2025 ASRJC Prelim H2Chem P4 QP

ASRJC JC2 Prelim 2025 9729/04/H2 ANDERSON SERANGOON JUNIOR COLLEGE 2025 JC2 Preliminary Examination NAME:________________________________ ( ) CLASS: 25 /____ CHEMISTRY 9729/04 Paper 4 Practical 19 August 2025 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class 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 a 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. Quantitative Analysis Notes are printed on pages 19 and 20. Shift Laboratory 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. For Examiner’s Use 1 / 19 2 / 18 3 / 8 4 / 10 Total / 55 This document consists of 19 printed pages and 1 blank page.

2 ASRJC JC2 Prelim 2025 9729/04/H2 1 Determination of an enthalpy change of neutralisation, ∆Hneut, by thermometric titration The enthalpy change of neutralisation between an acid and an alkali can be determined using thermometric titration. This involves using a fixed volume of alkali with progressive addition of small volumes of acid and monitoring the temperature of the reaction mixture during the process. FA 1 is an aqueous solution of 1.80 mol dm-3 of a monobasic acid, HA. FA 2 is aqueous sodium hydroxide, NaOH You are to carry out a thermometric titration to determine the enthalpy change of neutralisation for the reaction given below. HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) Question continues on Page 4.

3 ASRJC JC2 Prelim 2025 9729/04/H2 [Turn Over BLANK PAGE

4 ASRJC JC2 Prelim 2025 9729/04/H2 (a) Prepare a table in the space provided and record, to appropriate level of precision: • all volumes of FA 1 added, V • the maximum temperature, T, reached after each addition of FA 1. It is important that the volume of FA 1 recorded is the total volume you have added up to the point when the temperature reading was made. Note: If you overshoot on an addition, record the actual total volume of FA 1 added up to that point. Procedure 1. Place a polystyrene cup inside a second polystyrene cup and place both cups in a glass beaker. 2. Fill the burette with FA 1. 3. Use a measuring cylinder to transfer 25 cm3 of FA 2 into the polystyrene cup. 4. Stir the FA 2 solution in the cup gently with the thermometer. Read and record its temperature. 5. Use the burette to add 5.00 cm3 of FA 1 into the cup. Stir the mixture gently with the thermometer. Read and record both the maximum temperature and the actual total volume of FA 1 added. 6. Repeat step 5 until a total of 45.00 cm3 of FA1 has been added. For each addition of FA 1, read and record both the maximum temperature and the actual total volume of FA 1 added up to that point. Results [3]

5 ASRJC JC2 Prelim 2025 9729/04/H2 [Turn Over (b) Plot a graph of temperature, T, on the y-axis, against total volume of FA 1 added, V, on the x-axis on the grid in Fig.1.1. Your scale on the y-axis should allow for extrapolation above the highest temperature recorded. Draw two lines of best fit, taking into account the points when the temperature of the mixture was rising and the points when the temperature was falling. Each line should have a shape best suited to its plotted points. Extrapolate (extend) the two lines until they intersect. Fig. 1.1 [3]

6 ASRJC JC2 Prelim 2025 9729/04/H2 (c) From your graph, read the initial temperature of FA 2, Tinitial, and the maximum temperature of the mixture, Tmax. Use these values to calculate the temperature change in the reaction, ∆T. Read the volume of FA 1 added, Vneut, at the maximum temperature of the mixture. Record all these values below. Tinitial = ………………………………... Tmax = ……………………………….. ∆T = ……………………………….. Vneut = ………………………………. [4] (d) (i) Using your answers from (c), calculate the heat change, q, when FA 1 has completely neutralised 25 cm3 of sodium hydroxide. You should assume that the specific heat capacity of the solution is 4.18 J g−1 K−1, and that the density of the solution is 1.00 g cm−3. q = …………………..……………………[2]

7 ASRJC JC2 Prelim 2025 9729/04/H2 [Turn Over (ii) Calculate the enthalpy change of neutralisation, ∆Hneut, for the reaction. The equation for the reaction is shown. HA(aq) + NaOH(aq) → NaA(aq) + H2O(l) Include the sign of ∆Hneut in your answer. ∆Hneut = ……………………..………………[4] (e) Apart from using a thermometer with a greater level of precision, suggest one improvement that could be made to improve the accuracy of results in (d)(ii). ………………………………………………………………………………………………… …………………………………………………………………………………………….. [1] (f) A student decided to perform the same experiment in (a) but used aqueous ammonia instead of aqueous sodium hydroxide. Suggest what effect, if any, would replacing aqueous sodium hydroxide with aqueous ammonia have on the value of enthalpy change of neutralisation calculated in (d)(ii). effect ………………………………………………………………………………………… ……………………………………………………………………………………………….. explanation ………………………………………………………………………………..... …………………………………………………………………………………………….. [2] [Total: 19]

8 ASRJC JC2 Prelim 2025 9729/04/H2 2 Determination of the amount of water of crystallisation in sodium sulfite crystals, Na2SO3• xH2O FA 3 is an aqueous solution of 126.0 g dm–3 of hydrated sodium sulfite with the formula Na2SO3• xH2O. FA 4 is 0.100 mol dm–3 iodine, I2. FA 5 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. You are also provided with Solution S. Solid sodium sulfite is often provided as the hydrated salt, Na 2SO3• xH2O, where x is an integer. You will determine the value of x by using a solution of this sodium sulfite salt and reacting it with an excess of aqueous iodine. Na 2SO3 + I2 + H2O → Na2SO4 + 2I– + 2H+ The amount of iodine remaining will be determined by titration using a known concentration of sodium thiosulfate, Na2S2O3. I2 + 2S2O32–→ 2I– + S4O62– (a) (i) Dilution of FA 3 FA 3 is too concentrated and needs to be diluted. Use a burette to transfer 25.00 cm 3 of FA 3 into a 100 cm3 volumetric flask. Make the solution up to the mark with deionised water. Label this solution FA 6. Titration of FA 6 against FA 5 1. Fill the burette with FA 5. 2. Use a pipette to transfer 10.0 cm 3 of FA 6 into a 250 cm3 conical flask. 3. Use another pipette to transfer 25.0 cm3 of FA 4 into the same conical flask. 4. Swirl the flask to mix the contents. 5. Run FA 5 from the burette into the conical flask. Near the end-point, when the brown solution becomes pale, add about 1 cm3 of Solution S using a teat pipette. 6. Continue adding FA 5 . The end -point is reached when the solution first becomes colourless. 7. Record your titration results, to an appropriate level of precision, in the space provided on page 9. 8. Discard the reaction mixture. 9. Repeat steps 2 to 8 until consistent results are obtained.

9 ASRJC JC2 Prelim 2025 9729/04/H2 [Turn Over Titration results [3]