Rates of reaction - kinetics (A2)
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I am investigating the rate of reaction between iodine and propanone using an acid catalyst (sulphuric acid). I have two problems.
First, for making up my dilutions of iodine, my school has iodine solution (very small brown glass bottle which I assume is very concentrated) and KI. I know I need to make up the iodine solution in the KI, but not knowing the concentraion of the iodine solution I don't know how to do this to a known concentration, and don't know if the concentration of KI I use matters - or if I can treat this the same as using water to make dilutions.
Secondly, using the titration method, how can I measure the rate of reaction WRT propanone and the acid? Do I need to use a different solution (than sodium thiosulphate) to titrate with? If so, what should I use?
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Ulex replies
If you are going to offer this as an investigation, I think you have a bit of a problem. This experiment is thoroughly written up in many books and articles on A-level and equivalent practical work on kinetics. It has a long history, dating, as far as I recall, from the mid-1960s when it was used as a practical exam. With the greatest respect to yourself, I think it unlikely that you will come up with an original way of doing it when so many have worked on it for so long.
Having said this, it is a very elegant experiment so that if you a simply looking for a satisfying experiment to do, this would be a good choice. It isn’t really an investigation, though.
Two very pleasing methods are given in Nuffield A-level Chemistry, Students’ Book, 4th edition Topic 11, pp 249-250.
The titration method is one of these and you could extend it by changing the intial concentrations firstly of the propanone and secondly of the acid. Since you know that the reaction is zero order with respect to the iodine, determining the rate of reaction should be a straightforward matter.
To go back to your first problem – iodine is a solid at room temperature and is virtually insoluble in water. Your solution must therefore be in KI already or it may be ‘tincture of iodine’ which is a solution of iodine in ethanol. A very cautious sniff should tell you which!
If it is in ethanol, I don’t think it is useable in this experiment. If it is in KI, dilute a known volume of it a known number of times and titrate with sodium thiosulphate and, from the result, work out the concentration.
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updated: 01 February 2008
