The reaction between iodide ions and peroxodisulphate(VI) ions follows the overall equation:
S2O82-(aq) + 2I-(aq) ® 2SO42-(aq) + I2(aq)
Rate = k [iodide]m [peroxodisulphate(VI)]n
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.
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.
The following solutions are needed:
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
Rate of reaction with respect to Peroxodisulphate(VI) ions
The data from the experiments could be recorded in a table like the one below:
|Initial Rate |
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.
Record your experimental data in a table like the one below: