Rates of reaction - kinetics (A2)
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Hello again! I'm investigating the kinetics of the reaction between propanone and iodine by changing the concentrations of the reactants and the cataylst (sulphuric acid). I can't find any information about quantities of solutions needed when you change the concentration of iodine and sulphuric acid. Do I have to determine them from a pilot test? My teacher says I need an equation for the reation between propanone and iodine including the sulphuric acid. I thought you just had to put H2SO4 above the arrow? Could you also give me the equation for the reaction between sodium hydrogencarbonate and the products from the reaction between propanone and iodine. Many thanks!
Corrie writes ...
First of all, the catalyst here is H+(aq), from the acid, and being a catalyst, does not appear in the overall reaction equation. However, it will have to appear in at least one of the steps in the mechanism of the reaction if it is acting as a catalyst.
As far as amounts of solutions are concerned, this depends partly on how you intend to follow the rates of reaction: by titration, colorimetrically or by an initial rate method. All three have been used here, and rely on measuring the disappearance of the iodine. It sounds as if you are using the titration method, so I'll stick to that for now.
Let's say you start with a 0.02 M iodine solution. You'll need a sodium thiosulphate solution of similar concentration to give you a workable titration value. In order to study the effect of decreasing iodine concentration (as it is used up) on rate, you will need to ensure that the concentrations of acid and propanone in the mixture are in good excess, so that their concentrations don't change during the reaction. (If you think about it, the acid concentration shouldn't change anyway - why? But there is a little acid produced by the reaction as well.)
Having made up suitable stock solutions of acid, propanone (in water) and iodine (in KI(aq)), you will need to do some pilot experiments mixing them together in different proportions so that:
1. the titration value (of thiosulphate) for the first sample you remove is suitable, i.e. several cm3, so that you can obtain enough subsequent readings for a reasonable graph.
2. the reaction is fast enough to get sufficient readings done in the time available.
3. the acid and propanone are always in good excess throughout.
Having established the order with respect to iodine, you can now repeat the experiment, changing the concentration (e.g. double or half) of each of the other substances involved in turn. How you interpret the results will depend on the order you found for iodine!
The hydrogencarbonate solution into which you dump your samples before titrating them is used to neutralise the acid present, thereby stopping ('quenching') the reaction while you do the titration.
So the reaction here is just between H+ ions and hydrogencarbonate ions, producing water and carbon dioxide - I'll leave you to complete the equation!
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updated: 13 January 2007
