AS & A Level Chemistry - 9701 Paper 3 2014 Winter Zone 4

Hydrogen peroxide, $H_2O_2$, is used in hair bleach and for skin therapies. In this experiment you will determine the concentration of a solution of hydrogen peroxide by titration with acidified potassium manganate(VII).

FB 1 is 0.0250 mol dm$ extsuperscript{-3}$ potassium manganate(VII), KMnO$_4$.
FB 2 is dilute sulfuric acid, H$_2$SO$_4$.
FB 3 is aqueous hydrogen peroxide, H$_2O_2$.

(a) Method

Dilution of FB 3
● Pipette 25.0 cm$ extsuperscript{3}$ of FB 3 into the volumetric (graduated) flask.
● Make the solution up to the mark using distilled water.
● Shake the flask thoroughly.
● This diluted solution of hydrogen peroxide is FB 4.

Titration
● Fill the burette with FB 1.
● Pipette 10.0 cm$ extsuperscript{3}$ of FB 4 into a conical flask.
● Use a measuring cylinder to add 25 cm$ extsuperscript{3}$ of FB 2 into the same flask.
● Add FB 1 until a permanent pale pink colour is seen.
● Perform a rough titration and record your burette readings in the space below.

The rough titre is ................ cm$ extsuperscript{3}$.

● Carry out as many accurate titrations as you think necessary to obtain consistent results.
● Make sure 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 FB 1 added in each accurate titration.

Keep solution FB 2 for use in Question 3 and solution FB 3 for use in Questions 2 and 3.



(b) From your accurate titration results, obtain a suitable value for the volume of FB 1 to be used in your calculations.
Show clearly how you have obtained this value.

10.0 cm$ extsuperscript{3}$ of FB 4 required ............... cm$ extsuperscript{3}$ of FB 1. [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 calculated in (b).

moles of KMnO$_4$ = .................... mol

(ii) Complete the equation below for the reaction of potassium manganate(VII) with hydrogen peroxide. State symbols are not required.

.......... KMnO$_4$ + 5H$_2O_2$ + 3H$_2SO_4$ → K$_2$SO$_4$ + 2MnSO$_4$ + ......... H$_2O$ + 5O$_2$

(iii) Use your answers to (i) and (ii) to calculate the number of moles of hydrogen peroxide used in each titration.

moles of H$_2O_2$ = .................... mol

(iv) Calculate the concentration of H$_2O_2$ in FB 4, in mol dm$ extsuperscript{-3}$.

concentration of H$_2O_2$ in FB 4 = .................... mol dm$ extsuperscript{-3}$

(v) Calculate the concentration of H$_2O_2$ in FB 3, in mol dm$ extsuperscript{-3}$.

concentration of H$_2O_2$ in FB 3 = .................... mol dm$ extsuperscript{-3}$ [5]

In this experiment you will determine the enthalpy change, $\Delta H$, for the catalytic decomposition of hydrogen peroxide into water and oxygen.

$$\text{H}_2\text{O}_2(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l}) + \frac{1}{2}\text{O}_2(\text{g})$$

FB 3 is aqueous hydrogen peroxide, $\text{H}_2\text{O}_2$.
FB 5 is manganese(IV) oxide, $\text{MnO}_2$, the catalyst for the decomposition.

(a) Method

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

• For the first experiment, support the plastic cup inside the 250 $\text{cm}^3$ beaker.
• Use a measuring cylinder to transfer 40 $\text{cm}^3$ of distilled water into the plastic cup.
• Use a measuring cylinder to add 10$\text{cm}^3$ of FB 3 into the plastic cup.
• Measure and record the initial temperature of the mixture.
• Add a heaped spatula measure of FB 5 to the mixture in the plastic cup.
• Stir constantly until the maximum temperature is reached and record this temperature.
• Calculate and record the temperature rise.
• Wash and wipe out your plastic cup and rinse the thermometer, ready for the second experiment.

• For the second experiment, support the plastic cup inside the 250 $\text{cm}^3$ beaker.
• Use a measuring cylinder to transfer 30 $\text{cm}^3$ of distilled water into the plastic cup.
• Use a measuring cylinder to add 20 $\text{cm}^3$ of FB 3 into the plastic cup.
• Measure and record the initial temperature of the mixture.
• Add a heaped spatula measure of FB 5 to the mixture in the plastic cup.
• Stir constantly until the maximum temperature is reached and record this temperature.
• Calculate and record the temperature rise.
• Wash and wipe out your plastic cup and rinse the thermometer, ready for the third experiment.

• Carry out the third experiment in a similar way.
• Transfer 20 $\text{cm}^3$ of distilled water into the plastic cup.
• Add 30 $\text{cm}^3$ of FB 3 into the plastic cup.
• Measure and record the initial temperature of the mixture.
• Add a heaped spatula measure of FB 5 to the mixture in the plastic cup.
• Record the maximum temperature, then calculate and record the temperature rise.

• For the fourth experiment, use 10 $\text{cm}^3$ of distilled water and 40 $\text{cm}^3$ of FB 3. [5]

(b) Using the grid below, plot a graph of the temperature rise (y-axis) against the volume of FB 3 (x-axis).
Draw the line of best fit. [4]

(c) Calculation

(i) Use your graph to calculate the average temperature rise for each 1.0 $\text{cm}^3$ of FB 3 used.
Show your working clearly on the graph.

average temperature rise = ..................... $^\circ \text{C}$

(ii) Calculate the energy released for each 1.0 $\text{cm}^3$ of FB 3 used.
(Assume that 4.2 J are needed to raise the temperature of 1.0 $\text{cm}^3$ of solution by 1.0 $^\circ \text{C}$.)

energy released = ..................... J

(iii) Use your answer to 1(c)(v) to calculate the number of moles of hydrogen peroxide in 1.0 $\text{cm}^3$ of FB 3.
(If you were unable to calculate the concentration of $\text{H}_2\text{O}_2$ in FB 3, assume that it was 1.72 $\text{mol dm}^{-3}$. Note: this is not the correct value.)

number of moles of $\text{H}_2\text{O}_2$ = .......................... mol

(iv) Calculate the enthalpy change, in kJ mol$^{-1}$, for the reaction below.

$$\text{H}_2\text{O}_2(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l}) + \frac{1}{2}\text{O}_2(\text{g})$$

enthalpy change =...... .................kJ mol$^{-1}$
(sign            (value) [4]

(d) Which one of the four experiments that you carried out is likely to be the least accurate?
Explain your choice.

..............................................................................................................................................................
..............................................................................................................................................................
.............................................................................................................................................................. [1]

(a) You will carry out further experiments with aqueous hydrogen peroxide, FB 3.

FB 2 is dilute sulfuric acid, $H_2SO_4$.
FB 3 is aqueous hydrogen peroxide, $H_2O_2$.
FB 6 is a solution containing two cations and one anion.
FB 7 is a solution containing one cation and one anion.

(i) To a 1 cm depth of FB 6 in a boiling tube, add aqueous sodium hydroxide until it is in excess. Then heat the tube, gently and with care. Keep the mixture for test (ii). Record your observations. Identify the cations in FB 6.

observations ......................................................................................................................................
..........................................................................................................................................................
..........................................................................................................................................................
FB 6: cations are .................................................. and ..................................................

(ii) To the mixture obtained from (i) add a 1 cm depth of FB 3. Shake the tube. Record your observations.

observations ......................................................................................................................................
..........................................................................................................................................................
..........................................................................................................................................................

What type of reaction has taken place? Explain your answer.

..........................................................................................................................................................
...................................................................................................................................................

(iii) To a 1 cm depth of FB 3 in a test-tube, add an equal volume of sulfuric acid, FB 2. Then add a 1 cm depth of FB 7, followed by a few drops of starch solution. Record all your observations. Draw what conclusions you can about the ions in FB 7. If no conclusion is possible, write 'not known'.

observations ......................................................................................................................................
..........................................................................................................................................................
..........................................................................................................................................................
FB 7: cation .................................................. anion ..................................................


(b) FB 7, FB 8 and FB 9 are aqueous solutions, each containing one cation and one anion. Note that FB 7 was also used in (a)(iii).

(i) Carry out the following tests in test-tubes.
Use 1 cm depths of solutions.
Complete the table by recording your observations.

[Table_1]

(ii) Suggest the identify of the cation in FB 9. Explain your answer.

cation .................................................
explanation .......................................................

(iii) Give the ionic equation for the reaction between FB 7 and FB 9.
..........................................................................................................................................................

(iv) From your observations, identify FB 8.
.........................................................................................................................................................