Iodine Clock Reaction

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Clock reactions

If you choose a project that explores the kinetics of a chemical reaction you will need a way of measuring the rate of the reaction. Clock reactions provide an interesting way of doing this for some systems.

In a typical reaction the first part of a graph showing the concentration of product against time is approximately a straight line (see Figure 1). If you choose any value of concentration that lies on this straight line (say c1) the initial rate of reaction can be found by dividing this concentration by the time taken to reach it (t1).

[pic]

Figure 1

If you measure the time taken for the same concentration to be reached in a series of reactions, you will be finding the time for the same amount of product to be formed for each reaction. The shorter the time, the faster the reaction is occurring. You can therefore take 1/t as a relative measure of the initial rate of reaction.

The trick, of course, is knowing when the fixed amount of product has been formed. The following examples illustrate how this can be done.

Appearing blue
There are a number of so called 'iodine clock' reactions in which molecular iodine is one of the products. Probably the most famous of these is the reaction involving hydrogen peroxide and iodide

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