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Edexcel A-Level Chemistry A2 11.4 Experiments That Can Be Used to Investigate Reaction Rates By I an Initial Rate Method Carrying Out Separate Experiments Questions

Vary one initial concentration while holding others effectively constant, define a reproducible early endpoint or collect a full time series and process it consistently.

Syllabus
First assessment 2019
Course
Chemistry YCH11
Level
A2

Exam points

  • Use low clock-reagent concentration so 1/t approximates the initial rate before concentrations change.
  • Plot continuous concentration or volume data against time with correct axes and units.
  • Hold other reactants in excess or constant while varying one initial concentration.

Edexcel A-Level Chemistry A2 11.4 Experiments That Can Be Used to Investigate Reaction Rates By I an Initial Rate Method Carrying Out Separate Experiments Questions question 1

[Maximum number: 2]

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.

Under appropriate conditions, the reciprocal of time can be used as an approximate measure of the initial rate of the reaction.

Explain why the concentration of the sodium thiosulfate must be low compared with the initial concentrations of the other reagents.

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