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ClockRxnKinetics

ClockRxnKinetics - Experiment 19 Revision 1.3 Determination...

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Experiment 19 Revision 1.3 Determination of the Rate Constant for an Iodine Clock Reaction To learn about Integrated Rate Laws. To learn how to measure a Rate Constant. To learn about Clock Reactions. In this laboratory exercise, we will measure the Rate Constant k for the oxidation of Iodide (I - ) by Peroxysulfate (S 2 O 8 2- ). 2 I - (aq) + S 2 O 8 2- (aq) I 2 (aq) + 2 SO 4 2- (aq) (Eq. 1) This reaction is referred to as a Clock Reaction because the progress of (Eq. 1) is monitored by a secondary Clocking reaction which consumes the product I 2 as it is produced and triggers a color change when the Clocking Reagent is itself completely consumed. For this purpose, we use Thiosulfate (S 2 O 3 2- ) as the Clocking Reagent to reduce the I 2 : 2 S 2 O 3 2- (aq) + I 2 (aq) S 4 O 6 2- (aq) + 2 I - (aq) (Eq. 2) As long as any Thiosulfate Ion is present, none of the Iodine produced in (Eq. 1) remains; it is consumed as quickly as it is produced. (This, of course, requires that (Eq. 2) proceed very rapidly. This is in fact the case.) As soon as the Thiosulfate is used up, Iodine will begin to appear in the solution. The presence of Iodine is then dramatically detected by the formation of a blue Starch-Iodine molecular complex and the reaction is said to “Clock” at this point. (Actually, the starch molecules complex with I 3 - which is formed by a reaction between I 2 and I - .) We can set the Clock Point by simply adding differing amounts of Thiosulfate Ion to the solution. The Rate at which a chemical reaction proceeds is typically influenced by the amount of each reactant present and the temperature of the reaction vessel. And, typically, this relationship between the Reaction Rate and Reagent Concentration takes a simple form known as the Rate Law: Rate = k [A] n [B] m (Eq. 3) where A and B are generic reacting Species, k is a reaction specific proportionality constant known as the Rate Constant, and n and m are the Reaction Order. The Rate Law parameters k , n and m must be determined experimentally. Once the Rate Law has been established, the Rate of the chemical reaction can be determined. However, this is usually not the information which is sought. Consider how it would be received if while traveling over hill and dale to grandma’s house, one of the kids asks, “How long till we
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P a g e | 2 get there?”, and the father answers, “We are traveling at 60mph!”. Similarly, we usually want to know how long it will be until the reaction is complete, or, alternatively, how much of a given reagent remains after a specified period of time, not how fast the reaction is proceeding at a given moment. (This is a bit of an over statement, but . ..) The Rate itself is not an answer to either of the above queries. In order to answer questions like “How Long?” or “How Much Remains?”, we must relate the
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ClockRxnKinetics - Experiment 19 Revision 1.3 Determination...

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