YIJC 2021 Prelim P4 Answer

1 ãYIJC [Turn over YISHUN INNOVA JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION CANDIDATE NAME CLASS DATE H2 CHEMISTRY 9729/04 Paper 4 Practical Paper Tuesday 24 August 2021 2 hours 30 minutes Candidates answer on question paper. No Additional Materials are required 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. For Examiner’s use 1 19 2 7 3 17 4 12 Total 55 Shift Laboratory

2 ãYIJC Answer all the questions in the spaces provided. 1 To determine the enthalpy change of neutralisation, ∆Hn The enthalpy change of neutralisation between an acid and an alkali can be determined using a type of experiment known as thermometric titration. One way of performing a thermometric titration involves using a fixed volume of alkali with progressive addition of small volumes of acid and monitoring the temperature of the reaction mixture during the process. You are to determine the enthalpy change of neutralisation for the reaction given below. HA(aq) + NaOH(aq) ® NaA(aq) + H2O(l) FA 1 is 1.80 mol dm–3 HA FA 2 is aqueous sodium hydroxide, NaOH In this experiment, you will measure the temperature of the contents of a polystyrene 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 of aqueous HA and NaOH. You will use this value to calculate the heat change, q, for the experiment and hence determine a value for the enthalpy change of neutralisation for the reaction, ∆Hn. (a) Procedure 1. Place one polystyrene cup in a 250 cm3 beaker. 2. Rinse and fill the burette with FA 1. 3. Use a measuring cylinder to transfer 25 cm3 of FA 2 into the polystyrene cup. 4. Stir the solution in the cup with the thermometer. Read and record its temperature. 5. Run 5.00 cm3 of FA 1 into the cup. Stir the solution with the thermometer. Record the new temperature and volume of FA 1 added. 6. Run a second portion of 5.00 cm3 of FA 1 into the cup. Stir and record the new temperature and the total volume of FA 1 added. 7. Continue adding FA 1 in 5.00 cm3 portions. Stir and record each new temperature and the total volume of FA 1 until a total of 45.00 cm3 has been added. Results In an appropriate format in the space below, prepare a table to record the results of your experiment: • all measurements of volume used • all values of temperature, T, to an appropriate level of precision

3 ãYIJC Table of results Volume of FA 1 added / cm3 Temperature / °C 0.00 28.7 5.00 32.7 10.00 35.2 15.00 37.0 20.00 38.2 25.00 39.0 30.00 38.1 35.00 36.6 40.00 35.6 45.00 35.0 [4] M1 Table has correct headers and units. M2 Tabulates 10 sets of temp and vol up from 0.00cm3 to 45.00cm3 or Tabulates 9 sets of temp and vol up from 0.50cm3 to 45.00cm3, with initial Temp clearly recorded M3 All volumes are recorded to 2dp, temperatures to nearest 0.1 oC M4 Temperature increase from 5.00 to 25.00 cm3 and decreases after 25.00 cm3 of FA 1 is added (b) Plot a graph of temperature, T, on the y-axis, against total volume of FA 1 added, on the x-axis. The temperature axis should allow you to include a point at least 2 ºC greater than the maximum temperature recorded. Draw a best-fit straight line taking into account all of the points for the increase in temperature of the mixture. Draw another best-fit straight line taking into account all of the points for the cooling of the solution. Extrapolate the two lines and determine the • maximum temperature reached, Tmax, • maximum temperature increase, ∆Tmax, • corresponding volume of FA 1 added.

4 ãYIJC Tmax = ………………………… ∆Tmax = ………………………… Volume of FA 1 added = ………………………… [4]

5 ãYIJC M5 Axes correct way round + correct labels + units + scale (scale must be chosen to allow for the lines to be extrapolated to intersect) Graph has to occupy more than half the grid for both axes. M6 Plotting – all points within ±½ small square. (select two from each side to check) M7 Both graph lines are straight best-fit lines and lines are correctly extrapolated to intersect

6 ãYIJC M8 Tmax and volume of FA1 correctly read from the graph to ±½ small square and correct ∆T calculated (Tinitial may come from table or graph plot or intercept on y-axis) (c) Explain the shape of the graphs before and after the maximum temperature is reached. before maximum temperature is reached ………………………………………………………………………………………………………….… ……………………………………………………………………………………………………………. M9 As more acid was added, temperature rose as the (neutralization reaction between HA and NaOH) is exothermic / releases heat. after maximum temperature is reached ………………………………………………………………………………………………………….… ……………………………………………………………………………………………………………. M10 NaOH has been completely neutralized/reacted and no more heat is produced / heat is lost to surroundings /heat spread over a larger volume/cooled [2] (d) (i) Calculate the number of moles of HA present in the volume of FA 1 recorded in (b). M11 No of moles of HA = 1.80 x (21/1000) = 0.0378 mol number of moles of HA = ……………………. [1] (ii) Using your answers to (b), calculate the heat change, q, when FA 1 has completely neutralised 25 cm3 of sodium hydroxide. 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. Total volume = 25 + 21 = 46 cm3 Q = mcΔT = 46 x 4.18 x 11.0 = 2115.08 J = 2120 J M12 correct total volume M13 correct substitution of values into mcΔT (allow ecf)

7 ãYIJC q = …………………… [2] (iii) Determine the enthalpy change of neutralisation, ∆Hn, for the reaction. No of moles of water = no of moles of HA = 0.0378 mol ΔHn = – (q ÷ moles of water) = – (2115.08 x 10–3 ÷ 0.0378) = – 56.0 kJ mol–1 M14 Allow ecf If no negative sign, no marks ∆Hn = …………………… [1] (e) The maximum error in a thermometer reading is ±0.1 °C. Calculate the maximum percentage error in the increase in temperature recorded. M15 Percentage error = (0.1 x 2 ÷ 11) x 100% = 1.82% maximum percentage error = …………………… [1] (f) Apart from using a thermometer with a greater level of precision, suggest one improvement that could be made to the method carried out in (a) M16 Use a burette / pipette to measure volume of FA 2 / cover with lid / use smaller volumes close to Tmax ………………………………………………………………………………………………………….… ……………………………………………………………………………………………………………. [1] (g) A student decided to perform the same experiment in (a) but used aqueous ammonia instead of aqueous sodium hydroxide. Suggest what effect, if any, would replacing aqueous sodium hydroxide with aqueous ammonia have on the value of enthalpy change of neutralisation calculated in d(iii). effect ………………………………………………………………………………………………….…. The value of enthalpy change of neutralisation will be smaller in magnitude / less exothermic. (reject larger) explanation ……………………………………………………………………………………………... Aqueous ammonia is a weak base. Some of the energy released from neutralisation will be absorbed to ca