2021 DHS H2 Prelim Paper 4 Solutions

1 This question paper consists of 18 printed pages. © DHS 2021 9729/04 [Turn over Name: Centre/Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY 9729/04 Paper 4 Practical 24 August 2021 2 hours 30 minutes Candidates answer on the Question Paper. 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 23 and 24. 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 22 2 5 3 15 4 13 Total 55

2 © DHS 2021 9729/Prelim/04/21 Answer all questions in the spaces provided. 1 Determination of a value for the enthalpy change of precipitation, DHppt, of barium sulfate FA 1 is 1.00 mol dm-3 sodium hydroxide, NaOH. FA 2 is a saturated solution of barium hydroxide, Ba(OH)2. FA 3 is 1.00 mol dm-3 sulfuric acid, H2SO4. The addition of FA 2 to FA 3 results in neutralisation and precipitation reactions occurring as shown in equations 1 and 2 respectively. equation 1 H+(aq) + OH-(aq) ® H2O(l) DHneut equation 2 Ba2+(aq) + SO42-(aq) ® BaSO4(s) DHppt In the first experiment, you will determine the maximum temperature change as a result of both reactions occurring in a Styrofoam cup. In the second experiment, you will measure the temperature of the resulting mixture after each addition of a fixed volume of FA 1 solution to the remaining H+ ions in the Styrofoam cup. You will analyse your results graphically in order to determine an accurate value for the temperature change of the mixture, caused by the neutralisation reaction shown in equation 1. You will use this value to calculate the heat change for the second experiment and hence determine a value for the enthalpy change of neutralisation which will then be used to determine a value for the enthalpy change of precipitation for BaSO4. (a) (i) Experiment 1 1. Use a measuring cylinder to add 20.0 cm3 of FA 3 into a Styrofoam cup. Place this cup inside a second Styrofoam cup, which is placed in a 250 cm3 glass beaker. Stir and measure the temperature of this solution, T1,initial. Record this temperature in Table 1.1. 2. Use another measuring cylinder to add 5.0 cm3 of FA 2 into the Styrofoam cup. Using the thermometer, stir the mixture and record its maximum temperature, T1,max, in Table 1.1. You should expect a small rise in temperature. Retain the contents in the Styrofoam cup for experiment 2 in (b)(i). Complete Table 1.1 by calculating the value for DT1,max. Table 1.1 T1,initial / °C 30.0 T1,max / °C 31.0 DT1,max / °C +1.0 [1] (ii) Calculate the heat change, q1, for experiment 1. 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.

3 © DHS 2021 9729/Prelim/04/21 [Turn over [1] q1 = mcDT = (25)(4.18)(DT1,max) = w J (3sf) q1 = …………………………. Marker’s Comments This question was answered well by the majority. Some students identified the total volume of solution wrongly. (iii) Calculate the percentage error associated with the value of DT1,max and hence comment on the accuracy of the heat change, q1, calculated in (a)(ii). [1] % error = ±0.1 + 0.1DT1,max ´ 100 = ± ___ % Since there is a high percentage error associated with the value of DT1,max, the heat change, q1, calculated using DT1,max is not accurate. (b) (i) Experiment 2 1. Fill a burette with FA 1. 2. Record, in Table 1.2, the initial temperature of the mixture in the Styrofoam cup from experiment 1. 3. Run 5.00 cm3 of FA 1 from the burette into the Styrofoam cup. Using the thermometer, stir the mixture continuously until it reaches its maximum temperature. Record this temperature, T2,max, in Table 1.2. 4. Repeat step 3 until a total volume of 50.00 cm3 of FA 1 is added. Table 1.2 total volume of FA 1 added after each addition, VFA 1 / cm3 T2,max / °C 0.00 30.8 5.00 32.8 10.00 34.2 15.00 35.4 20.00 36.2 25.00 37.0 30.00 37.8 35.00 38.4 40.00 38.4 45.00 37.8 50.00 37.2 [2]

4 © DHS 2021 9729/Prelim/04/21 (ii) Plot a graph of T2,max on the y-axis against VFA 1 on the x-axis. Draw two best-fit lines, • the first is a smooth curve taking into account all of the points before, • the second is a straight line taking into account all of the points after the temperature of the mixture has started to drop. Extrapolate (extend) both lines until they intersect (cut) each other. [3] (iii) From your graph, read VFA 1 and T2,max of the intersection point. Record these values in the spaces provided. Deduce the maximum temperature change, DT2,max. VFA 1 = ……………………. cm3 T2,max = ………………………°C DT2,max = ………………………°C VFA 1 = 37.50 cm3 T2,max = 38.7 °C DT2,max = 38.7 - 30.8 = +7.9 °C [2]

5 © DHS 2021 9729/Prelim/04/21 [Turn over (iv) Calculate the heat change, q2, for experiment 2 using the values you deduced in (b)(iii). 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. q2 = …………………………. q2 = mcDT = (25+Vmax)(4.18)(DT2,max) = x J (3sf) [1] (v) Hence determine a value for the enthalpy change, for the neutralisation reaction shown in equation 1, DHneut. H2SO4 + 2NaOH ® Na2SO4 + 2H2O NaOH º H2O Moles of H2O formed = moles of NaOH added = Vmax ´ 10-3 ´ 1 = y mol DHneut =- xy ´ 10-3 = -z kJ mol-1 DHneut = …………………………. [2] (c) (i) The FA 2 solution was prepared as follows. An excess of solid Ba(OH)2.8H2O was added to 100 cm3 of water and the mixture was allowed to stand. The undissolved solid was then removed by filtration to obtain the saturated solution of barium hydroxide. Calculate the concentration of barium and hydroxide ions in the FA 2 solution given that the solubility of Ba(OH)2.8H2O in water is 56 g dm-3. [Mr: Ba(OH)2.8H2O, 315.3] [1] solubility of Ba(OH)2.8H2O = 56315.3 = 0.17761 mol dm-3 [Ba2+] in FA 2 = 0.178 mol dm-3 [OH-] in FA 2 = 0.17761 ´ 2 = 0.355 mol dm-3 [Ba2+] = …………………………. [OH-] = …………………………. (ii) Hence calculate the heat evolved from the neutralisation reaction between FA 2 and FA 3 in experiment 1. [1] moles of water formed = moles of OH- neutralised = 0.35522 ´ 0.005 = 0.0017761 mol Heat evolved from neutralisation = 0.0017761 ´ |DHneut| ´ 103 = r J

6 © DHS 2021 9729/Prelim/04/21 (iii) Use your answers in (a)(ii) and (c)(ii) to determine the heat change due to the precipitation of BaSO4 in experiment 1. Hence determine a value for the enthalpy change for the precipitation reaction shown in equation 2, DHppt. [3] Heat evolved from precipitation = q1 - heat evolved from neutralisation = s J Heat change of solution due to the precipitation of BaSO4 = +s J moles of BaSO4 = moles of Ba2+ = 0.17761 ´ 0.005 = 8.8804 ´ 10−4 mol DHppt = -heat evolved from precipitation8.8804 ´ 10-4 ´ 10−3 = -p kJ mol-1 Heat change due to the precipitation of BaSO4 = …………………………. DHppt = …………………………. (iv) Explain, in terms of the chemistry involved, the sign of DHppt. Hence explain