The Effect of Concentration on Reaction Rate

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The Effect of Concentration on Reaction Rate Introduction: In this experiment, we utilized the ability for the iodide ion to become oxidized by the persulphate ion. Our general reaction can be described as: (NH4)2S2O8 + 2KI Ã I2 + (NH4)2SO4 + K2SO4 (1a) However, we know that in an aqueous solution, all of these compounds except iodine will dissociate into their ionic components. Thus we can rewrite the equation in a more convenient manner: S2O82- + 2I- Ã I2 + 2SO42- (1b) It is important however to note that the NH4 and K ions are still in the solution, they are just unreactive. In order to measure the rate of the reaction, the conventional method would be to measure the species in question at certain times. However, this would be inconvenient, especially for a three hour laboratory period. Since the iodide ion can be oxidized by the persulphate ion, we can use sodium thiosulphate to be an indicator of the presence of iodine in the solution. For this experiment, we can simply calculate the rate of the reaction by timing the amount of iodine being produced in several runs. The reaction between iodine and sodium persulphate can be depicted as: I2 + 2Na2S2O3 Ã 2NaI + Na2S4O6 (2a) Similarly, this reaction above can also be simplified due to dissociation of all the ions except for iodine and persulphate. I2 + 2S2O3 Ã 2I- + S4O62- (2b) An interesting property of reaction (1) is that it produces a brilliant violet colour. However, this violet colour only results in the presence of iodine, or in other words, when iodine is being produced in the reaction. If sodium thiosulphate is added to reaction (1), than as long as there are two moles of thiosulphate for every mole of iodine, the solution will be colourless because the iodine is being used up in reaction (2). However, as time passes, the thiosulphate must run out at some point, and when it does, the violet colour will appear.

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