2021 NJC Prelims Paper 4 Answers Final

1 NJC SH2 Preliminary Examination 9729/04/21 NATIONAL JUNIOR COLLEGE SH 2 Year − End Practical Examination Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 4 Practical Candidate answer on the Question paper. 9729/04 Tuesday 17 August 2021 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your identification number and name. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in 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. The number of marks is given in brackets [ ] at the end of each question or part question. This paper consists of 20 printed pages including this cover page. Shift Laboratory For Examiner’s use 1 / 15 2 / 15 3 / 14 4 / 10 Presentation /1 Total / 55 Presentation [1] shows appropriate units and 3 or 4 sf in all final ans in Q1 & 2. Any calculations not attempted loses this mark.

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3 NJC SH2 Preliminary Examination 9729/04/21 [Turn over 1 Determination of the concentration of a solution of sodium hydroxide and the enthalpy change of neutralisation, 'Hn FA 1 is 1.0 mol dm−3 sulfuric acid, H2SO4. FA 2 is a solution of sodium hydroxide, NaOH, of unknown concentration. According to the Arrhenius theory of acids and bases, an acid produces H+(aq) ions and a base produces OH–(aq) ions, in aqueous solution. Using the Arrhenius theory, an acid-base neutralisation reaction involves reacting these two ions together to produce water molecules. The equation for this neutralisation reaction is given below. H+(aq) + OH(aq) → H2O(l) 'Hn In this question, you will carry out a series of experiments where different volumes of FA 1 and FA 2 are mixed together. You will determine the temperature change of the mixture, 'T of each experiment and then analyse your results graphically in order to determine the x concentration of NaOH in FA 2 x maximum temperature change, 'Tmax x value for the enthalpy change of neutralisation, 'Hn

4 NJC SH2 Preliminary Examination 9729/04/21 (a) Method 1 Support the Styrofoam cup in a 250 cm3 beaker. 2 Using a measuring cylinder, place 20.0 cm3 of FA 2 into the Styrofoam cup. 3 Measure the temperature of FA 2 in the Styrofoam cup. Record the initial temperature of the solution of FA 2 as Tinitial. 4 Place 40.0 cm3 of FA 1 into another measuring cylinder. 5 Transfer the FA 1 in the measuring cylinder into the Styrofoam cup, stir and record the maximum temperature obtained in the reaction as Tmax. 6 Rinse and dry the Styrofoam cup and the thermometer. 7 Repeat steps 2 to 6 using volumes of FA 2 listed in Table 1.1. Fill in Table 1.1 with the appropriate volumes of FA 1 to be used in each experiment such that the total volume of the reaction mixture is 60.0 cm3. Table 1.1 Experiment 1 2 3 4 5 6 Volume of FA 2 / cm3 20.0 30.0 40.0 50.0 33.0 36.0 Volume of FA 1 / cm3 40.0 30.0 20.0 10.0 27.0 24.0 [1] In an appropriate format in the space provided, prepare a table in which to record for each experiment x the initial temperature, Tinitial x the maximum temperature, Tmax x the change in temperature, 'T Experiment Initial Temperature, Tinitial / oC Maximum Temperature, Tmax / oC 'T/ oC 1 28.4 34.4 +6.0 2 28.4 37.6 +9.2 3 28.4 38.0 +9.6 4 28.4 33.0 +4.6 5 28.4 38.8 +10.4 6 28.4 39.4 +11.0 [2] [1] Total volume for expt 1 to 6 = 60.0 cm3 and vol of FA 2 for expt 5 and 6 chosen between 30.0 to 40.0 cm3 [1] Clear header with units [1] All recorded values to 1 d.p.

5 NJC SH2 Preliminary Examination 9729/04/21 [Turn over (b) (i) On the grid given below, plot a graph of 'T (y–axis) against volume of FA 2 used (x–axis) using the data you have obtained in (a). By considering the points you have plotted, carry out two more experiments (experiment 5 and 6) which will enable you to identify the volume of FA 2 that gives the maximum temperature change, 'Tmax. [2] Axis labelled 'T in oC and volume of FA 2 used in cm3 Linear scales chosen to use more than half of each axis 1 All points recorded (for 6 experiments). Correct plotting – each point accurately plotted (within 1⁄2 small square and in the correct square). 1 (ii) Given that 'T = 0 at volume of FA 2 = 0.0 and 60.0 cm3, draw two straight lines of best fit. The first best fit line should be drawn using the plotted points where 'T is increasing and the second best fit line should be drawn using the plotted points where 'T is decreasing. Extrapolate these two lines until they cross. [1] (iii) From your graph, determine the maximum temperature change, 'Tmax, and the volume of FA 2 required to obtain this. 'Tmax = ………………………………. oC VFA2 = ………...…………………... cm3 +11.4 36.0 10 20 0 30 40 50 60 Vol of FA2 /cm3 ∆T / ᵒC 2 4 6 8 10 12

6 NJC SH2 Preliminary Examination 9729/04/21 [1] Calculations (c) Using your answers in b(iii), (i) calculate the concentration of NaOH in FA 2 concentration of NaOH in FA 2 = ……………………………………….. [2] (ii) calculate the heat evolved at 'Tmax. You may assume that the specific heat capacity of the reaction mixture is 4.18 J g–1 K–1 and that its density is 1.00 g cm–3. heat evolved = ……………………………….. [1] (d) Using your answer in (c)(ii), calculate the enthalpy change of neutralisation, 'Hn. 'Hn = …………………………………….. [1] 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) Using VFA2 = 36.0 cm3 volume of H2SO4 reacted = 60.0 – 36.0 = 24.0 cm3 Amount of H2SO4 reacted = 24.0/1000 x 1.00 = 0.0240 mol Amount of NaOH reacted = 0.0240 x 2 = 0.0480 mol Concentration of NaOH = ଴.଴ସ଼଴ ଷ଺.଴ x 1000 = 1.333 = 1.33 mol dm–3 1.33 mol dm–3 heat evolved = 60.0 x 4.18 x 11.4 = 2859.12 = 2860 J Amount of water produced = amount of NaOH reacted = 0.0480 mol 'Hn = – ଶ଼ହଽ.ଵଶ ଴.଴ସ଼଴ = – 59565 J mol–1 = – 59.6 kJ mol–1 2860 J – 59.6 kJ mol–1

7 NJC SH2 Preliminary Examination 9729/04/21 [Turn over Analysis and Evaluation (e) State and explain the effect on the magnitude of 'Tmax if ethanedioic acid of the same concentration was used instead of sulfuric acid in the experiment in (a).