Iodine Clock Reaction

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Kinetics of the Harcourt-Essen Reaction

Name: Manpreet Kaur

Candidate Number: 7123

AS and A2 Aims:


1. Investigate the effect of temperature on the rate of reaction.

For this aim 3 sets of results will be obtained by timing how long it takes for the colour change to occur in different temperatures determined by the use of an electric water bath. With these results, the effect of temperature on the rate of reaction will be investigated.

2. Determine the activation enthalpy with and without the catalyst ammonium molybdate(VI) and with different protic acids and use this to compare the effectiveness.

To find out which catalyst is most effective, this aim will be carried out as an iodine clock reaction. The goal of this aim is to find out what catalyst is best to make this reaction occur at the fastest rate.

3. Determine the effects of the presence of ethanol on the rate equation.

It is known that ethanol effects hydrogen peroxide and therefore it has an effect on the rate equation. This aim will find out the effect of ethanol by carrying out the iodine clock reaction with and without ethanol present and the results will be compared to draw a conclusion.


4. Investigate the order of reaction with respect to hydrogen peroxide, iodide and acid.

The goal of this aim is to find out what order of reaction each chemical produced by making a graph for each of the reactants and analysing the line of best fit.

5. Investigate the rate equation, rate constant and possible mechanism for this reaction.

Investigating the possible mechanism will depict why the reaction occurs as it does. This aim is theory based and it will inform as to how the chemicals react and how this forms the products. By finding the rate equation, the dependence of rate on the concentration can be worked out. The rate constant will help to work out the rate equation.

6. Determine how the presence of a catalyst changes the mechanism of the Harcourt-Essen reaction and whether the mechanism is effected by monoprotic, diprotic or tripotic acids.

Catalysts speed up chemical reactions and so this theory based aim will find out how this happens and which catalysts is the most efficient at increasing the rate of reaction.

7. To find out how the concentration of iodide ions and peroxodisulphate (VI) ions affects the rate of reaction.

Potassium iodide reacts with hydrogen peroxide to form iodine, in this aim the concentration of iodide and peroxodisulphate ions will be measured to see how this effects the rate of reaction.

Chemical Ideas:

The equation for this experiment is as follows:

H2O2(l) + 2I-(l) + 2H+ ( I2(s) + 2H2O(l)

Half equations:
2I- ( I2 + 2e-
H202 + 2H+ + 2e- ( 2H2O

In this reaction the iodide ions are oxidised by the hydrogen peroxide and turned into iodine, the hydrogen peroxide gains electrons and is turned into water.

The iodine clock reaction occurs in two parts the first of which is the reactions between iodine and sodium thiosulfate, which produces iodide ions and a tetrathionate anion. The blue-black colour of the starch-iodide complex shows up when all of the thiosulphate ions have reacted with the iodine. The reaction is shown below:

I2(s) + 2S2O32-(l) ( 2I- + S4O6(l)-

The iodide ions being reacting with the starch once the thiosulphate ions have been exhausted, this produces the starch-I5- complex that gives the blue-black colour that is recorded to give the amount of hydrogen peroxide that has reacted and the time that the reaction took. The equation is shown below:

I2 + starch ( starch-I5- + I-

The specifics of the reaction between the starch and iodine are not completely known but it is thought that the coils of amylose are where the iodine fits. The iodine transfers charge to the starch, which...
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