2025 JC2 Prelims H2 Chem Paper 4 QP TJC

9729 / TJC Prelim / 2025 [Turn Over TEMASEK JUNIOR COLLEGE 2025 JC2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS CENTRE NUMBER S INDEX NUMBER Chemistry 9729/04 Paper 4 Practical 28 August 2025 2 hours 30 minutes Candidates answer on the Question Paper. READ THESE INSTRUCTIONS FIRST Write your name, CG, centre number and index number 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 19 and 20. 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. This document consists of 19 printed pages and 1 blank page. For Examiner’s Use 1 2 3 4 Total Shift Laboratory

2 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2025 Answer all the questions in the spaces provided. 1 Determination of the percentage by mass of iron in the wire Iron wire contains impurities. In this experiment, you will investigate the percentage by mass of iron in a sample of iron wire. A sample of iron wire is reacted with an excess of sulfuric acid to produce a solution of iron( II) sulfate. You will titrate the solution of iron( II) sulfate with potassium manganate( VII) of known concentration to determine the amount of iron( II) ions present and hence percentage by mass of iron in the wire. You may assume that impurities do not react with potassium manganate(VII). Iron(II) ions react with manganate(VII) ions according to the equation shown. 5Fe2+ + MnO4– + 8H+ → 5Fe3+ + Mn2+ + 4H2O FA 1 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 2 is a diluted solution of FeSO4 prepared as follows: • 48.9 g of iron wire was reacted with sulfuric acid to make 1.00 dm3 of solution. • 34.00 cm3 of the solution was then made up to 250 cm3 with deionised water. FA 3 is dilute sulfuric acid, H2SO4. (a) (i) Procedure 1. Fill the burette with FA 1. 2. Pipette 25.0 cm3 of FA 2 into a 250 cm3 conical flask. 3. Use a measuring cylinder to add 25 cm3 of FA 3 into the conical flask. 4. Add FA 1 from the burette until the solution in the conical flask turns to a permanent pale pink colour. 5. Record your titration results, to an appropriate level of precision in the space on page 3. 6. Repeat steps 2 to 5 until consistent results are obtained.

3 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2025 [Turn over Titration results [3] (ii) From your titrations, obtain a suitable volume of FA 1 to be used in your calculations. Show clearly how you obtained this volume. volume of FA 1 = ……………………………………… [3] (b) (i) Calculate the amount of iron(II) ions present in 25.0 cm3 of FA 2. amount of iron(II) ions = …………………………………… [1]

4 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2025 (ii) Calculate the mass of iron present in 25.0 cm3 of FA 2. [Ar: Fe, 55.8] mass of iron = …………………………………… [1] (iii) Calculate the percentage by mass of iron in the sample of iron wire. percentage by mass of iron = …………………………………… [2] (c) A student suggested that when a piece of iron wire was dissolved in a known amount of excess sulfuric acid, the amount of iron that reacted with the acid could be determined by titrating the mixture containing the remaining acid with a known concentration of sodium hydroxide. Explain whether the student was correct. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ………………………………………………………………………………………………...…[2] [Total: 12]

5 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2025 [Turn over 2 Determination of enthalpy change for the reaction between sodium carbonate with water and carbon dioxide You will determine the enthalpy change, Hr, for the reaction of sodium carbonate, Na2CO3, with water and carbon dioxide to form sodium hydrogencarbonate, NaHCO3, via reactions 1 and 2. Na2CO3(s) + H2O(l) + CO2(g) ⟶ 2NaHCO3(s) Hr In reaction 1, you will react sodium hydrogencarbonate with dilute hydrochloric acid and find the temperature change. NaHCO3(s) + HCl(aq) ⟶ NaCl(aq) + H2O(l) + CO2(g) H1 In reaction 2, the reaction of sodium carbonate with dilute hydrochloric acid will be considered. Na2CO3(s) + 2HCl(aq) ⟶ 2NaCl(aq) + H2O(l) + CO2(g) H2 You will then use the results to calculate the enthalpy change, Hr. FA 4 is sodium hydrogencarbonate, NaHCO3. FA 5 is 2.0 mol dm–3 hydrochloric acid, HCl. Handle all chemicals with care and follow all safely precautions at all times. (a) Reaction 1: Reaction between sodium hydrogencarbonate and dilute hydrochloric acid Read through the method and prepare two suitable tables in the space provided on page 6 to record: • all weighings to an appropriate level of precision • all values of temperature, to an appropriate level of precision • all values of time, recorded to the nearest minute. It is important that you measure each temperature at the specified time. 1. Weigh the container with FA 4 and record the balance reading in your table on page 6. 2. Place the polystyrene cup in the 250 cm3 beaker. 3. Use the 25 cm 3 measuring cylinder to transfer 25 cm 3 of the acid, FA 5, into the polystyrene cup. The acid is in excess. 4. Place the thermometer in the acid and record the initial temperature in your second table on page 6. Tilt the cup if necessary, so that the bulb of the thermometer is fully covered. This is the temperature at t = 0 minute. Start timing. 5. Record the temperature of the acid at 1 minute and at 2 minutes. 6. At time t = 2½ minutes, carefully tip all the FA 4, in small portions to avoid spray, into the acid and stir to dissolve it. Avoid inhalation of the acid vapour. 7. Record the temperature of the solution from t = 3 to 8 minutes at 1 minute intervals. 8. Reweigh the container with any residual FA 4. Record the balance reading and the mass of FA 4 used.

6 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2025 Results Mass Temperature [3]

7 DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN DO NOT WRITE IN THIS MARGIN 9729 / TJC Prelim / 2025 [Turn over (b) (i) Plot a graph of temperature on the y-axis against time on x-axis on the grid below. You will use the graph to determine the theoretical temperature change at 2½ minutes. The scale for temperature should extend at least 1 ºC below your lowest recorded temperature. Draw two straight lines of best fit on your graph, one for the temperature of the acid before adding FA 4 and the other for the warming of the solution once the reaction is complete. Extrapolate the two lines to 2½ minutes. [3] (ii) Determine the change in temperature at 2½ minutes. temperature change at 2½ minutes = ……………..………… ºC [1