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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

Practise Edexcel IAL Chemistry 11.12 by identifying the blue-black starch endpoint, explaining thiosulfate timing and using reciprocal time to compare reaction rates.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • State the starch–iodine endpoint as blue-black once thiosulfate is consumed.
  • explain how thiosulfate timing and reciprocal time provide a clock-reaction rate

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

[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 1H2O2(aq)+2H+(aq)+2I−(aq)→2H2O(l)+I2(aq)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\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 S2O32−(aq)+I2(aq)→S4O62−(aq)+2I−(aq)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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