2019 TJC H2 Chem Prelim P4 ANS

1 9729/04/TJC Prelims/2019 [Turn over CANDIDATE NAME CIVICS GROUP / CENTRE NO. / INDEX NO. / CHEMISTRY 9729/04 Paper 4 Practical 2 September 2019 2 hours 30 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your Civics Group and name on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. Qualitative Analysis Notes are printed on pages 17 and 18. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. Shift Laboratory This document consists of 18 printed pages and 2 blank pages. For Examiner’s Use 1 / 18 2 /12 3 /15 4 / 10 Total / 55 2019 JC2 PRELIMINARY EXAMINATIONS HIGHER 2

2 9729/04/TJC Prelims/2019 [Turn over 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 4. (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 Final burette reading /cm3 20.70 20.80 Initial burette reading /cm3 0.00 0.00 Volume of FA 2 /cm3 20.70 20.80   ● Appropriate headings and units. ● All accurate burette readings recorded to 0.05 cm3. ● Has two uncorrected, accurate titres value within ±0.10 cm3. ●● 2m – accuracy [5]

3 9729/04/TJC Prelims/2019 [Turn over (b) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. ● Average titre = 𝟐𝟎.𝟕𝟎+𝟐𝟎.𝟖𝟎 𝟐 = 20.75 cm3 25.0 cm3 of FA 4 required _20.75_ 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 MnO4 present in 20.75 cm3 of FA 2 = 𝟐𝟎.𝟕𝟓 𝟏𝟎𝟎𝟎 × 𝟎. 𝟎𝟐𝟎𝟎 = 4.15 x 104 mol amount of KMnO4 = 4.15 x 104 mol [1] (ii) Calculate the amount of hydrogen peroxide present in 25.0 cm3 of FA 4. 2MnO4͞ (aq) ≡ 5H2O2(aq) ● Amount of H2O2 in 25 cm3 of FA 4 = 4.15 x 104 x 𝟓 𝟐 = 1.04 x 103 mol (3sf) amount of H2O2 in 25.0 cm3 of FA4 = 1.04 x 103 mol [1] (iii) Calculate the concentration, in mol dm-3, of H2O2 in FA 4. ● Concentration of H2O2 in FA 4 = 1.04 x 103 x 𝟏𝟎𝟎𝟎 𝟐𝟓 = 0.0415 mol dm-3 (3sf) concentration of H2O2 in FA 4 = 0.0415 mol dm-3 [1]

4 9729/04/TJC Prelims/2019 [Turn over (iv) Calculate the concentration, in mol dm-3, of H2O2 in FA 1. Amount of H2O2 in 250 cm3 of FA 4 = Amount of H2O2 in 25 cm3 of FA 1 = 1.04 x 103 x 𝟐𝟓𝟎 𝟐𝟓 mol = 0.0104 mol ● Concentration of H2O2 in FA 1 = 0.0104 x 𝟏𝟎𝟎𝟎 𝟐𝟓 = 0.415 mol dm-3 (3sf) OR Dilution factor = 250 / 25 = 10 ● Concentration of H2O2 in FA 1 = 0.0415 x 10 = 0.415 mol dm-3 concentration of H2O2 in FA 1 = 0.415 mol dm-3 [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 e ach of the resulting mixtures were then titrated against potassium manganate(VII) and the results are 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, your 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]

5 9729/04/TJC Prelims/2019 [Turn over (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. [1] ●Clearly labelled axes. Appropriate scale starting from origin and allowing the plotted points to be more than half given grid. ●correctly plotted points ●2 best-fit straight lines Vmax(KMnO4) = 12.8 cm3 Vmax(H2O2) = 20.0 cm3 ● 1 mark for both values (only if the 2 lines intersect at x-axis)

6 9729/04/TJC Prelims/2019 [Turn over (iii) Determine the concentration of hydrogen peroxide in mol dm-3 in this experiment. 2Fe2+ ≡ H2O2 Amount of Fe2+ in 25.0 cm3 of solution = 𝟐𝟓 𝟏𝟎𝟎𝟎 × 𝟎. 𝟎𝟒 = 1.00 x 10–3 mol ●Amount of H2O2 in 20 cm3 of solution = 1.00 x 10–3 x ½ = 5.00 x 10–4 mol (●using Vmax(H2O2)) Concentration of H2O2 solution = 5.00 x 10–4 x 𝟏𝟎𝟎𝟎 𝟐𝟎 = 0.0250 mol dm-3 (allow ecf from (ii)) ● All calculations to 3 significant figures ● correct units for all calculated values Concentration of hydrogen peroxide = 0.0250 mol dm-3 [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 measuring 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 contai