Stop the sampled reaction without introducing a second catalyst, state the blue-black starch endpoint and convert thiosulfate moles to iodine using the 2:1 ratio.
A group of students investigated the kinetics of a 'clock' reaction.
The reaction investigated was that between hydrogen peroxide and iodide ions in the presence of acid.
Reaction \(1 \quad \mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+2 \mathrm{I}^{-}(\mathrm{aq}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{I}_{2}(\mathrm{aq})\)
In this 'clock' reaction, a fixed volume of aqueous sodium thiosulfate, Na2 S2O3, and a small amount of starch were added to the reaction mixture.
The added thiosulfate ions react with the iodine produced in Reaction 1.
Reaction \(22 \mathrm{~S}_{2} \mathrm{O}_{3}^{2-}(\mathrm{aq})+\mathrm{I}_{2}(\mathrm{aq}) \rightarrow \mathrm{S}_{4} \mathrm{O}_{6}^{2-}(\mathrm{aq})+2 \mathrm{I}^{-}(\mathrm{aq})\)
When all the thiosulfate ions have reacted, the presence of iodine is detected by the formation of a starch-iodine complex. The students recorded the time taken for this complex to form.
State the final colour of the mixture containing the starch-iodine complex.
An answer that makes reference to the following point:
- Blue(-)black
Accept (dark) blue /black/ black-blue