by measuring the absorbance of I2 as it forms with a visible spectrometer without the use of thiosulfate or starch. Direct correlations between the slopes of the graphs of absorbance vs. time and the concentrations of reactants illustrate the reactionís first order behavior with respect to the individual reactants. The calculated initial rate law based on the slopes further proves the reaction to be first order for each reactant. The iodine clock reaction can now be conducted without the starch and thiosulfate to allow students to calculate the rate law of this reaction.

Key Words: Laboratories and Demonstrations; general chemistry; kinetics

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