Aim To study the effect of concentration of iodide ion solution on the rate of iodide ion I- oxidation by peroxodisulphate ion S2O82- using iodine clock reaction

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To study the effect of concentration of iodide ion solution on the rate of iodide ion I- oxidation by peroxodisulphate ion S2O82- using iodine clock reaction


In an iodine clock reaction, two clear solutions would be added together in a common container where no apparent reaction takes place. After a short delay, at a sudden, the clear solution would turn into a blue-black solution.

In this experiment, two clear solutions – potassium iodide, KI and sodium peroxodisulphate, Na2S2O8 – would be added together, with delaying additives, where a blue-black product solution would be observed.

The aim of this experiment is to measure the different time taken to form the blue-black solution for different concentration of potassium iodide solution used. The different rates of reaction for each concentration can then be determined because they are the reciprocal of the times taken.

The chemical reaction that takes place is a redox reaction where iodide ion is oxidized and peroxodisulphate ion is reduced. The full ionic equation for the reaction is represented by:

I-  (aq)    +        S2O82-  (aq)        →      I2  (aq)    +              2SO42- (aq)

* all potassium and sodium ions are spectator ions.

However, without any delaying mechanism, the formation of the blue-black starch complex is instantaneous. In order for the experimenter to be able to measure the time for the formation of the blue-black solution, and by extension rate of reaction, a delaying mechanism needs to be introduced.

Sodium thiosulphate would be added, in additional to potassium iodide and sodium peroxodisulphate, to convert iodine back into iodide as represented by the ionic equation:

I2  (aq)    +        2S2O32-  (aq)        →      I-  (aq)    +              S4O62-  (aq)

After all thiosulphate is exhausted, iodine would then be free to form a blue-black starch complex.


According to the Collision Theory1, for a reaction to happen, the two reacting particles involved must:

  1. collide with one another

  1. the collision must be energetic enough to overcome the activation energy of the reaction

  1. the collision must occur with the correct geometrical alignment, i.e it must bring the reactive parts of the molecules into contact in the correct way.


The theory also states that effective collision can be affected by the concentration of reactants and temperature. In this experiment, the concentration factor is being studied.

In terms of the iodine clock reaction, the concentration of iodide ions would, according to the Collision Theory, affect the rate of conversion of iodide to iodine. As the concentration decreases, the iodide particles per volume decrease.

Consequently, as the iodide particles per volume decrease, with all other variables remain equal, the probability of collision between iodide and peroxodisulphate ions, would decrease. By extension, the number of effective collisions would also decrease.

As the number of effective collisions decrease, the rate of reaction would decrease, signaled by the increase in time taken for formation of the blue-black solution.

The presence of delayer - thiosulphate - would not have a net effect on the reaction because the concentration of thiosulphate is fixed throughout the experiment.

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  1. Independent Variable      : Concentration of iodide solution (or concentration of KI solution used)

  1. Dependent Variable : Time taken for formation of blue-black solution, rate of reaction

iii)        Fixed Variable        : Volume of sodium peroxodisulphate Na2S2O8 solution used, volume of

sodium thiosulphate Na2S2O3 solution used, volume of starch

solution used, volume of iodide I-  solution used

Pre-Experiment Preparation / Method Designing

Using trial and error approach, a pre-experiment exercise was performed to determine the suitable amount of solute and concentration of chemical solutions used. Most notably, the amount of sodium thiosulphate used was ...

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