You can make the time taken to produce a particular amount of iodine obvious if you add thiosulphate ions to the mixture at the beginning, as thiosulphate ions turn iodine back to iodide ions. So no starch-iodine colour will appear until all of the thiosulphate has been used up, you will suddenly see the clear mixture transform into a blue-black. By measuring the length in time it takes for this to happen you will be able to find out how long it took to produce the equivalent amount of iodine.
In this experiment I will be using this clock method in order to find out how the reaction rate depends on the concentration of iodide ions in the reaction mixture.
You may find it useful to plot your results on a graph via computer, but I have learnt that it’s a much better idea to draw graphs by hand, as computers can't always include all the vital information.
Method:
1: Firstly I made up the reaction mixture no.1 from the table below, and measured how long it took for the blue-black iodine-starch colour to appear. I then continued this with the other mixtures in the table, which only differ in the concentration of iodide ions, the water is added to keep the volume at a constant. The temperature was also kept the same to ensure constant conditions.
The potassium iodide, sodium thiosulphate, starch and the water were already attained, so all I had to concentrate on was the concentration of iodide ions, and for mixtures 2,3,4 and 5 the water.
2: Finally make sure that the mixture is mixed well with a thermometer after the peroxodisulphate(VI) has been added, and start the stopwatch and watch very closely as it happens very suddenly, and if not looking can go unnoticed.
Mixture Volume of Volume of Volume of Volume of Volume of
KI(aq) /cm water /cm Na S O (aq) starch K S O (aq)
/cm solution /cm /cm
1 5 0 2 1 2
2 4 1 2 1 2
3 3 2 2 1 2
4 2 3 2 1 2
5 1 4 2 1 2
Table of results:
Mixture Time
1 1.08
2 1.03
3 1.53
4 3.18
- 8.04
Finding the order of reaction:
Using the results above I can now perform the calculations. To find the order of the reaction with respect to iodide ions, I will have to know how the initial rate of the reaction varies with iodide ion concentration. In order to do this I will have to do the following;
- Work out the amount of I produced before the blue-black colour appears.
- Use this amount of I and the clock time to work out the initial rate of reaction for each mixture.
- Plot a graph of the initial rate of reaction against [I ].
- Use the graph to determine the order of the reaction with respect to I ions.