Chemistry 9701/36 — October/November 2011

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

questions

marks

120

minutes

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

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

Iodine and propanone react together at room temperature in the presence of an acid catalyst.

\text{CH}_3\text{COCH}_3(\text{aq}) + \text{I}_2(\text{aq}) \rightarrow \text{CH}_3\text{COCH}_2\text{I}(\text{aq}) + \text{HI}(\text{aq})

In the following experiment you will investigate how the rate of reaction alters when the concentration of a reagent is changed. When the reaction is complete, the solution turns colourless as all the iodine has been used up. It is possible to determine the rate of the reaction by measuring how long it takes for the mixture to go colourless.

The product of the reaction, $\text{CH}_3\text{COCH}_2\text{I}$ , is a strong irritant to the eyes. Make sure that at the end of each experiment you wash out the reaction flask with plenty of water.

FB 1 is $2.0\text{ mol dm}^{-3}$ aqueous propanone, $\text{CH}_3\text{COCH}_3$ .
FB 2 is $3.0\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FB 3 is $0.0050\text{ mol dm}^{-3}$ aqueous iodine, $\text{I}_2$ .

(a)

Method

Read through the method and prepare a table for your results before starting any practical work.

Experiment 1

Fill the burette labelled FB 1 with the propanone solution, FB 1.
Fill the second burette with distilled water.
Run $20.0\text{ cm}^3$ of FB 1 into a $100\text{ cm}^3$ beaker.
Using a $25\text{ cm}^3$ measuring cylinder, add $20\text{ cm}^3$ of FB 2 to the beaker.
Using a second $25\text{ cm}^3$ measuring cylinder, measure $10\text{ cm}^3$ of FB 3.
Add the measured FB 3 to the solution in the $100\text{ cm}^3$ beaker and start timing immediately.
Stir the mixture once and place the beaker on a white tile.
Stop timing as soon as the solution goes colourless. Record this reaction time to the nearest second.
Wash out the beaker thoroughly.

Experiment 2

Run $14.0\text{ cm}^3$ of FB 1 into the $100\text{ cm}^3$ beaker.
Run $6.0\text{ cm}^3$ of distilled water into the $100\text{ cm}^3$ beaker.
Using the measuring cylinder, add $20\text{ cm}^3$ of FB 2 to the beaker.
Using the second measuring cylinder, measure $10\text{ cm}^3$ of FB 3.
Add the measured FB 3 to the solution in the $100\text{ cm}^3$ beaker and start timing immediately.
Stir the mixture once and place the beaker on a white tile.
Stop timing as soon as the solution goes colourless and record the reaction time as before.
Wash out the beaker thoroughly.

Experiment 3

Repeat the experiment as before using the volumes below.

$8.0\text{ cm}^3$ of FB 1
$12.0\text{ cm}^3$ of distilled water
$20\text{ cm}^3$ of FB 2
$10\text{ cm}^3$ of FB 3

Record all your results for experiments 1, 2 and 3 in the space below showing the volume of propanone solution, FB 1, the volume of distilled water and the reaction time.

(b)

Carry out two experiments to investigate further how the reaction time changes with different concentrations of propanone. Remember that the combined volume of propanone solution and distilled water must always be $20.0\text{ cm}^3$ . Record these results in the space below.

(c)

(i)

Calculate the number of moles of iodine that were added in each experiment.

(ii)

Calculate the initial concentration of the iodine in $50\text{ cm}^3$ of the reaction mixture.

(d)

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

\text{'rate'} = \frac{\text{concentration of iodine from (c)(ii)}}{\text{reaction time}} \times 10^5

Use your experimental results to complete the following table to include the reaction time and the 'rate'.

volume of propanone solution, FB 1 / $\text{cm}^3$

(e)

On the grid opposite, plot the 'rate' against the volume of propanone solution, FB 1. Draw a line of best fit through the points you have plotted.

Fig. 1.1

(f)

(i)

From your results what conclusion can you draw about the relationship between the rate of this reaction and the concentration of propanone?

(ii)

Suggest an improvement to the experiment that would allow you to be more confident about this conclusion.

(g)

Carry out one additional experiment to investigate how the 'rate' is altered when the concentration of iodine is halved. Record the volume of each solution used and the reaction time in the space below. Calculate the 'rate' using the equation in (d).

(h)

From your results in (d) and (g), what conclusion can you draw about the relationship between the rate of reaction and the concentration of iodine?

(i)

In these experiments you used a burette to measure $20.0\text{ cm}^3$ of FB 1. Calculate the percentage error in measuring this volume.

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