YIJC 2025 Prelim P4 QP (for exchange) H2 Chem

©YIJC 9729/04/JC2/PE/2025 [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CG INDEX NO CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions 9729/04 25 August 2025 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your name and class in the spaces at the top of this page. 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 18 printed pages and 2 blank pages. Shift Laboratory For Examiner’s Use 1 / 16 2 / 17 3 / 6 4 / 16 Total / 55

2 ©YIJC 9729/04/JC2/PE/2025 Answer all the questions in the spaces provided. 1 Determination of concentrations of sodium hydroxide and of sodium carbonate in a mixture Sodium hydroxide reacts with hydrochloric acid according to the equation below: reaction 1 NaOH + HCl → NaCl + H2O Sodium carbonate reacts with hydrochloric acid in two separate stages. The reactions that occur are: reaction 2 Na2CO3 + HCl → NaCl + NaHCO3 reaction 3 NaHCO3 + HCl → NaCl + CO2 + H2O You are required to find the concentrations of sodium hydroxide and of sodium carbonate in FA 1 by means of a double-indicator titration. In a double-indicator titration, two different indicators are used, separately, in the same titration. For this experiment, you will be using thymolphthalein indicator followed by methyl orange indicator . Thymolphthalein indicates the end -point when reactions 1 and 2 are complete, while methyl orange indicates the end-point when reaction 3 is complete. FA 1 is a solution containing sodium hydroxide, NaOH, and sodium carbonate, Na2CO3. FA 2 is an aqueous solution containing 1.50 mol dm–3 hydrochloric acid, HCl. Solution T is thymolphthalein indicator. Solution M is methyl orange indicator. (a) Dilution of FA2 Use a burette to transfer 20.00 cm3 of FA 2 into a 250 cm3 volumetric flask. Make the solution up to the mark with deionised water and shake well to mix. Label this solution FA 3. This is a solution of hydrochloric acid of a concentration that is suitable for the titration. (b) (i) Titration of FA 1 against FA 3 You are to titrate FA 1 with FA 3 using Solution T , thymolphthalein indicator, and Solution M, methyl orange indicator. Record your titration results to an appropriate level of precision in the table provided on page 3. 1. Fill a second burette with FA 3. Record the initial burette reading. 2. Pipette 25.0 cm3 of FA 1 into a 250 cm3 conical flask. Place the cap back over the FA 1 bottle to prevent absorption of carbon dioxide from the atmosphere. 3. Add 4 to 5 drops of Solution T.

3 ©YIJC 9729/04/JC2/PE/2025 [Turn over 4. Titrate the solution in the conical flask with FA 3 from the burette. The first end-point is reached when the solution turns from blue to colourless. Record the burette reading. Keep the contents in the flask for step 5. 5. To the solution from step 4, add 4 to 5 drops of Solution M. Continue to titrate with FA 3 from the burette. The second end-point is reached when the solution turns from yellow to orange. Record the burette reading. 6. Repeat steps 2 to 5 until consistent results for the total volume of FA 3 used for the titration are obtained. The volume of FA 3 used to reach the first end -point in step 4 may not be consistent across titrations. Titration results Initial burette reading / cm3 Burette reading at first end-point / cm3 Burette reading at second end-point / cm3 Volume of FA 3 used to complete reactions 1 and 2 / cm3 Volume of FA 3 used to complete reaction 3 / cm3 Total volume of FA 3 used for titration / cm3 [4] (ii) You should consider only the total volume of FA 3 used for the titration when deciding which sets of titration results to use. From your titrations, obtain • a suitable volume of FA 3 used to complete reactions 1 and 2, and • a suitable total volume of FA 3 used for the titration to be used in your calculations. Show clearly how you obtained these volumes. volume of FA 3 required to reach first end-point = cm3 total volume of FA 3 required for the titration = cm3 [1]

4 ©YIJC 9729/04/JC2/PE/2025 (c) (i) Calculate the concentration of HCl in FA 3. concentration of HCl in FA 3 = [1] (ii) Calculate the amount of sodium carbonate, Na2CO3, present in 25.0 cm3 of FA 1. amount of Na2CO3 in 25.0 cm3 of FA 1 = [2] (iii) Calculate the amount of sodium hydroxide, NaOH, present in 25.0 cm3 of FA 1. amount of NaOH in 25.0 cm3 of FA 1 = [2] (iv) Using your answers from (c)(ii) and (c)(iii), calculate the concentrations, in mol dm –3, of Na2CO3 and NaOH in FA 1. If you were unable to obtain an answer in (c)(ii) or in (c)(iii), use the values 0.00120 mol and 0.00100 mol respectively. These are not the correct answers. concentration of Na2CO3 in FA 1 = [1] concentration of NaOH in FA 1 = [1]

5 ©YIJC 9729/04/JC2/PE/2025 [Turn over (d) (i) Both thymol blue and thymolphthalein change colour over a similar pH range. Thymolphthalein changes from blue to colourless, while thymol blue changes from blue to yellow. Suggest one reason why thymolphthalein is a more suitable indicator than thymol blue for the titration. [1] (ii) Common indicators used in titrations can be weak organic acids or bases. Methyl orange is an example of an indicator that can behave as a weak base. Deduce the effect on the volume of FA 3 required to reach the second end-point when too much Solution M is added after the first end-point. [1] (e) In step 2 of (b)(i), you were instructed to place the cap back over the FA 1 bottle immediately after use. This is to prevent the absorption of carbon dioxide, CO2, from the atmosphere which reacts with sodium hydroxide. 2NaOH + CO2 → Na2CO3 + H2O A student performed the experiment in (b)(i) and forgot to replace the cap on the FA 1 bottle. Suggest what effect, if any, using the uncapped FA 1 would have on the total volume of FA 3 required for the titration. Explain your answer. effect explanation [1] (f) The discontinuous method can also be used to investigate the composition of NaOH and Na2CO3 by conducting two separate titrations on two separate samples, each with a different indicator. Suggest how the accuracy of the calculated composition of NaOH and Na2CO3 may be affected. [1] [Total: 16]

6 ©YIJC 9729/04/JC2/PE/2025 2 To determine the enthalpy change of a reaction between sodium carbonate, water and carbon dioxide It is difficult to directly determine the enthalpy change of the reaction between sodium carbonate, water and carbon dioxide, as shown in equation 1. equation 1 Na2CO3(s) + H2O(l) + CO2(g) → 2NaHCO3(s) ∆H1 Sodium carbonate and sodium hydrogencarbonate each react with hydrochloric acid. equation 2 NaH