Rates of reaction - kinetics (A2)
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I am studying the autocatalytic reaction between potassium manganate and ethanedioic acid. To work out how the rate varies with concentration I took 10 cm3 portions from the reaction mixture every 5 minutes and added it to 10 cm3 potassium iodide to stop the reaction (manganate(VII) ions reacted with the iodide ions to produce iodine). Then I titrated the mixture with sodium thiosulphate so I could work out how much iodine was produced and therefore the concentration of manganate(VII) ions that there were at this stage in the reaction. From this I could find out how the concentration and rate vary with time, and prove that the process is autocatalytic. I did all this and got the volume of sodium thiosulphate from the titration, but I don’t know what to do with this value and how to do all the calculations to get to my conclusion. How do I work it out from here if I don’t have the volume of iodine, only of KI known?
Igloo says ...
Presumably the potassium iodide solution being used is sufficiently concentrated to ensure that adding 10 cm3 of it provides an excess of iodide ions, and therefore guarantees that all the manganate(VII) ions have been destroyed, producing iodine in the process:
2MnO4- (aq) + 16H+ (aq) + 10I3 (aq) ---> 2Mn2+ (aq) + 8H2O(l) + 5I2 (aq)
This iodine is then titrated with sodium thiosulphate solution:
I2 (aq) + 2Na2S2O3 (aq) ---> 2NaI(aq) + Na2S4O6(aq)
If you study these two equations carefully you should see that 2 moles of manganate(VII) in the original reaction mixture will eventually give rise to a need for 10 moles of sodium thiosulphate, but that in general, the more manganate(VII) ions present the greater the volume of sodium thiosulphate solution finally needed.
In fact it should be clear that the volume of sodium thiosulphate solution required in the titration is directly proportional to the number of moles of manganate(VII) ions remaining in the reaction mixture.
Thus there is no need to calculate the concentration of manganate(VII) ions in the mixture for each portion removed (though it would be possible to do so). All you have to do is to plot the volume of thiosulphate used (the titration reading) against the time, and you should obtain the shape of curve appropriate for an autocatalytic reaction.
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updated: 28 April 2004
