Edexcel IAL Chemistry 11.4 Initial-rate and continuous experiments

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

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.

[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 $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.

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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11.4—Experiments that can be used to investigate reaction rates by: i an initial-rate method, carrying out separate experiments question 1