2025 DHS H2 Chem Prelim P4 QP

This question paper consists of 20 printed pages. © DHS 2025 9729/04 [Turn over Name: Centre/Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper. 9729/04 25 August 2025 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your centre number, index number, name and class 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 sci entific 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. Shift Laboratory For Examiner’s Use 1 26 2 18 3 11 Total 55

2 © DHS 2025 9729/04 Answer all questions in the spaces provided. 1 Determination of the concentrations of two solutions by graphical analysis Sulfuric acid is a strong acid which neutralises sodium hydroxide in an exothermic reaction as shown in equation 1. equation 1 H2SO4 + 2NaOH → Na2SO4 + 2H2O In this question, you will prepare six mixtures, each containing the same total volume of solution but with different volumes of sulfuric acid and sodium hydroxide added. For each mixture, you will measure the temperature change. Graphical analysis of your results will enable you to determine the concentration s of the sulfuric acid and of the sodium hydroxide used. FA 1 is aqueous sodium hydroxide, NaOH. FA 2 is aqueous sulfuric acid, H2SO4. (a) Prepare a table in the space provided on page 4 in which to record, to an appropriate degree of precision: • the volumes of FA 1 (VFA 1) and FA 2 (VFA 2) used • all values of temperature TFA 1, TFA 2, Tave, Tmax and Tmax Experiment 1 1. Using a measuring cylinder, transfer 10.0 cm3 of FA 1 into a Styrofoam cup. Place this cup inside a second Styrofoam cup, which is placed in a 250 cm3 glass beaker. 2. Place the thermometer into the FA 1 in the cup. Tilt the cup if necessary to ensure the bulb of the thermometer is fully covered. Record the temperature of FA 1, TFA 1. 3. Using another measuring cylinder, measure 60.0 cm3 of FA 2. 4. Rinse and dry the thermometer. Measure and record the temperature of FA 2, TFA 2. 5. Add the FA 2 from the measuring cylinder to the FA 1 in the Styrofoam cup. 6. Using the thermometer, stir the mixture continuously until it reaches its maximum temperature. Record this temperature, Tmax. 7. Rinse and dry the thermometer. Wash out the Styrofoam cup thoroughly with tap water and then with deionised water. Stand the cup upside down on a paper towel to drain.

3 © DHS 2025 9729/04 [Turn over Experiments 2 to 6 Repeat experiment 1 five times, adding 20.0 cm3, 30.0 cm3, 40.0 cm3, 50.0 cm3 and 60.0 cm3 of FA 1 respectively in step 1. In each experiment, you should adjust the volume of FA 2 in step 3 to ensure the same total volume of solution is used. You should alternate the use of the two Styrofoam cups. Calculate the following values for each experiment • Tave, the average of TFA 1 and TFA 2 • Tmax, where Tmax = Tmax – Tave Record all volumes and temperatures in your table on page 4.

4 © DHS 2025 9729/04 Results [4]

5 © DHS 2025 9729/04 [Turn over (b) (i) Plot a graph of Tmax on the y–axis against VFA 1 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 straight 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. Extrapolate (extend) both lines until they intersect. Fig. 1.1 [3]

6 © DHS 2025 9729/04 (ii) From your graph, read VFA 1 and Tmax of the intersection point. Record these values in the spaces provided. VFA 1 = ……………………. cm3 Tmax = ………………………°C [3] (iii) Calculate the heat change, q, at the point of neutralisation in your experiment, using your answer in (b)(ii). 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] (iv) The literature value for the enthalpy change of neutralisation of a strong alkali by a strong acid is −57.0 kJ mol−1. Calculate the amount of water formed at the point of neutralisation in your experiment using your answer in (b)(iii). Hence, determine the concentrations of the sulfuric acid and of the sodium hydroxide used. amount of water formed = …………………………. mol [NaOH] in FA 1 = …………………………. mol dm−3 [H2SO4] in FA 2 = …………………………. mol dm−3 [4]

7 © DHS 2025 9729/04 [Turn over (v) The actual concentration of the sodium hydroxide used is 2.40 mol dm−3. Calculate the percentage error in your value of concentration. [1] (c) (i) The calculated values of the concentrations of the sulfuric acid and of the sodium hydroxide in (b)(iv) are expected to be lower than the actual values. Explain why. ………………………………………………………………………………………………...... ………………………………………………………………………………………………...... ………………………………………………………………………………………………...... …………………………………………………………………………………………….... [1] (ii) Suggest two improvements to the method that would give more accurate values for the concentrations of the sulfuric acid and of the sodium hydroxide. ………………………………………………………………………………………………...... ………………………………………………………………………………………………...... ………………………………………………………………………………………………...... …………………………………………………………………………………………….... [1]

8 © DHS 2025 9729/04 (d) Planning The enthalpy change of neutralisation when sodium hydroxide reacts with sulfuric acid, Hneut, can be determined using a thermometric titration. This involves adding portions of aqueous sulfuric acid progressively to a fixed volume of aqueous sodium hydroxide in a Styrofoam cup. The temperature of the resulting solution is then measured after each addition. A suitable graph can be plotted to find T at equivalence point and hence Hneut. You may assume that you are provided with: • 50 cm3 of 2.03 mol dm−3 aqueous sodium hydroxide, FB 1 • 50 cm3 of 1.00 mol dm−3 aqueous sulfuric acid, FB 2 (i) Determine a suitable volume of FB 1 to be used for the thermometric titration. Explain your reasoning, showing any relevant calculations. volume of FB 1 = ……………………… cm3 [2] (ii) Suggest a suitable graph to be plotted to determine T at equivalence point. ………………………………………………………………………………………………...... …………………………………………………………………………………………….... [1]

9 © DHS 2025 9729/04 [Turn over (iii) Plan a procedure to determine Hneut using thermometric titration. You may assume you are provided with: • 50 cm3 of 2.03 mol dm−3 aqueous sodium hydroxide, FB 1 • 50 cm3 of 1.00 mol dm−3 aqueous sulfuric acid, FB 2 • the equipment normally found in a school or college laboratory. In your plan you should include brief details of: • the apparatus you would use • the volumes of FB 1 and FB 2 you would use • the procedure you would follow • the measurements you would make to allow the suitable graph identified in (d)(ii) to be drawn.