Chemical kinetics is the branch of chemistry that is concerned with the mechanisms and rates of chemical reactions. The mechanism of a chemical reaction is a description of what happens to each molecule at a very detailed level—which bonds are broken, which new bonds are formed, and how the three-dimensional shapes of the chemicals change during the course of the reaction. The rate of the reaction is a measure of its speed. The rate of a chemical reaction can be measured by how quickly the reactants disappear, or by how quickly the products are generated. The iodine clock reaction is a favorite demonstration in chemistry classes because it has an element of drama. Two clear solutions are mixed, producing a new clear solution. Then, after a period of several seconds, the solution turns dark blue. A demonstration of this reaction is shown in the video below.

Experiment Reactions

As mentioned, chemical kinetics measures how fast a reaction is occurring. For most chemical reactions, the rate is so fast that special equipment is needed to measure it. For the iodine clock reaction, on the other hand, the rate can be easily measured by monitoring the color change of the reaction. To perform the iodine clock reaction, you will mix potassium iodide, sulfuric acid, starch, and thiosulfate. The time it takes for the reaction mix to turn blue will be measured with a timer.

The reactions that form the basis for the iodine clock reaction are shown below.

Equation 1

IO3 (aq) + HSO3 (aq) → I (aq) + H+ (aq) + SO42– (aq)

Equation 2

H+(aq) + I(aq) + IO3(aq) → I2(aq) + H2O(l)

Equation 3

I2 (aq) + HSO3(aq) + H2O (l) → I (aq) + SO42–(aq) + H+(aq)

Equation 4

I2(aq) + Starch → Dark-blue colored complex

This experiment involves a reaction that is sometimes called an iodine clock reaction. There are a number of different combinations of chemicals that give a reaction of this type. What happens, essentially, is that there are two different reactions: one in which iodine is produced (a slow reaction) and one in which the iodine produced in the first reaction is used up (a fast reaction). By carefully controlling the quantities of reactants, you can obtain a situation in which the reactant in the second reaction is used up first, allowing iodine to form at that point. At very low concentrations the iodine then combines with starch to suddenly give a deep blue-black color, at a time determined by the conditions used. Hence the term “iodine clock”. The time elapsed from when the solutions were first mixed together until the point when the blue-black color appears is measured, and from this time measurement the rate of the reaction can be determined. You will alter the conditions of concentrations of reactants in Part I and temperature in Part II in order to determine their effect on reaction rate.

Procedure Handout

Please read carefully the lab procedure before moving on to the pre-lab quiz. The pre-lab quiz is specifically focused around a detail of the lab procedure. The handout is found at the bottom of this page as an attachment.