2024 DHS H2 Chemistry Prelim P4 (Ans)

This document consists of 15 printed pages. © DHS 2024 9729/04 [Turn over Suggested Solutions DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper. 9729/04 19 August 2024 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 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 20 and 21. 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 16 2 11 3 13 4 15 Total 55

2 © DHS 2024 9729/04 Answer all questions in the spaces provided. 1 Determination of the enthalpy change of decomposition of magnesium carbonate The enthalpy change of decomposition of magnesium carbonate, H1, cannot be determined experimentally. Reaction 1 MgCO3(s) → MgO(s) + CO2(g) H1 However, the enthalpy changes for Reaction 2 and Reaction 3 can be determined experimentally as H2 and H3 respectively. Reaction 2 MgCO3(s) + H2SO4(aq) → MgSO4(aq) + H2O(l) + CO2(g) H2 Reaction 3 MgO(s) + H2SO4(aq) → MgSO4(aq) + H2O(l) H3 You will carry out an experiment to determine the enthalpy change for Reaction 3, H3, and use this value, together with H2, to calculate the enthalpy change of decomposition of magnesium carbonate, H1. FA 1 is solid magnesium oxide powder, MgO. FA 2 is 1.0 mol dm−3 sulfuric acid, H2SO4. (a) Determination of H3 In this experiment, you will be adding solid magnesium oxide powder to sulfuric acid in a Styrofoam cup. You will then measure the temperature of the reaction mixture over time. Plotting an appropriate graph using the data obtained will allow you to determine H3. Prepare a table in the space provided on page 3 in which to record: • all weighings to an appropriate level of precision, • all values of temperature, T, to an appropriate level of precision, • all values of time, t, recorded to 0.5 min. Procedure 1. Weigh the capped bottle containing solid FA 1. Record this mass. 2. Place a clean and dry Styrofoam cup on the 250 cm3 glass beaker. 3. Using a 25 cm3 measuring cylinder, add 25 cm3 of FA 2 into the Styrofoam cup. 4. Stir the solution in the cup with the thermometer. Read and record its temperature, T (time, t = 0.0 min). 5. Continue to stir the solution. Read and record T every 0.5 min. 6. At exactly 2.0 minutes, transfer all the solid FA 1 into the Styrofoam cup. Stir the mixture but do not read T. 7. Read and record T at t = 2.5 min and then every 0.5 min until t = 7.0 min. 8. Continue stirring occasionally throughout this time. 9. Reweigh the emptied bottle and its cap. Record this mass.

3 © DHS 2024 9729/04 [Turn over (i) Results Mass of capped bottle containing solid FA 1 / g 5.918 Mass of emptied bottle and cap / g 5.420 Mass of FA 1 used / g 0.498 t / min T / °C 0.0 30.0 0.5 30.0 1.0 30.0 1.5 30.0 2.0 − 2.5 35.0 3.0 42.0 3.5 45.8 4.0 46.2 4.5 45.6 5.0 45.2 5.5 44.8 6.0 44.2 6.5 43.6 7.0 43.0 [3] (ii) Plot a graph of temperature, T, on the y −axis, against time, t, on the x−axis, on the grid in Fig.1.1. Draw a best−fit straight line taking into account all of the points before t = 2.0 min. Draw another best −fit straight line taking into account all of the points after the temperature of the mixture has started to decrease steadily. Extrapolate both lines to t = 2.0 min.

4 © DHS 2024 9729/04 Fig. 1.1 [3] (iii) From your graph, read the minimum temperature, Tmin, and the maximum temperature Tmax, at 2.0 min. Record these values in the spaces provided below. Deduce the temperature change, T, at t = 2.0 min. Tmin = 30.0 °C Tmax = 48.2 °C T = 18.2 °C [1]

5 © DHS 2024 9729/04 [Turn over (iv) Excess sulfuric acid was used in (a). Without carrying out any additional tests, what observation could you have made during your experiment to confirm this? All the magnesium oxide has completely/all dissolved or no more solid remaining. [1] (v) Calculate the heat change, q, that occurred during the reaction in (a). 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 = mc|T| = (25)(4.18)|+18.2| = 1901.9 J = 1.90 kJ (3sf) [1] (vi) Use your answer to (a)(v) to calculate H3, the enthalpy change for the reaction between magnesium oxide and sulfuric acid. [Ar: Mg, 24.3; O, 16.0] Moles of MgO used = 0.498 40.3 = 0.01235 mol H3 = − 1.9019 0.01235 = −153.908 = −154 kJ mol−1 [1] (b) Determination of H2 Note: You do not need to perform this experiment. In this experiment, the initial temperature of 25 cm 3 of sulfuric acid in a Styrofoam cup is measured and recorded as 30 C. After which, 2.00 g of solid magnesium carbonate powder is added to the sulfuric acid in the cup. The maximum temperature reached during the reaction is measured and recorded as 40 C. (i) Using suitable calculations, show that sulfuric acid is in excess. [Ar: Mg, 24.3; C, 12.0; O, 16.0] Moles of MgCO3 used = 2.00 84.3 = 0.02372 mol Moles of H2SO4 used = 25 1000 × 1.00 = 0.025 mol MgCO3 ≡ H2SO4 Hence, H2SO4 is in excess. [1] (ii) Hence, calculate H2, the enthalpy change for the reaction between magnesium carbonate and sulfuric acid. q = mc|T| = (25)(4.18)|+10.0| = 1045 = 1.045 kJ (3sf) H2 = − 1.045 0.02372 = −44.046 = −44.0 kJ mol−1 [2]

6 © DHS 2024 9729/04 (c) Using your answers in (a)(vi) and (b)(ii), construct an energy cycle to calculate H1, the enthalpy change for Reaction 1. MgCO3(s) MgO(s) + CO2(g) MgSO4(aq) + H2O(l) + CO2(g) By Hess’ Law, ∆H1 = ∆H2 – ∆H3 ∆H1 = −44.055 − (−153.908) = +109.853 = +110 kJ mol–1 [2] (d) The method used to determine ∆H3 in (a) was more accurate than the method used to determine ∆H2 in (b). Suggest why the method used for Reaction 2 was less accurate. No correction made for loss of heat on cooling in Reaction 2. [1] [Total: 16] H3 + H2SO4(aq) H2 + H2SO4(aq) H1

7 © DHS 2024 9729/04 [Turn over 2 Planning Calcium hydroxide, Ca(OH)2, is a white powder which is produced when calcium oxide is mixed with water. Calcium hydroxide is poorly soluble in water and its solubility is described by the following equilibrium. Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH−(aq) (a) A 25.0 cm 3 saturated solution containing Ca(OH) 2 dissolved in 0.020 mol dm−3 NaOH at 25 °C was titrated with HCl. Given the average titre volume of HC l is 2 0.00 cm 3 and the solubility of Ca(OH)2 in 0.020 mol dm−3 NaOH at 25 ºC is approximately 5.00 x 10−3 mol dm−3, show that a suitable concentration of HCl needed for this titration is 0.0375 mol dm−3. Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH–(aq) NaOH → Na+ + OH– Total [OH–] = 0.020 + 2(0.005) = 0.0300 mol dm–3 Amount of OH– in 25.0 cm3 = 0.03 x (25.0/1000) = 7.50 x 10–4 mol Amount of HCl required = 7.50 x 10–4 mol [HCl] = 7.50 x 10–4 / 0.0200 = 0.0375 mol dm–3 [2] (b)