The Reaction: The rate of the reaction between peroxydisulfate ion (often called persulfate for short) and iodide ion (rxn #1) is 0.4 S^-1. In order to time the progress of this reaction, you will run a "clock" reaction simultaneously in the same solution. The clock reaction is a rapid reaction between thiosulfate ion and iodine (rxn #2). Note that iodine is produced in reaction #1 and consumed in reaction #2. Here's how the reaction timing works: The initial reaction mixture will contain relatively large amounts of persulfate and iodide, a small, precisely known amount of thiosulfate, and a few drops of starch indicator (which will turn blue in the presence of iodine). When all the reagents are mixed together at time zero, reaction #1 will begin and iodine will be created as a product. But all of the iodine created will quickly be consumed in reaction #2 and it will not have time to interact with the starch. This will be the situation until all the thiosulfate (S2O3^2-) is used up. Once the thiosulfate is gone, the iodine created in reaction #1 will begin to accumulate in the solution and the iodine and starch will stick together to make a blue-colored complex. So, when your reaction solution turns blue, you are really measuring the time it takes for all the thiosulfate to be consumed. Because you know precisely the moles of thiosulfate you started with, you should be able to use the stoichiometric relationship between thiosulfate and persulfate to calculate how many moles of persulfate were consumed in that same interval of time, which will allow you to calculate the initial rate of the reaction. (Note that during this short time interval, the S2O8^2- and the I2 change very little compared to their initial concentrations). You will then repeat the experiment using different initial concentrations of persulfate and iodide to see how that affects the rate of the reaction. This information will allow you to determine the experimental rate law for the reaction. Finally, you will repeat the experiment at different temperatures to see how that affects the rate of reaction and the rate constant, k. By plotting a graph of ln k as a function of 1/T, you will be able to calculate the activation energy for the reaction.