Investigating The Rate Of Reaction Between Peroxydisulphate (Vi) Ions And Iodide Ions
...and iodide ions. This will be achieved by using an iodine clock method and colorimetric analysis.
2. Draw a graph of rate against concentration for each reactant (Hydrogen peroxide, potassium iodide and H+ ions).
3. Finding the order for each reactant
4. Finding the rate-determining step.
5. Proposing a mechanism for the reaction.
6. Using Arrhenius' equation to find the activation enthalpy.
Background
The basic reaction for this can be illustrated with the following equation:
3I-(aq) + H2O2(aq) + 2H+(aq) → I3-(aq) + 2H2O(aq) (1)
The half equations for this reaction can be written as follows:
3I- I3- + 2e-
H2O2 + 2H+ + 2e- 2H2O
This reaction demonstrates that iodide ions are oxidised by hydrogen peroxide to tri-iodide ions.
This is stage one of a sequence of reactions, which continues below:
I3¬-(aq) + 2S2O32-(aq) ¬ 3I-(aq) + S4O6-(aq) (2)
This shows that the tri-iodide ions are reduced back to iodide ions by the thiosulphate ions. Thus, the iodine that is formed in reaction (1) is immediately transformed into iodide ion and we do not see the blue-black colour of the starch-iodide complex until all of the thiosulphate ion has reacted with I 2(aq) and is exhausted.
I3- (aq) + starch Starch-I5- complex + I-(aq)
Once the thiosulphate ion has been exhausted, the tri-iodide ion can react with the starch, forming the Starch-I5- complex, giving the blue-black colour. When this occurs, we will then know the amount of hydrogen peroxide that has reacted and the time it took to react.
These equations will, thus, enable the slow step (rate-determining step) to be determined, which is another aim of this experiment.
Though details of the starch and iodine reaction are not yet fully known, it is thought that iodine fits inside the coils of amylose. The transfer of charge...
View Full Essay
