Chemistry 9701/34 — October/November 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 · Analysis, Conclusions and Evaluation · Presentation of Data and Observations · Qualitative Analysis

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

You are to investigate how the rate of reaction between acidified hydrogen peroxide and aqueous iodide ions depends on the concentration of the hydrogen peroxide.

When hydrogen peroxide and potassium iodide are mixed in the presence of an acid, iodine, $\text{I}_2$ , is produced and the colour of the solution changes from colourless to a blue-black colour if starch indicator is present.

\text{H}_2\text{O}_2(\text{aq}) + 2\text{I}^-(\text{aq}) + 2\text{H}^+(\text{aq}) \rightarrow 2\text{H}_2\text{O}(\text{l}) + \text{I}_2(\text{aq})

If the reaction mixture contains sodium thiosulfate, the iodine produced in the reaction above is immediately reduced back to iodide ions. The solution only turns blue-black when all of the sodium thiosulfate has been used up.

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

FB 1 is $0.23\text{ mol dm}^{-3}$ hydrogen peroxide, $\text{H}_2\text{O}_2$ .
FB 2 is $0.10\text{ mol dm}^{-3}$ potassium iodide, $\text{KI}$ .
FB 3 is $0.050\text{ mol dm}^{-3}$ sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3$ .
FB 4 is $1.0\text{ mol dm}^{-3}$ sulfuric acid, $\text{H}_2\text{SO}_4$ .
starch indicator
distilled water

Read through the instructions carefully before starting any practical work.

(a)

Experiment 1

Fill a burette with FB 3.
Use the measuring cylinder labelled A to place $25\text{ cm}^3$ of FB 2 and $25\text{ cm}^3$ of distilled water into a conical flask.
Add to the conical flask $10.00\text{ cm}^3$ of FB 3 from the burette and 6 drops of starch indicator.
Use the measuring cylinder labelled B to place $50\text{ cm}^3$ of FB 1 and $20\text{ cm}^3$ of FB 4 into a $100\text{ cm}^3$ beaker.
Pour the mixture from the beaker into the conical flask and immediately start timing.
Swirl the flask to ensure good mixing and place the flask on a white tile.
Stop timing when a blue-black colour suddenly appears in the solution.
Record, in the table on page 4, the reaction time, in seconds, to the nearest second.
Empty, rinse and drain the conical flask.

Experiment 2

Use the measuring cylinder labelled A to place $25\text{ cm}^3$ of FB 2 and $35\text{ cm}^3$ of distilled water into a conical flask.
Add to the conical flask $10.00\text{ cm}^3$ of FB 3 from the burette and 6 drops of starch indicator.
Use the measuring cylinder labelled B to place $40\text{ cm}^3$ of FB 1 and $20\text{ cm}^3$ of FB 4 into a $100\text{ cm}^3$ beaker.
Pour the mixture from the beaker into the conical flask and immediately start timing.
Swirl the flask to ensure good mixing and place the flask on a white tile.
Stop timing when a blue-black colour suddenly appears in the solution.
Record, in the table on page 4, the reaction time, in seconds, to the nearest second.
Empty, rinse and drain the conical flask.

Experiments 3 – 5

Carry out experiments 3 – 5 in the same way but using the volumes of solutions shown in the table.
Complete the units in the table.
Calculate all values of $\frac{1000}{\text{reaction time}}$ to three significant figures.

Table 1.1

(b)

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

\text{'rate'} = \frac{1000}{\text{reaction time}}

On the next page plot a graph of 'rate' against the volume of FB 1.

Start each of the axes at zero.

Draw the line of best fit.

Graph Grid

(c)

The concentration of hydrogen peroxide in FB 1 is $0.23\text{ mol dm}^{-3}$ .

The total volume of each reaction mixture is $130\text{ cm}^3$ .

(i)

Calculate the initial concentration of hydrogen peroxide for each of the following experiments. Show your working.

Experiment	volume of FB 1 / $\text{cm}^3$	concentration of hydrogen peroxide / $\text{mol dm}^{-3}$
1	50
5	10

(ii)

Use your results in (i) to show that the initial concentration of hydrogen peroxide is directly proportional to the volume of FB 1 used in the experiment.

(d)

A website states that the rate of reaction between acidified hydrogen peroxide and potassium iodide is directly proportional to the concentration of hydrogen peroxide.

Use your graph to decide whether the statement on the website is correct or not.

Explain your answer.

(e)

Experiment 1 was repeated using $0.025\text{ mol dm}^{-3}$ sodium thiosulfate instead of FB 3.
Suggest how this would affect the reaction time.
Explain your answer using the chemical equations on page 3.

(f)

Suggest a factor, other than volumes of solutions used, that could have significantly affected the rate of reaction in each of the experiments.

(g)

A student carrying out a similar investigation decides to repeat one of the experiments a number of times. The reaction times for these repeated experiments are listed below.

run	time / s
1	56
2	54
3	62
4	56
5	53

(i)

From these experimental results calculate an appropriate mean reaction time, correct to 1 decimal place.

\text{mean reaction time} = \dots\dots\dots\dots\dots\text{ s}

(ii)

Assume that the uncertainty in the mean reaction time is $\pm 2$ seconds.
Calculate this uncertainty as a percentage of the mean reaction time.

\text{percentage uncertainty} = \dots\dots\dots\dots\dots\text{ \%}

(h)

The experimental method you have used can be adapted to investigate how the rate of reaction would vary if the concentration of potassium iodide or the concentration of sulfuric acid were changed.

In the first line of the tables below, the volumes of FB 2, distilled water, FB 3, FB 1 and FB 4 used in Experiment 2 are recorded.

Complete the following table, suggesting volumes for each of the reagents that could be used in a further experiment to investigate how the rate of reaction varies with a change in the volume of potassium iodide, FB 2.

Do not carry out this experiment.

Experiment	volume of FB 2 / $\text{cm}^3$	volume of distilled water / $\text{cm}^3$	volume of FB 3 / $\text{cm}^3$	volume of FB 1 / $\text{cm}^3$	volume of FB 4 / $\text{cm}^3$
2	25	35	10.00	40	20

Do not carry out this experiment.

Experiment	volume of FB 2 / $\text{cm}^3$	volume of distilled water / $\text{cm}^3$	volume of FB 3 / $\text{cm}^3$	volume of FB 1 / $\text{cm}^3$	volume of FB 4 / $\text{cm}^3$
2	25	35	10.00	40	20

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