Chemistry 9701/31 — May/June 2012

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Presentation of Data and Observations · Manipulation, Measurement and Observation · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q1Presentation of Data and ObservationsManipulation, Measurement and ObservationAnalysis, Conclusions and EvaluationFree sample

When iodide ions are mixed with peroxodisulfate ions, $\text{S}_2\text{O}_8^{2-}$ , iodine is formed.

2\text{I}^-(\text{aq}) + \text{S}_2\text{O}_8^{2-}(\text{aq}) \rightarrow \text{I}_2(\text{aq}) + 2\text{SO}_4^{2-}(\text{aq})

The rate of this reaction can be measured by adding thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$ , and some starch indicator to the mixture. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide ions.

\text{I}_2(\text{aq}) + 2\text{S}_2\text{O}_3^{2-}(\text{aq}) \rightarrow 2\text{I}^-(\text{aq}) + \text{S}_4\text{O}_6^{2-}(\text{aq})

When all the thiosulfate ions have reacted, the iodine which continues to be produced then turns the starch indicator blue-black. The rate of reaction may be determined by timing how long it takes for the reaction mixture to turn blue-black.

You are to investigate how the rate of reaction is affected by changing the concentration of the peroxodisulfate ions.

FA 1 is $0.0200\text{ mol dm}^{-3}$ aqueous potassium peroxodisulfate, $\text{K}_2\text{S}_2\text{O}_8$ .
FA 2 is $1.00\text{ mol dm}^{-3}$ aqueous potassium iodide, $\text{KI}$ .
FA 3 is $0.00500\text{ mol dm}^{-3}$ aqueous sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
starch indicator

Read through the instructions carefully and prepare a table for your results on page 3 before starting any practical work.

(a)

Method

Experiment 1

Fill the burette labelled FA 1 with aqueous potassium peroxodisulfate, FA 1.
Run $20.00\text{ cm}^3$ of FA 1 into a $100\text{ cm}^3$ beaker.
Using a $25\text{ cm}^3$ $25 cm^{3}$ measuring cylinder add the following to a second $100\text{ cm}^3$ $100 cm^{3}$ beaker:
- $20\text{ cm}^3$ of FA 2
- $10\text{ cm}^3$ of FA 3
Add 10 drops of starch indicator to the second beaker.
Add the contents of the first beaker to the second beaker and start timing immediately.
Stir the mixture once and place the beaker on a white tile.
Stop timing as soon as the solution turns blue-black.
Record this reaction time to the nearest second in the table that you have prepared on page 3.
Wash out both beakers.

Experiment 2

Fill a second burette with distilled water.
Run $10.00\text{ cm}^3$ of FA 1 into the first $100\text{ cm}^3$ beaker.
Run $10.00\text{ cm}^3$ of distilled water into the beaker containing FA 1.
Using a $25\text{ cm}^3$ $25 cm^{3}$ measuring cylinder add the following to a second $100\text{ cm}^3$ $100 cm^{3}$ beaker:
- $20\text{ cm}^3$ of FA 2
- $10\text{ cm}^3$ of FA 3
Add 10 drops of starch indicator to the second beaker.
Add the contents of the first beaker to the second beaker and start timing immediately.
Stir the mixture once and place the beaker on a white tile.
Stop timing as soon as the solution turns blue-black.
Record this reaction time to the nearest second in the table that you have prepared on page 3.
Wash out both beakers.

Experiments 3 – 5

Carry out three further experiments to investigate how the reaction time changes with different volumes of potassium peroxodisulfate.
Remember that the combined volume of FA 1 and distilled water must always be $20.00\text{ cm}^3$ .
Do not use a volume of FA 1 that is less than $6.00\text{ cm}^3$ .

Record all your results in a single table. You should include the volume of potassium peroxodisulfate, the volume of distilled water and the reaction time.

(b)

In order to convert the times measured in the experiments into rates of reaction, it is necessary first to work out the concentration of $\text{I}_2(\text{aq})$ that would have been produced in the reaction time if the thiosulfate had not been present. You must show your working.

(i)

Calculate how many moles of thiosulfate ions, $\text{S}_2\text{O}_3^{2-}$ , were added in each experiment.

(ii)

Calculate how many moles of iodine, $\text{I}_2$ , must have been produced to react with this amount of thiosulfate ions.

(iii)

Calculate the concentration of iodine from (ii) in the total reaction volume.

(c)

The rate of the reaction can be represented by the following formula.

\text{'rate'} = \frac{\text{concentration of }\text{I}_2\text{ from (b)(iii)}}{\text{reaction time}} \times 10^6

Use your experimental results to complete the following table. Include the volume of FA 1, the reaction time and 'rate' with their units.

If you were unable to answer (b)(iii), you may assume that the concentration of iodine is $4.25 \times 10^{-4}\text{ mol dm}^{-3}$ (This is not the correct value).

Table 1.1

(d)

On the grid opposite, plot the 'rate' against the volume of FA 1. Draw a line of best fit.

Grid 1.1

(e)

In your experiments, the volume of FA 1 represents the concentration of potassium peroxodisulfate. From your results, what conclusion can you draw about the relationship between the rate of reaction and the concentration of potassium peroxodisulfate?

(f)

Assume that the error in the time measured for each reaction was $\pm 0.5\text{ s}$ .

(i)

Calculate the maximum percentage error in the reaction time you recorded in Experiment 1.

(ii)

Assuming this is the only source of error, calculate the minimum reaction rate for Experiment 1.

(iii)

Suggest an additional source of error in these experiments and what improvement could be made to reduce this error.

(g)

(i)

Carry out one additional experiment using the following volumes of each reagent. Use the same method as in (a), mix FA 2, FA 3, the distilled water and the starch together and start the reaction by adding FA 1 to this mixture.

$10.00\text{ cm}^3$ of FA 1
$20\text{ cm}^3$ of FA 2
$20\text{ cm}^3$ of FA 3
10 drops of starch

Record the time for the reaction to go blue-black.

(ii)

Explain the relationship between this time and the one you recorded in Experiment 2.

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