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Experiment: Determine the Rate Law and Activation Energy of the Iodine Clock Reaction

The “iodine clock” reaction involves the mixing of two colorless solutions to produce a solution which remains colorless for a precise amount of time, then suddenly changes to a deep purple-blue color (as seen in the demonstration below). The time is controlled by the temperature and/or the concentrations of the reactants.

The reaction you will study involves the oxidation of iodide ion (I-) to dissolved tri-iodide ion (I3-):

6 H+ + IO3- + 8 I- $\to$ 3 I3- + 3 H2O

Triiodide ion combines with starch indicator to produce the characteristic purple-blue color. Timing how long it takes for the blue color to appear allows the rate of the reaction to be measured.

Experimental Goals

In this lab simulation, your goals will be to:

1. Determine the rate law for the iodine clock reaction by using the method of initial rates to determine the reaction order for each reaction (IO3-, I- and H+) and the rate constant for the reaction.
2. Determine the activation energy for the iodine clock reaction by measuring the rate constant for the reaction at different temperatures.
3. Use the experimentally determined rate law to examine the plausibility of proposed mechanisms for the iodine clock reaction.

Lab Instructions

Download lab instructions for carrying out this experiment as either a MS-Word or PDF file.

These instructions contain an extensive background explaining the basis for the iodine clock reaction and how to use an online lab simulation (Links to an external site.) of the iodine clock reaction (provided by Prof. Gary Bertrand, Missouri University of Science and Technology) to collect experimental data. They also contain details of how to analyze the data to determine the rate law and other kinetic parameters.

When you have completed the experimental analysis you will answer a set of postlab questions included in the instructions file.