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Hydrogen peroxide, $H_2O_2$, can be oxidised to give oxygen, $O_2$. This reaction happens rapidly in the presence of acidified potassium manganate(VII), $KMnO_4$.
$$2MnO_4^- (aq) + 5H_2O_2 (aq) + 6H^+ (aq) \rightarrow 2Mn^{2+} (aq) + 5O_2 (g) + 8H_2O (l)$$
You will determine the concentration of a solution of hydrogen peroxide. You will first dilute the solution and then carry out a titration using acidified potassium manganate(VII), $KMnO_4$.
FA 1 is aqueous hydrogen peroxide, $H_2O_2$.
FA 2 is 0.0200 $mol$ $dm^{-3}$ potassium manganate(VII), $KMnO_4$.
FA 3 is 1.0 $mol$ $dm^{-3}$ sulfuric acid, $H_2SO_4$.
(a) Method
Dilution
- Pipette 25.0 $cm^3$ of FA 1 into the 250 $cm^3$ volumetric flask.
- Add distilled water to make 250 $cm^3$ of solution and shake the flask thoroughly.
- Label this diluted solution of hydrogen peroxide FA 4.
Titration
- Fill the burette with FA 2.
- Rinse the pipette thoroughly with distilled water and then with a little FA 4.
- Pipette 25.0 $cm^3$ of FA 4 into a conical flask.
- Use the measuring cylinder to add 25 $cm^3$ of FA 3 to the conical flask.
- 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 2 added in each accurate titration.
Keep FA 3 and FA 4 for use in Question 2.
(b) From your accurate titration results, obtain a suitable value for the volume of FA 2 to be used in your calculations. Show clearly how you obtained this value.
25.0 $cm^3$ of FA 4 required ...................... $cm^3$ of FA 2. [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 potassium manganate(VII) present in the volume of FA 2 calculated in (b).
$ ext{moles of } KMnO_4 = ............................... ext{mol}$
(ii) Use your answer to (i) and the equation at the top of page 2 to calculate the number of moles of hydrogen peroxide present in 25.0 $cm^3$ of FA 4.
$ ext{moles of } H_2O_2 = ................................. ext{mol}$
(iii) Calculate the concentration, in $mol$ $dm^{-3}$, of $H_2O_2$ in FA 4.
$ ext{concentration of } H_2O_2 ext{ in FA 4 = ................................ $mol$ $dm^{-3}$}$
(iv) Calculate the concentration, in $mol$ $dm^{-3}$, of $H_2O_2$ in FA 1.
$ ext{concentration of } H_2O_2 ext{ in FA 1 = ................................ $mol$ $dm^{-3}$}$ [4]
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In Question 1, hydrogen peroxide was oxidised to form oxygen. Hydrogen peroxide can also be reduced to form water. This occurs, for example, when hydrogen peroxide, $H_2O_2$, reacts with iodide ions, $I^−(aq)$, to form iodine, $I_2(aq)$.
$$H_2O_2(aq) + 2H^+(aq) + 2I^−(aq) \rightarrow I_2(aq) + 2H_2O(l)$$
The rate of this reaction can be measured by adding acidified hydrogen peroxide, $H_2O_2$, to a mixture of iodide ions, $I^−$, thiosulfate ions, $S_2O_3^{2−}$, and starch indicator. As the iodine is produced, it reacts immediately with the thiosulfate ions and is reduced back to iodide ions.
$$I_2(aq) + 2S_2O_3^{2−}(aq) \rightarrow 2I^−(aq) + S_4O_6^{2−}(aq)$$
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.
In this experiment you will measure two reaction times and use one of these to calculate the concentration of the thiosulfate ions in FA 6.
FA 3 is 1.0 mol dm$^{−3}$ sulfuric acid, $H_2SO_4$.
FA 4 is the diluted hydrogen peroxide, $H_2O_2$, you prepared in 1(a).
FA 5 is 1.0 mol dm$^{−3}$ potassium iodide, KI.
FA 6 is aqueous sodium thiosulfate, $Na_2S_2O_3$.
starch indicator
Read through the instructions carefully before starting any practical work.
(a) Method
• Use the measuring cylinder to transfer 20 cm$^3$ of FA 3 into the 100 cm$^3$ beaker.
• Use the measuring cylinder to add 10 cm$^3$ of FA 4 into the 100 cm$^3$ beaker.
• Rinse the measuring cylinder with distilled water and drain.
• Use the measuring cylinder to transfer 20 cm$^3$ of FA 5 into the 250 cm$^3$ beaker.
• Use the measuring cylinder to transfer 20 cm$^3$ of FA 6 into the 250 cm$^3$ beaker.
• Use the measuring cylinder to transfer 10 cm$^3$ of starch indicator into the 250 cm$^3$ beaker.
• Add the contents of the 100 cm$^3$ beaker to the 250 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 turns blue-black.
• Record this reaction time to the nearest second.
• Rinse both beakers and drain.
Keep all FA solutions and starch indicator for use in (c).
reaction time = ......................... s [2]
(b) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) The expression for the rate of reaction is shown.
$$\text{rate} = \frac{\text{concentration of iodine at the reaction time}}{\text{reaction time}}$$
The concentration of iodine at the reaction time is the concentration of iodine that would have been present at the time the blue-black colour appeared if no thiosulfate ions had been added.
In this experiment, you should assume the rate = $2.61 \times 10^{−5}$ mol dm$^{−3}$ s$^{−1}$.
Use this value for the rate to calculate the concentration of iodine that would have been present at the reaction time if no thiosulfate ions had been added.
concentration of $I_2$ = ............................ mol dm$^{−3}$
(ii) Use your answer to (i) and the total reaction volume, to calculate the number of moles of iodine that would have been present at the reaction time if no thiosulfate ions had been added.
(If you were unable to calculate an answer to (i), you should use the value of $1.32 \times 10^{−3}$ mol dm$^{−3}$. This may not be the correct value.)
moles of $I_2$ = ............................ mol
(iii) Calculate the number of moles of thiosulfate ions that reacted with the moles of $I_2$, calculated in (ii).
$$I_2(aq) + 2S_2O_3^{2−}(aq) \rightarrow 2I^−(aq) + S_4O_6^{2−}(aq)$$
moles of $S_2O_3^{2−}$ = ............................ mol
(iv) Use your answer to (iii) to calculate the concentration of thiosulfate ions in FA 6.
concentration of $S_2O_3^{2−}$ in FA 6 = ............................ mol dm$^{−3}$ [4]
(c) Repeat the experiment in (a) using the following quantities of each reagent.
• add to the 100 cm$^3$ beaker
20 cm$^3$ of FA 3
10 cm$^3$ of FA 4
• add to the 250 cm$^3$ beaker
20 cm$^3$ of FA 5
10 cm$^3$ of FA 6
10 cm$^3$ of starch indicator
10 cm$^3$ of distilled water
• Add the contents of the 100 cm$^3$ beaker to the 250 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 turns blue-black.
• Record this reaction time to the nearest second.
reaction time = ............................ s [1]
(d) (i) Explain the relationship between the value of the reaction time in (a) and the value of the reaction time in (c).
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(ii) A student states that the error in the total volume of the reaction mixture in (a) is the same as the error in the total volume of the reaction mixture in (c).
State whether or not you agree with the student and explain your answer.
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
.......................................................................................................................................... [4]
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(a) FA 7, FA 8, FA 9 and FA 10 are solutions that each contain a metal cation from those listed in the Qualitative Analysis Notes on page 10. Carry out tests using reagents that will allow you to identify which metal ions are present. Draw a single table to record your observations. Complete the table below with your conclusions.
[Table]
| solution | FA 7 | FA 8 | FA 9 | FA 10 |
| -------- | --- | --- | --- | --- |
| metal ion present | | | | |
(b) Carry out the following test and record your observations.
[Table]
| test | observations |
| --- | --- |
| To a 1 cm depth of FA 10 in a boiling tube add a 0.5 cm depth of 20 'vol' hydrogen peroxide, then add one drop of aqueous sodium hydroxide. | |
(c) FA 11 is a solid that contains one anion from those listed in the Qualitative Analysis Notes on page 11.
Place all of FA 11 into a boiling tube and add a 3 cm depth of distilled water. Stir until all the solid has dissolved.
(i) Carry out the following test and record your observations.
[Table]
| test | observations |
| --- | --- |
| To a 1 cm depth of the solution of FA 11 in a test-tube add dilute sulfuric acid. | |
(ii) Use your observations from (i) to identify the anion in FA 11.
The anion is ................... .
(iii) Use the remaining solution of FA 11 in the boiling tube to carry out a further test to support your identification of the anion. Record details of this test.
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