Edexcel A-Level Chemistry A2 11.12 Core Practicals 9a and 9b Following the Rate of the Iodine Propanone Reaction By a Titrimetric Method and Investigating Questions

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.

Exam points

Explain why hydrogencarbonate neutralises acid while NaOH would continue catalysis.
State the starch–iodine endpoint as blue-black once thiosulfate is consumed.
Calculate iodine amount from thiosulfate using the 2 mol to 1 mol stoichiometric ratio.

[Maximum number: 1]

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, $\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}$ , 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.

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Edexcel A-Level Chemistry A2 11.12 Core Practicals 9a and 9b Following the Rate of the Iodine Propanone Reaction By a Titrimetric Method and Investigating Questions question 1