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14102410-Peroxysulfate-Kinetics - CHEM 3364 Spring 2008 1...

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CHEM 3364 Spring 2008 1 Kinetics of an Iodine Clock Reaction Objective: Determine the experimental rate law for a peroxysulfate decomposition reaction. Find its activation energy. Consider appropriate reaction mechanisms that match the experimental rate law. Introduction: Thermodynamics allows us to predict whether a reaction is favorable. Exactly how the reaction occurs on a microscopic level, however, cannot be determined from G. The process by which a reaction occurs, what we call the reaction’s mechanism, can often be deduced by measuring how fast the reaction occurs. We can do this because a reaction’s rate depends on the rate of effective collisions between reacting species. This collision rate is directly related to the concentrations of those species. The overall reaction rate is obtain by appropriately combined the collision rates of species that participate in any step up to and including the slowest or rate- determining step. The relationship between the reaction’s rate and the concentrations of species affecting the rate, R, is given by the rate law, which takes the general form R = k[A] α •••[D] δ where k is the reaction’s rate constant and the terms in brackets, [ ], are concentrations. The superscripts α δ are called reaction orders. The purpose of this experiment is to characterize the reaction between iodide, I - , peroxydisulfate (or persulfate), S 2 O 8 2- , and thiosulfate, S 2 O 3 2- . This reaction system is an example of what is commonly known as a “clock reaction.” At one step during the reaction, a product is made which reacts with an indicator solution; the resulting change in color is used to time the reaction. Preliminary work will focus on identifying the individual steps comprising the overall reaction. An investigation of the reaction’s kinetics provides additional information. Solution Preparation You will need to prepare 200 mL each of the solutions listed below. With the exception of the peroxysulfate solution that must be prepared each week, these solutions, if well sealed, can be stored and used for several weeks. If you run out of any solution, you can prepare an additional amount. Be sure to note, however, the cautionary statements in Tasks I and II. 0.20 M KI 0.20 M KCl 0.0050 M Na 2 S 2 O 3 0.10 M Na 2 SO 4 0.10 M K 2 S 2 O 8 (this must be prepared each week) Prepare these solutions using appropriate volumetric glassware. Two additional solutions – a saturated solution of I 2 and a starch indicator solution – are available in the lab.
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CHEM 3364 Spring 2008 2 Preliminary Investigations Much of the chemistry in this system can be deduced using simple experiments in which two or more reagents are mixed together and the results recorded. Each preliminary investigation is followed by one or more questions. The answers to these questions are important so give them careful consideration. When asked to write tentative reactions, put them in the following general form: R 1 + R 2 P 1 + P 2 where R represents a reactant and P represents a product. Whenever possible, replace R and P with known reactants and products. If you cannot identify a reactant or product (but know that
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