Practical investigations
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I'm currently doing my A2 investigation on the reaction between potassium iodide ions (1.00 mol dm-3) and potassium peroxodisulphate (0.04 mol dm-3), using the iodine-clock method. I've got my experimental data, but now I have to find the rate and I am stuck on the first step - finding the concentration. I have no clue in how to work out the concentration of iodide ions in each reaction mixture.
Igloo says ...
Remember that at the start the iodide ions are being oxidised to iodine by the peroxodisulphate ions, but that the iodine formed is immediately reduced by thiosulphate ions to form iodide ions again. So as long as ther eis thiosulphate present, the iodide ion concentration is constant.
To find the initial concentration of iodide ions you need to find out by how much your potassium iodide solution has been diluted. If, for example, you are using 5.0 cm3 of potassium iodide solution, and, after adding the sodium thiosulphate, the potassium peroxodisulphate, the starch and the water, the total volume is 15.0 cm3, then it has been diluted by a factor of 3. So the initial concentration of iodide ions is 1.00/3 = 0.33 mol dm-3.
If the experiment is repeated using different initial concentrations of iodide ions (by varying the volume of KI solution used), you will be able to plot a graph of initial rate of reaction against initial concentration of iodide ions. This graph can be used to determine the order of reaction with respect to I- ions.
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updated: 20 January 2006
