RVHS Prelim H2 Chemistry P4 QP

River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination [Turn over RIVER VALLEY HIGH SCHOOL JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME CLASS 2 4 J CENTRE NUMBER S INDEX NUMBER H2 CHEMISTRY 9729/04 Paper 4 Practical 26 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 Centre number, index number, class and name on all the work that 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. DO NOT WRITE IN ANY BARCODES. 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. The number of marks is given in brackets [ ] at the end of each question or part question. Shift Laboratory For Examiner’s Use 55 This document consists of 20 printed pages.

2 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination 1 Determination of the equilibrium constant Aqueous iron(II) ions, Fe2+(aq), are usually kept in acidic conditions to prevent them from readily oxidising to aqueous iron(III) ions, Fe3+(aq). Fe2+(aq) ions react with Ag +(aq) ions in a redox reaction. The following equilibrium is established. Fe2+(aq) + Ag+(aq) = Fe3+(aq) + Ag(s) The concentration of Ag+(aq) at equilibrium can be found by a titration with a standard solution of aqueous potassium thiocyanate, KSCN(aq). The equilibrium constant for the reaction can be found using the following equation. Kc = 3+ eqm 2+ + eqm eqm [Fe (aq)] [Fe (aq)] ×[Ag (aq)] FA 1 is the equilibrium mixture. FA 2 is 0.0100 mol dm–3 potassium thiocyanate, KSCN. The equilibrium mixture has been prepared for you using the following instructions. You do not need to perform the preparation. Step 1 Add 100.0 cm3 of 0. 100 mol dm–3 Ag+(aq) to 100.0 cm3 of acidified 0.100 mol dm–3 Fe2+(aq) in a 500 cm3 conical flask and stopper the flask. Step 2 Ag precipitate forms. Leave the resultant solution for 4 hours. This is the equilibrium mixture, FA 1. When the equilibrium mixture is titrated against a standard solution of potassium thiocyanate, white ppt of silver thiocyanate is formed as shown in equation 1. equation 1 Ag+(aq) + SCN–(aq) = AgSCN(s) The end point is indicated by formation of complex ion between thiocyanate and iron(III) ion when one excess drop of aqueous potassium thiocyanate is added. In this experiment, you will determine the equilibrium constant, Kc, for the reaction between silver(I) and iron(II) ions. You will titrate FA 1 against FA 2. You are to rinse all glassware with deionised water thoroughly before starting any practical work.

3 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination [Turn over (a) (i) Titration of FA 1 against FA 2. 1. Fill the burette with FA 2. 2. Use a pipette to transfer 10.0 cm 3 of FA 1 into a 250 cm 3 conical flask. Do not disturb the silver ppt. 3. Run FA 2 from the burette into this flask. A white ppt forms. 4. Continue adding FA 2 slowly. The end-point is reached when the solution turns from colourless to permanent pale orange. The white ppt remains and the mixture appears off-white. 5. Record your titration results, to an appropriate level of precision, in the space provided. Results [5] (ii) From your titrations, obtain a suitable volume of FA2, VFA2, to be used in your calculations. Show clearly how you obtained this volume. volume of FA 2 used = …………………………cm3 [1]

4 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination (iii) Calculate the concentration of Ag+(aq) in the equilibrium mixture. concentration of Ag+(aq) = ……………………………… [1] (iv) Hence, determine the concentration of Fe3+(aq) in the equilibrium mixture. concentration of Fe3+(aq) = ……………………………… [2] (v) Determine the value of Kc. Include units in your answer Kc = ……………………………… [2]

5 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination [Turn over (b) Student A proposed that this titration method is not a valid method for the investigation of the value of Kc as the position of equilibrium will shift during titration, giving inaccurate results. Apart from considering the effect of temperature on Kc, explain whether you agree with student A. ……………………………………………………………………………………... ……………………………………………………………………………………... ……………………………………………………………………………………... [1] (c) Suggest one modification to the procedure which would make the value of the Kc calculated in (a)(v) more accurate. ……………………………………………………………………………………... ……………………………………………………………………………………... [1] [Total: 13]

6 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination 2 Investigation of the enthalpy change of neutralisation You will investigate the enthalpy change of neutralisation, ∆ Hneut, between aqueous barium hydroxide of known concentration and a dilute organic acid. You will use your results to suggest the identity of the organic acid. The acid is a halogenodicarboxylic acid containing one halogen atom, X, per molecule. Ba(OH)2(aq) + CHX(COOH)2(aq) → CHX(COO)2Ba(aq) + 2H2O(l) FA 3 is 0.950 mol dm−3 barium hydroxide, Ba(OH)2. FA 4 is a solution containing 141.3 g dm−3 of the organic diacid, CHX(COOH)2. (a) Method Prepare a table in the space provided on page 6, to appropriate degree of precision: • All volumes of FA 4 added, V • The maximum temperature, T, reached after each addition of acid, FA 4. It is important that the volume of FA 4 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 4 added up to that point. Procedure 1. Support the cup in the 250 cm3 beaker. 2. Pipette 25.0 cm3 of FA 3 into the cup. 3. Place the thermometer into FA 3. Read and record its temperature. 4. This is the temperature when the volume of FA 4 is 0.00 cm3. 5. Fill the burette with FA 4. 6. Run 5.00 cm3 of FA 4 into the cup containing FA 3. 7. Stir the mixture. Record the highest temperature observed. 8. Run further 5.00 cm3 portions of FA 4 into the same cup. 9. On each addition of FA 4 stir the contents of the cup. Record the highest temperature after each addition. 10. Repeat 8 and 9 until a total of 40.0 cm3 of FA 4 has been added. [2]

7 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination [Turn over (b) (i) Plot a graph of temperature (y -axis) against volume of FA 4 added (x-axis) on the grid. Select a scale on the y -axis to include a temperature of 2 ºC above your maximum thermometer reading. Draw two lines of best fit, the first for the increase in temperature and the second for after the maximum temperature has been reached. Extrapolate the two lines so they intersect. This intersection corresponds to the volume of FA 4 required to form a neutral solution. [3]

8 River Valley High School 9729/04/PRELIMS/25 2025 Preliminary Examination (ii) Use your graph to determine the volume of FA 4, Vneut, required to neutralise 25.0 cm3 of FA 3. Show your working on the graph clearly. Vneut = …………………………cm3 [1] (iii) Calculate the energy change, in J