Rates of reaction - kinetics (A2)
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I am investigating the reaction between iodine and propanone in the presence of sulphuric acid catalyst, by changing the concentrations of the reactants.
I am using colorimetry and am producing 6 different concentrations of propanone. I am planning to put each concentration in a cuvette and I shall measure the absorbance every 30 secs for 6 mins. How do I plot this on a graph? Do I need to do a calibration curve, if so how?
Igloo writes
First estimate the initial concentration of iodine in your mixture. This allows you to decide on the range of concentrations for your calibration curve.
For example, let's assume that its original concentration is 0.1 mol dm-3 and you are pipetting, say 25 cm3, into a flask in which the total volume eventually becomes 100 cm3 (when you've added the propanone, sulphuric acid and water(?)). In this case, the 25 cm3 will have been dispersed in 100 cm3 of solution and will have been diluted four times, so its new concentration is 0.025 mol dm-3 (just before it starts to be destroyed by the propanone). By the time you have finished taking readings, the concentration may well be zero. (Note that you may be using different concentrations and volumes - so you must allow for that.)
Next, make up, shall we say, five solutions of aqueous iodine - in our case 0.025, 0.020, 0.015, 0.010, 0.005 and 0.000 mol dm-3 (this last one could be pure water). You may decide to have more solutions than this in between. Measure accurately the absorbance of each of these. Take your time. Their concentration should remain constant! Next, plot a calibration curve of absorbance (y-axis) against concentration.
Finally, carry out your experiments as you've described, using your calibration curve to convert all your absorbance readings into iodine concentrations, and follow this up by plotting concentration against time (well, this is one way to present your results, if you're planning to measure the half life of the reaction).
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updated: 09 November 2004
