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Rate Law Determination Lab

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Sue Stathopulos

Investigating the Effect of Concentration on an Iodide Persulphate Reaction and Rate Law DeterminationthLab Performed on Monday February 25 2013IntroductionThe purpose of this lab is to observe the effect of concentration on reaction rates to ultimately 2determine the rate law for the reaction between the iodide ion I and the persulphate ion SO The 28general reaction can be described asNHSO2KII2KSO4228aqaq2aq24aqHowever we know that in aqueous solution the ions will dissociate except for iodine into their ionic components The ions that are not a part of the actual reaction do not affect the ionic compounds in any ways as they are unreactive The reaction can be rewritten as22SO2II2SO28aqaq2aq4aqIn this experiment the experimenters are interested in using the rate of oxidation of the iodide ion by the persulphate ion to determine the rate law for this reaction The rate of reaction between the two ions will be measured near the beginning of the reaction before the concentrations of the components in the system have changed significantly New trials will be run such that one concentration of a specie is altered from that of the original reaction and the rate will be measured once again To determine the reaction rate the amount of product formed or reactant used within a time interval must be measured For this experiment the time required for a specific amount of diiodine I to be produced will be measured2To monitor when a specific amount of diiodine has been produced the diiodine will have to react with a thiosulphate ion2 2I2SO2ISO2 aq23aqaq46aqAs long as thiosulphate ions are still present in the aqueous solution as the reaction proceeds the reaction will remain colourless in the presence of starch since the thiosulphate ions will be using up the diiodine molecules According to the chemical reaction formula two moles of thiosulphate is required to react with one mole of diiodine The first appearance of free diiodine is detected by its reaction with starch This means that the initial amount of thiosulphate ions added into the reaction has been used up so there is nothing for the diiodine molecules to react with other than starch The reaction between diiodine and starch will yield a violetdarkblue colour due to the starchiodine complex formed The reaction should be timed from the moment the two solutions are mixed together to the moment a violetdarkblue colour is visible Therefore the amount of iodine produced within this time interval will be known since the amount of thiosulphate used is known The process of thiosulphate ions reducing iodine back to iodide before the iodine can form its complex with starch to yield a violet colour is called a clock reaction because this process is used to clock the reaction between persulphate and iodide2SO231AverageRateoftheReaction2To determine the order of reaction with respect to the iodide ion the concentration of persulphate must remain constant as the concentration of iodide ion varies Levine 2002 The relationship between the change of concentration of iodide ion to the time it takes for a constant amount of thiosulphate to be used up will aid the experimenters in determining the order of the reaction with respect to I Levine 20022
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