AS & A Level Chemistry - 9701 Paper 3 2014 Summer Zone 1

Hydrogen peroxide, $H_2O_2$, is unstable and decomposes to give water and oxygen. In addition to the usual units of concentration, mol dm\(^{-3}\), the concentration of a solution of hydrogen peroxide can also be given in ‘volume strength’ or ‘vol’. For example, in Question 3 you will use a solution of ‘20 vol’ $H_2O_2$. This term means that when 1 dm\(^3\) of this solution is completely decomposed it generates 20 dm\(^3\) of oxygen at room temperature and pressure.

The aim of the following titration is to determine the volume strength of a solution of hydrogen peroxide. To do this you will titrate an acidified solution of hydrogen peroxide with potassium manganate(VII) solution.

FA 1 is 0.0200 mol dm\(^{-3}\) potassium manganate(VII), $KMnO_4$.
FA 2 is aqueous hydrogen peroxide, $H_2O_2$.
FA 3 is 1.0 mol dm\(^{-3}\) sulfuric acid, $H_2SO_4$.

(a) Method

• Fill the burette with FA 1.
• Pipette 25.0 cm\(^3\) of FA 2 into the conical flask.
• Use the measuring cylinder to add 25 cm\(^3\) of FA 3 to the conical flask.
• Run FA 1 from the burette into the conical flask until the pink colour remains.
• Perform a rough titration and record your burette readings in the space below.
  
  The rough titre is ..................... cm\(^3\).
  
  Carry out as many accurate titrations as you think necessary to obtain consistent results.
  Make certain any recorded results show the precision of your practical work.
  Record, in a suitable form below, all of your burette readings and the volume of FA 1 added in each accurate titration.
  

An acidified solution of hydrogen peroxide is able to oxidise iodide ions, I⁻(aq), to iodine, I₂(aq).

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

In this experiment, you will investigate how the rate of this reaction depends on the concentration of the hydrogen peroxide and on the concentration of the iodide ions.

The rate of this reaction can be measured by adding thiosulfate ions, S₂O₃²⁻, and 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 has reacted, the iodine then turns the starch indicator blue-black. The rate of reaction may be determined by timing how long it takes the reaction mixture to turn blue-black.

FA 2 is aqueous hydrogen peroxide, H₂O₂.
FA 3 is 1.0 mol dm⁻³ sulfuric acid, H₂SO₄.
FA 4 is 0.50 mol dm⁻³ potassium iodide, KI.
FA 5 is 0.025 mol dm⁻³ sodium thiosulfate, Na₂S₂O₃.
starch indicator

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

(a) Method

Experiment 1
• Use the measuring cylinder to transfer the following volumes into the same 100 cm³ beaker.
○ 10 cm³ of FA 3
○ 20 cm³ of FA 4
○ 10 cm³ of FA 5
○ 10 cm³ of distilled water
• Add 10 drops of starch indicator to the beaker.
• Rinse the measuring cylinder with water and shake dry.
• Use the measuring cylinder to measure 10 cm³ of FA 2.
• Add the contents of the measuring cylinder to the 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.
• Wash out the beaker and the measuring cylinder with water and shake dry.

Experiment 2
• Use the measuring cylinder to transfer the following volumes into the same 100 cm³ beaker.
○ 10 cm³ of FA 3
○ 20 cm³ of FA 4
○ 10 cm³ of FA 5
• Add 10 drops of starch indicator to the beaker.
• Rinse the measuring cylinder with water and shake dry.
• Use the measuring cylinder to measure 20 cm³ of FA 2.
• Add the contents of the measuring cylinder to the 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.
• Wash out the beaker and the measuring cylinder with water and shake dry.

Experiment 3
• Use the measuring cylinder to transfer the following volumes into the same 100 cm³ beaker.
○ 10 cm³ of FA 3
○ 10 cm³ of FA 4
○ 10 cm³ of FA 5
○ 10 cm³ of distilled water
• Add 10 drops of starch indicator to the beaker.
• Rinse the measuring cylinder with water and shake dry.
• Use the measuring cylinder to measure 20 cm³ of FA 2.
• Add the contents of the measuring cylinder to the 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.

Record all your results in a single table. You should include the volume of hydrogen peroxide, the volume of potassium iodide, the volume of distilled water and the reaction time. You should also include the 'rate of reaction' which is given by the following expression.

$$\text{rate of reaction} = \frac{1}{\text{reaction time}}$$

(b) From your results, what can you conclude about how the 'rate of reaction' is affected by,

(i) the concentration of hydrogen peroxide,
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(ii) the concentration of potassium iodide?
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(c) It is not possible to draw a reliable conclusion about the effect of changing the concentration of a reagent on the 'rate of reaction' from only two experiments.

Suggest three experiments that could be carried out in addition to Experiments 2 and 3 to investigate the effect of changing the concentration of FA 4, potassium iodide. In each case give the volumes of each solution/liquid that would be used. Do not carry out these experiments.
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(d) A student was unable to complete all three experiments on the same day but had to return to the investigation the following morning. The student noticed that the results were significantly different.

Give two possible reasons for these differences and explain how the student could have overcome these problems.
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(e) There are many sources of error in these experiments. In the following, assume that the only error involves measuring the reaction time.

(i) If the error in each recorded time is ±1 s, calculate the percentage error in the time recorded in Experiment 1.
percentage error = ............ %

(ii) Another student decided to repeat Experiment 1 but used 0.050 mol dm⁻³ sodium thiosulfate in place of FA 5.
How would this change the percentage error in the recorded time? Explain your answer.
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(a) FA 6 is a solution of two different salts. It contains two different cations, one of which is listed in the Qualitative Analysis Notes on page 10. It contains two anions, both of which are listed in the Qualitative Analysis Notes on page 11.
(i) Choose reagents that will allow you to identify one of the cations. Carry out suitable tests using these reagents and record your results in the space below.
[Image of table with five rows labeled I to V]
One of the cations in FA 6 is ................. .

(ii) Carry out the following tests to identify the two anions present in FA 6.
[Table]
| test | observations |
|------------------|-------------------|
| To a 1 cm depth of FA 6 in a test-tube add a 1 cm depth of aqueous silver nitrate, then |
| add aqueous ammonia. |
| To a 1 cm depth of FA 6 in a test-tube add a 1 cm depth of aqueous barium chloride (or aqueous barium nitrate), then |
| add dilute nitric acid. |
The anions in FA 6 are ............. and ............. .

(b) FA 7 is an acidified solution of iron(II) sulfate, FeSO4(aq).
Carry out the following tests and record your observations.
[Table]
| test | observations |
|------------------|-------------------|
| (i) To a 1 cm depth of FA 7 in a test-tube add aqueous sodium hydroxide and leave for a few minutes. |
| (ii) To a 1 cm depth of FA 7 in a boiling tube add a 1 cm depth of dilute sulfuric acid followed by a 1 cm depth of '20 vol' hydrogen peroxide. Stir the mixture, then |
| (iii) pour a 1 cm depth of the mixture into a clean boiling tube and add a 3 cm depth of aqueous sodium hydroxide. |
[Image of table with six rows labeled I to VI]
(iv) What type of reaction takes place in (ii)?
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(v) Explain your observations in (iii).
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