Investigating the Kinetics
of the
Reaction between Iodide ions and Peroxodisulphate(VI) ions

The reaction between iodide ions and peroxodisulphate(VI) ions follows the overall equation:

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

In this investigation you can:

  1. Determine the Rate Equation for the peroxodisulphate(VI) - iodide reaction.

    Rate = k [iodide]m [peroxodisulphate(VI)]n


    That is, determine the values of m and n.

  2. Determine the Activation Enthalpy for this reaction.

In brief, this is how the experiment works...

The experiment makes use of the initial rates method. All solutions are maintained at a constant temperature of 25 C. The experiment is carried out several times each with a different concentration of iodide ions, keeping all the other components the same. The time taken to produce the same amount of iodine in each is measured. This is done by including a fixed amount of thiosulphate ions in the reaction mixture along with some starch solution. As iodine forms it reacts immediately with the thiosulphate ions and is removed. At the instant when all the thiosulphate is used up, the iodine produced causes the formation a blue-black complex with starch. This set of experiments is now repeated but changing the concentration of peroxodisulphate(VI) ions and again keeping all other components the same.

In each experimental run the total volume will be 15 cm3. This means that the iodide ion concentration in the first set of experiments (and the peroxodislphate(VI) ion concentration in the second) will be proportional to its volume used.

For determining the activation enthalpy, just one of the runs above is chosen and repeated across a range of different temperatures.

Treatment of the results

The iodide ion concentration in each mixture can be calculated. From the volume and concentration of the thiosulphate solution, the amount of iodine (I2) can be calculated. It is the same for each. Knowing the time taken for the production of this amount of iodine in each experimental run, it is possible to work out an initial rate of reaction for each. A graph of initial rate of reaction against iodide ion concentration can be plotted. The order of reaction with respect to iodide ions can be determined from this graph. The same can be done for the second set of experiments, but to determine the order of reaction with respect to peroxodisulphate(VI) ions.

Repeating just one of the experimental runs but at a range of temperatures provides the data for the activation enthalpy to be determined. For each temperature, obtain an initial rate of reaction as above. The Arrhenius equation can now be employed to find the activation enthalpy. A graph of log Rate against 1/T is plotted (T is the Absolute temperature in K) and the activation enthalpy calculated from its gradient.

Chemicals required

The following solutions are needed:

Experimental amounts

The tables which follow give the amounts of reactants used in each set of experiments for finding the orders of reaction.

Rate of reaction with respect to Iodide ions

MixtureVolume
KI(aq)
cm3
Volume
Water
cm3
Volume
Na2S2O3(aq)
cm3
Volume
Starch soln.
cm3
Volume
K2S2O8(aq)
cm3
150212
241212
332212
423212
514212

Rate of reaction with respect to Peroxodisulphate(VI) ions

MixtureVolume
KI(aq)
cm
Volume
Water
cm
Volume
Na2S2O3(aq)
cm
Volume
Starch soln.
cm
Volume
K2S2O8(aq)
cm
120215
221214
322213
423212
524211

Experimental method - Orders of Reaction

This is what you do...

  1. Place the potassium iodide solution, sodium thiosulphate solution and water (except for experimental run 1) into a boiling tube. Add 1 cm3 of starch solution and ensure the contents are mixed.
  2. Measure out the potassium peroxodisulphate(VI) solution into a separate test tube.
  3. Maintain both tubes and contents at 25 C.
  4. Pour the potassium peroxodisulphate(VI) solution into the mixture in the boiling tube. Immediately start the timing. Shake the mixture carefully to ensure complete mixing of the reactants. Record the time taken for the blue-black colour to appear.
  5. Repeat the above procedure for the mixtures 2 to 5.
  6. Now repeat the whole experiment to find the order of reaction with respect to peroxodisulphate(VI) ions.

Recording the experimental data...

The data from the experiments could be recorded in a table like the one below:

Mixture  Time 
(s)
[I-(aq)]
(mol dm-3)
Initial Rate 
(mol s-1)
1   
2   
3   
4   
5   

Experimental method - Activation Enthalpy

This is what you do...

Choose a run from one of the above sets of experiments which you feel gives reliable results. Repeat this experiment across a range of temperatures, say, 15 C, 25 C, 35 C, 45 C, 55 C, 65 C.

Recording the experimental data...

Record your experimental data in a table like the one below:

Temperature
C
Time
(s)
Initial
Rate
(mol s-1)
1/T
(K-1)
15   
etc.