2019 TJC H2 Chem Prelim P4 QP

2 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelims / 2019 DO NOT WRITE IN THIS MARGIN Answer all the questions in the spaces provided. 1 Determination of the concentration of a solution of hydrogen peroxide Hydrogen peroxide, H 2O2, can be oxidised to give oxygen, O 2. This reaction happens rapidly in the presence of acidified potassium manganate(VII), KMnO4. 2MnO4͞ (aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 5O2(g) + 8H2O(l) To determine the concentration of a solution of hydrogen peroxide, y ou will first dilute the solution and then carry out a titration using acidified potassium manganate(VII), KMnO4. FA 1 is aqueous hydrogen peroxide. FA 2 is 0.0200 mol dm-3 potassium manganate(VII), KMnO4. FA 3 is 1.0 mol dm-3 sulfuric acid, H2SO4. Keep FA 1 for use in Question 3 (a) Method Dilution ● Pipette 25.0 cm3 of FA 1 into the 250 cm3 volumetric flask. ● Add distilled water to make 250 cm3 of solution and shake the flask thoroughly. ● Label this diluted solution of hydrogen peroxide as FA 4. Titration ● Fill the burette with FA 2. ● Pipette 25.0 cm3 of FA 4 into a conical flask. ● Use the measuring cylinder to add 25 cm3 of FA 3 to the conical flask. ● Titrate FA 4 against FA 2 and record your readings in the space below. ● Carry out as many accurate titrations as you think necessary. Results [5]

3 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelims / 2019 DO NOT WRITE IN THIS MARGIN (b) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. 25.0 cm3 of FA 4 required _____________ cm3 of FA 2 for titration [1] (c) Calculations Show your working and give appropriate significant figures in the final answer for each step of your calculations. (i) Calculate the amount of potassium manganate(VII) present in the volume of FA 2 calculated in (b). amount of KMnO4 = [1] (ii) Calculate the amount of hydrogen peroxide present in 25.0 cm3 of FA 4. amount of H2O2 in 25.0 cm3 of FA4 = [1] (iii) Calculate the concentration, in mol dm-3, of H2O2 in FA 4. concentration of H2O2 in FA 4 = [1]

4 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelims / 2019 DO NOT WRITE IN THIS MARGIN (iv) Calculate the concentration, in mol dm-3, of H2O2 in FA 1. concentration of H2O2 in FA 1 = [1] (d) Another experiment was conducted to determine the concentration of hydrogen peroxide. Different volumes of hydrogen peroxide were added to identical 25.0 cm 3 samples of 0.0400 mol dm -3 aqueous iron( II) solution. Iron(II) is oxidised by hydrogen peroxide as shown by the following equation. 2Fe2+ + H2O2 + 2H+  2Fe3++ 2H2O The remaining iron (II) ions in each of the resulting mixtures were then titrated against potassium manganate(VII) and the results were shown in the table below: Volume of H2O2 added / cm3 Volume of KMnO4 / cm3 2.00 11.60 12.00 5.20 16.00 2.60 25.00 3.40 30.00 6.70 35.00 10.00 (i) Plot, on the grid on page 5, the values for the volume of potassium manganate(VII) (y-axis) against the volume of hydrogen peroxide added (x-axis). Draw two straight lines of best fit, taking into account all of your plotted points. Your graph should enable you to determine the values for (d)(ii). [3] (ii) Hence obtain values for  the volume of potassium manganate(VII) required, Vmax(KMnO4), to react completely with 25.0 cm3 of iron(II) solution if no hydrogen peroxide is added.  the volume of hydrogen peroxide required, V max(H2O2), to react completely with 25.0 cm 3 of iron(II) solution if no potassium manganate(VII) is added.

5 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelims / 2019 DO NOT WRITE IN THIS MARGIN Vmax(KMnO4) = Vmax(H2O2) = [1]

6 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelims / 2019 DO NOT WRITE IN THIS MARGIN (iii) Determine the concentration of hydrogen peroxide, in mol dm-3, in this experiment. Concentration of hydrogen peroxide = [4] [Total: 18] 2 Determination of the enthalpy change of reaction, ΔHr You are to determine the enthalpy change of reaction, ΔHr¸ for the following reactions: Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l) ΔH1 NaHCO3(s) + HCl(aq) → NaCl(aq) + CO2(g) + H2O(l) ΔH2 FA 5 is anhydrous sodium carbonate, Na2CO3 FA 6 is solid sodium hydrogen carbonate, NaHCO3 You are also provided with 2.0 mol dm–3 HCl (a) Reaction of FA 5, Na2CO3, with an excess of 2.0 mol dm-3 hydrochloric acid Method Read through the instructions carefully before starting any practical work. 1. Support the styrofoam cup in the 250 cm3 beaker. 2. Use the m easuring cylinder to transfer 50 cm3 of 2.0 mol dm –3 hydrochloric acid into the styrofoam cup. 3. Place the thermometer in the styrofoam cup and record the temperature of the solution. 4. Weigh the weighing bottle containing FA 5, anhydrous sodium carbonate. 5. With the aid of a dry spatula, a dd the contents of the weighing bottle to the styrofoam cup in three separate portions, taking care that the mixture does not overflow. [Safety precaution: DO NOT add FA 5 all at once. The reaction is very vigourous.] 6. Stir and record the highest temperature obtained. 7. Reweigh the weighing bottle containing residual FA 5. Record in an appropriate form below, all of your weighings and temperature measurements together with the mass, m1, of FA 5 added and the maximum temperature rise, ΔT1.

7 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelims / 2019 DO NOT WRITE IN THIS MARGIN Results [2] (b) (i) Calculate the temperature rise per gram of FA 5, Na2CO3, used in the experiment. ∆𝑇1 𝑚1 = [2] (ii) Calculate the enthalpy change, ΔH1, for the reaction Na2CO3(s) + 2HCl(aq) → 2NaCl (aq) + CO2 (g) + H2O(l) ΔH1 = – (22.79 x ∆𝑇1 𝑚1 ) kJ mol-1 ΔH1 = [1]

8 DO NOT WRITE IN THIS MARGIN [Turn over DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelims / 2019 DO NOT WRITE IN THIS MARGIN (c) Reaction of FA 6, NaHCO3, with an excess of 2.0 mol dm-3 hydrochloric acid Method Read through the instructions carefully before starting any practical work. 1. Support another styrofoam cup in the 250 cm3 beaker provided. 2. Use the measuring cylinder to transfer 50 cm 3 of 2.0 mol dm –3 hydrochloric acid into the styrofoam cup. 3. Place the thermometer in the styrofoam cup and record the temperature of the solution. 4. Weigh the empty weighing bottle labelled “FA 6”. 5. Weigh approximately 7.0 – 8.0 g of FA 6, anhydrous sodium hydrogen carbonate into the empty weighing bottle labelled “FA 6”. 6. With the aid of a dry spatula, add the contents of the weighing bottle to the styrofoam cup in three separate portions, taking care that the mixture does not overflow. [Safety precaution: DO NOT add FA6 all at once. The reaction is very vigourous.] 7. Stir and record the lowest temperature obtained. 8. Reweigh the weighing bottle containing residual FA 6. Record in an appropriate form below, all of your weighings and temperature measurements together with the mass, m2, of FA 6 added and the maximum temperature fall, ΔT2. Results

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