Chemistry 9701/32 — May/June 2012

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

questions

marks

120

minutes

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Iodide ions are oxidised by iron(III) ions in the presence of acid.

2\text{Fe}^{3+}(\text{aq}) + 2\text{I}^-(\text{aq}) \rightarrow 2\text{Fe}^{2+}(\text{aq}) + \text{I}_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 reagents.

FB 1 is $0.0500\text{ mol dm}^{-3}$ aqueous acidified iron(III) chloride, $\text{FeCl}_3$ .
FB 2 is $0.0500\text{ mol dm}^{-3}$ aqueous potassium iodide, $\text{KI}$ .
FB 3 is $0.00500\text{ mol dm}^{-3}$ aqueous sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
FB 4 is 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

Using a $25\text{ cm}^3$ $25 cm^{3}$ measuring cylinder add the following to a $100\text{ cm}^3$ $100 cm^{3}$ beaker:
- $10\text{ cm}^3$ of FB 2
- $20\text{ cm}^3$ of FB 3
- $10\text{ cm}^3$ of FB 4
Fill the burette labelled FB 1 with aqueous acidified iron(III) chloride, FB 1.
Run $20.00\text{ cm}^3$ of FB 1 into a second $100\text{ cm}^3$ 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

Using a $25\text{ cm}^3$ $25 cm^{3}$ measuring cylinder add the following to a $100\text{ cm}^3$ $100 cm^{3}$ beaker:
- $10\text{ cm}^3$ of FB 2
- $20\text{ cm}^3$ of FB 3
- $10\text{ cm}^3$ of FB 4
Fill a second burette with distilled water.
Run $10.00\text{ cm}^3$ of FB 1 into a second $100\text{ cm}^3$ beaker.
Run $10.00\text{ cm}^3$ of distilled water into the beaker containing FB 1.
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 iron(III) chloride.
- Remember that the combined volume of FB 1 and distilled water must always be $20.00\text{ cm}^3$ .
- Do not use a volume of FB 1 that is less than $6.00\text{ cm}^3$ .

Record all your results in a single table. You should include the volume of iron(III) chloride, 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 change in the iron(III) ion concentration that occurs from the time when the mixtures are combined to the time when the solution turns blue-black. You must show your working.

(i)

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

moles of $\text{S}_2\text{O}_3^{2-}$ = ................. mol

(ii)

Calculate how many moles of iodine, $\text{I}_2$ , reacted with the number of moles of thiosulfate ions calculated in (i).

moles of $\text{I}_2$ = ................. mol

(iii)

Calculate how many moles of iron(III) ions, $\text{Fe}^{3+}$ , reacted to form the number of moles of iodine calculated in (ii).

moles of $\text{Fe}^{3+}$ = ................. mol

(iv)

Use your answer from (iii) to calculate the decrease in concentration of iron(III) ions in the total reaction mixture ( $60\text{ cm}^3$ ) from the start of the experiment to the point when the solution turned blue-black.

decrease in $\text{Fe}^{3+}$ concentration = ................. $\text{mol dm}^{-3}$

(c)

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

\text{'rate'} = \frac{\text{decrease in }\text{Fe}^{3+}\text{ concentration from (b)(iv)}}{\text{reaction time}} \times 10^6

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

If you were unable to answer (b)(iv), you may assume that the decrease in $\text{Fe}^{3+}$ concentration is $2.25 \times 10^{-3}\text{ mol dm}^{-3}$ (This is not the correct value).

Table 1.1

(d)

On the grid opposite, plot ‘rate’ against the volume of FB 1. Draw a line of best fit.

Grid 1

(e)

In your experiments, the volume of FB 1 represents the concentration of iron(III) chloride. From your results, what conclusion can you draw about the relationship between the rate of reaction and the concentration of iron(III) chloride?

(f)

One source of error in this experiment arises from measuring the volumes of solutions.

(i)

Calculate the maximum percentage error in the volume of FB 1 used in Experiment 1.

maximum percentage error = ................. %

(ii)

Other than errors involving measurements of volumes, suggest an additional source of error in these experiments.

(iii)

In this experiment, thiosulfate ions reduce iodine while iron(III) ions oxidise iodide ions. What other reaction might take place that would affect your confidence in the conclusions you made in (e)?

(g)

(i)

Carry out one additional experiment using the following volumes of each reagent. Use the same method as in (a), mix FB 2, FB 3 and FB 4 together and start the reaction by adding this mixture to FB 1 and the distilled water.

$10.00\text{ cm}^3$ of FB 1
$20.00\text{ cm}^3$ of distilled water
$10\text{ cm}^3$ of FB 2
$10\text{ cm}^3$ of FB 3
$10\text{ cm}^3$ of FB 4

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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