Practical investigations
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I am currently doing my chemistry coursework on the iodine clock reaction with hydrogen peroxide and potassium iodide. I've worked out the order of reaction with respect to each reactant by plotting concentration against rate. I've worked out the rate equation from this, however, I'm not entirely sure how to work out k. I think it's something to do with the gradients of each graph but I've confused myself! Please help!
Ulex replies
You are almost there! The Nuffield Book of Data, p.119, gives two rate equations for this reaction – see item 8. Check that you agree with one of these (in one of these rate equations, the acid is included, in the other it isn’t). The bit of calculus on the left of the rate equation means ‘rate’, incidentally. If you know the rate and each of the concentrations corresponding to that rate, the only unknown is the rate constant.
The problem is that everything must be in the right units. Concentrations must be in mol dm-3 and the rate must be in mol dm-3 s-1. This will enable you to assign sensible units to the value of k.
You could do this for a range of rate values and find the mean of your answers. You could also find k graphically but this would involve a lot more calculation.
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updated: 20 January 2006
