AS & A Level Chemistry - 9701 Paper 3 2014 Winter Zone 6

You are to determine the concentration of a solution of sodium thiosulfate, $Na_2S_2O_3$.
To do this you will first produce a known amount of iodine by reacting iodate(V) ions, $IO_3^-$, with an excess of iodide ions, $I^-$. The equation for this reaction is below.
$$IO_3^- + 5I^- + 6H^+ \rightarrow 3I_2 + 3H_2O$$
The amount of iodine produced in this reaction can be found by titrating with thiosulfate ions. The equation for this reaction is below.
$$I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-}$$

$FB 1$ is aqueous sodium thiosulfate, $Na_2S_2O_3$.
$FB 2$ is aqueous potassium iodate(V) containing 3.60 g dm$^{-3}$ $KIO_3$.
$FB 3$ is sulfuric acid, $H_2SO_4$.
$FB 4$ is aqueous potassium iodide, $KI$.
starch indicator

(a) Method

\begin{itemize}

\item Fill a burette with $FB 1$.
\item Pipette 25.0 cm$^3$ of $FB 2$ into the conical flask.
\item Use the measuring cylinder to add 25 cm$^3$ of $FB 3$ into the conical flask.
\item Use the measuring cylinder to add 10 cm$^3$ of $FB 4$ into the conical flask. Brown iodine solution is produced.
\item Add $FB 1$ from the burette until most of the iodine has been removed and the solution in the conical flask is yellow.
\item Add 10 drops of starch indicator to the contents of the conical flask. The solution will turn blue-black.
\item Continue adding $FB 1$, from the burette, until the blue-black colour just disappears.
\item Carry out a rough titration and record your burette readings in the space below.

\end{itemize}

The rough titre is ......................... cm$^3$.

\begin{itemize}

\item Carry out as many accurate titrations as you think necessary to obtain consistent results.
\item Make certain any recorded results show the precision of your practical work.
\item Record, in a suitable form below, all of your burette readings and the volume of $FB 1$ added in each accurate titration.

\end{itemize}

$\begin{array}{|c|}\hline\text{I} \\ \hline \text{II} \\ \hline \text{III} \\ \hline \text{IV} \\ \hline \text{V} \\ \hline \text{VI} \\ \hline\end{array}$

(b) From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you have obtained this value.

25.0 cm$^3$ of $FB 2$ required ..................... cm$^3$ of $FB 1$.

(c) Calculations

Show your working and appropriate significant figures in the final answer to each step of your calculations.

(i) Calculate the number of moles of $KIO_3$ present in 25.0 cm$^3$ of $FB 2$.
$[A_r$: $O, 16.0$; $K, 39.1$; $I, 126.9]$
moles of $KIO_3$ = .......................... mol

(ii) The equations for the production of iodine and its titration with thiosulfate are shown below.
$$IO_3^- + 5I^- + 6H^+ \rightarrow 3I_2 + 3H_2O$$
$$I_2 + 2S_2O_3^{2-} \rightarrow 2I^- + S_4O_6^{2-}$$
Use these equations to calculate the number of moles of thiosulfate present in the volume of $FB 1$ you calculated in (b).
moles of $S_2O_3^{2-}$ = .......................... mol

(iii) Calculate the concentration, in mol dm$^{-3}$, of sodium thiosulfate in $FB 1$.
concentration = .......................... mol dm$^{-3}$

In Question 1 iodide ions were oxidised by iodate(V) ions. Iodide ions can also be oxidised by peroxodisulfate ions, $S_2O_8^{2-}$, and you are to investigate the rate of this reaction.

$$2I^- + S_2O_8^{2-} ightarrow I_2 + 2SO_4^{2-}$$

The rate of this reaction can be measured by adding thiosulfate ions, $S_2O_3^{2-}$, and starch solution to the mixture. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide according to the reaction below.

$$I_2 + 2S_2O_3^{2-} ightarrow 2I^- + S_4O_6^{2-}$$

After all the thiosulfate has reacted, the iodine turns the starch indicator blue-black. The rate of reaction may be determined by timing how long it takes for the mixture to turn blue-black.

The rate of the reaction can be represented by rate = $\frac{1}{\text{reaction time}}$.

(a) Method

FB 4 is aqueous potassium iodide, KI.
FB 5 is 0.020 mol $dm^{-3}$ potassium peroxodisulfate, $K_2S_2O_8$.
FB 6 is 0.010 mol $dm^{-3}$ sodium thiosulfate, $Na_2S_2O_3$.
Starch indicator

Read through the method before you start any practical work and prepare a suitable table for your results in the space at the top of page 5.

• Empty and wash thoroughly the burette that you used in Question 1.

Experiment 1

• Use the measuring cylinder to add 20 $cm^3$ of FB 4 into a 100 $cm^3$ beaker.
• Use the 10 $cm^3$ pipette to add 10.0 $cm^3$ of FB 6 to the beaker.
• Add 10 drops of starch indicator to the beaker.
• Fill a burette with FB 5.
• Run 20.00 $cm^3$ of FB 5 into a second 100 $cm^3$ 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 goes blue-black.
• Record this reaction time to the nearest second.
• Wash out both beakers and shake dry.

Experiment 2

• Use the measuring cylinder to add 20 $cm^3$ of FB 4 into a 100 $cm^3$ beaker.
• Use the 10 $cm^3$ pipette to add 10.0 $cm^3$ of FB 6 to the beaker.
• Add 10 drops of starch indicator to the beaker.
• Fill the second burette with distilled water.
• Run 10.00 cm3 of FB 5 into the second 100 $cm^3$ beaker.
• Run 10.00 $cm^3$ of distilled water into this second beaker containing FB 5.
• 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 goes blue-black.
• Record this reaction time to the nearest second.
• Wash out both beakers and shake dry.

Record your results in the space below. You should show the volume of FB 5, the volume of water and the reaction time for each experiment.

[3]

(b) Carry out three further experiments to investigate how the reaction time changes with different volumes of peroxodisulfate. Remember that the combined volume of peroxodisulfate solution, FB 5, and distilled water must always be 20.00 $cm^3$. Do not use a volume of FB 5 that is less than 4.00 $cm^3$.

Record the volume of FB 5, the volume of water and the reaction time for each experiment.

[4]

(c) Use your results from (a) and (b) to complete the table below. You should show the volume of FB 5, the reaction time and the volume of FB 5 × reaction time.

| I | | | |
|---|---|---|---|
| II | | | |
| III| | | |
| IV | | | |

[2]

(d) A student who had carried out these experiments concluded that the rate of reaction was directly proportional to the volume of FB 5.

Using your values from (c), explain whether your results agree with this conclusion.
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[1]

(e) Another student thought that the experiment could be made more accurate by giving longer reaction times. To do this he repeated Experiment 1 with the same volumes but using 0.100 mol $dm^{-3}$ sodium thiosulfate instead of FB 6. He found that the reaction never turned blue-black. Explain why.
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[1]

(f) Describe how you could modify this experiment to investigate the effect of the concentration of iodide ions, FB 4, on the rate of the reaction.
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[2]

FB 7 and FB 8 are aqueous solutions. One of the elements present in the different ions in FB 7 and FB 8 is the same.

Carry out the following tests on FB 7 and FB 8 and complete the table.

test | observations
---|---
(i) To a 1 cm depth of FB 7 in a test-tube add a 1 cm depth of aqueous silver nitrate.

(ii) To a 1 cm depth of FB 7 in a test-tube add aqueous sodium hydroxide.

(iii) To a 1 cm depth of FB 7 in a test-tube add a 1 cm depth of aqueous sodium hydroxide and then a 1 cm depth of hydrogen peroxide. Leave to stand.

(iv) Before starting this test, the Bunsen burner must be turned off. To a 2 cm depth of dilute sulfuric acid add a few drops of FB 8 then add a 1 cm depth of ethanol. Leave to stand in the water bath.

(b) What element is present in both FB 7 and FB 8?
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(c) Solid FB 9 and aqueous solution FB 10 both contain the same cation.

(i) Transfer approximately half of the FB 9 into a hard-glass test-tube and heat.
observations ..................................................................................................................................................
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(ii) Transfer the remaining FB 9 into a boiling tube and carefully add dilute sulfuric acid.
observations ..................................................................................................................................................
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(iii) To a 1 cm depth of FB 10 in a test-tube, add a piece of magnesium ribbon. Leave to stand.
observations ..................................................................................................................................................
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(iv) To a 1 cm depth of FB 10 in a test-tube, add an equal depth of concentrated hydrochloric acid (care: this is very corrosive).
observations ..................................................................................................................................................
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(v) Suggest which cation is present in both FB 9 and FB 10.
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(vi) What change in the oxidation state of magnesium is occurring during reaction (c)(iii)?
Oxidation state change from .............................. to ....................................

(d) In which part of the Periodic Table are the elements identified as being present in FB 7, FB 8, FB 9 and FB 10?
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