The Clock Reaction

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A popular and entertaining demonstration is the Clock Reaction. Clock reactions serve two purposes. With their relatively slow kinetics, they are used in a laboratory setting to investigate kinetic reactions. In an outreach presentation they are eye catching. In the clock reaction, two clear, colorless solutions are mixed, and after about a minute, the solution suddenly turns color. It’s fun to watch the reaction of the audience.

We use the clock reaction as a metaphor for the passage of time from having fun with chemical magic and entertaining demonstrations as young kids to a more serious approach to chemistry as professionial chemists in our twenties, thirties, forties …. and then some! At that point in our outreach program we transition to more practical, yet entertaining, demonstrations. For example, watch the Combustion, Acidic Oxide, and Basic Oxide videos.

The clock reaction also serves as a transition to our demonstration of the Scientific Method called “The Box”, which we discussed in a previous posting. After mixing the two solutions, nothing initially appears to happen, but we know something is going on from the sudden color change. Clock reactions can be used to emphasize that human senses do not directly detect what is going on “inside the chemical reaction box.” Chemists must measure what they see directly and use the scientific reasoning process to infer what human senses cannot detect directly. This leads us to “The Box” demonstration.

Most clock reactions are based on the formation of elemental iodine which forms a blue-black complex with starch. One of the most popular versions is called “The Old Nassau” reaction developed by Hubert Alyea and his students at Princeton University. After mixing, a yellow-orange color (mercuric iodide) develops followed by the black iodine-starch complex; these are the colors of Princeton. This is also a popular demonstration at Halloween.

We do a similar version, called the Landolt Reaction. The iodine-starch complex is formed, but the solution doesn’t contain mercury. It is preferable due to disposal concerns (however, there is a “mercury-less” version of the Old Nassau). The chemistry of the Landolt is summarized in the pdf document associated with the video.

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