JPJC Prelim 2025 Paper 4 Answers H2 Chem

© Jurong Pioneer Junior College [Turn Over NAME Suggested Answer Scheme 1 Determination of the enthalpy change for the reaction between NaOH and CO2 In this experiment you will determine the enthalpy change, H1, for the reaction shown in equation 1. equation 1 NaOH(aq) + CO2(g) → NaHCO3(aq) H1 Sodium hydrogencarbonate and sodium hydroxide each react with sulfuric acid as shown in equations 2 and 3. equation 2 2NaHCO3(s) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) + 2CO2(g) H2 equation 3 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) H3 You will determine the values of H2 and H3 and use Hess’s law to calculate H1. (a) (i) Reaction of sodium hydrogen carbonate with sulfuric acid FA 1 is solid sodium hydrogen carbonate FA 2 is 1.0 mol dm–3 sulfuric acid, H2SO4 In this experiment, you will measure the initial temperature of sulfuric acid in a polystyrene cup. After a sample of sodium hydrogen carbonate is added to sulfuric acid which is in excess, you will measure the minimum temperature reached in the experiment. You will then determine the temperature fall caused by the reaction. In the space provided on page 3, prepare tables in which to record for the experiment: • all weighings to an appropriate level of precision, • all required temperature measurements to an appropriate level of precision. Procedure: 1. Weigh the capped bottle containing solid FA 1. 2. Using a measuring cylinder, transfer 50.0 cm 3 of FA 2 into a polystyrene cup. Place this cup inside a second polystyrene cup, which is placed in a 250 cm3 glass beaker. 3. Place the thermometer in this FA 2 and record the initial temperature, Ti. 4. Adding small quantities at a time, tip all the FA 1 into the FA 2 in the polystyrene cup. 5. Using the thermometer, stir the mixture continuously until it reaches its minimum temperature. Record this temperature, Tmin. 6. Calculate and record the temperature fall. 7. Weigh the capped bottle with any residual FA 1 and record the mass of FA 1 added.

2 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2025 Results Mass of capped bottle with FA 1 /g 11.658 Mass of capped bottle with residual FA 1 / g 5.661 Mass of FA 1 added / g 5.997 Initial temperature/ Ti oC 30.5 Minimum temperature/ Tmin / oC 24.2 Temperature fall / oC 6.3 [3] (a) (ii) Calculate the heat change, q, for your experiment in 1(a)(i). 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 = 50.0  4.18  1.00  6.3 = 1317 J = 1.32 kJ [1] (iii) Determine the enthalpy change for the reaction of sodium hydrogen carbonate with sulfuric acid, H2. In this reaction, sulfuric acid is used in excess. 2NaHCO3(s) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) + 2CO2(g) H2 Include the sign of H2 in your answer. [Ar: C, 12.0; H, 1.0; Na, 23.0; O, 16.0] nNaHCO3 = 5.997 84.0 = 0.0714 mol nH2SO4 reacted= 0.0714 2 = 0.0357 mol H2 = + 1.32 0.0357 = + 36.9 kJ mol−1 [2] (b) Reaction of sodium hydroxide with sulfuric acid FA 2 is 1.0 mol dm–3 sulfuric acid, H2SO4 A student conducted this experiment to determine the enthalpy change for the reaction of sodium hydroxide with sulfuric acid, H3. The student added 40.0 cm 3 of FA 2 into 30.0 cm 3 of 2.00 mol dm −3 NaOH in a polystyrene cup and recorded a temperature rise of 11.0 C. (i) Calculate the heat change, q, for the student’s experiment. You should assume that the specific heat capacity of the solution is 4.18 J g−1 K−1, that the density of the solution is 1.00 g cm−3. q = 70.0  4.18  1.00  11.0 = 3219 J = 3.22 kJ [1]

3 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2025 [Turn Over (ii) Determine the enthalpy change for the reaction of sodium hydroxide with sulfuric acid, H3. In this reaction, sulfuric acid is used in excess. 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) H3 Include the sign of H3 in your answer. nNaOH = 30 2.001000 = 0.0600 mol nH2SO4 reacted = 0.0600 2 = 0.0300 mol H3 = 3.22 0.03− = − 107 kJ mol−1 [2] (c) The enthalpy change of solution of NaHCO3 is +18.8 kJ mol−1. NaHCO3(s) + aq → NaHCO3(aq) Hsol = +18.8 kJ mol−1 2NaHCO3(s) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) + 2CO2(g) H2 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) H3 Using your answer in 1(a)(iii), 1(b)(ii) and Hsol given in this question, use Hess’s law to calculate H1. NaOH(aq) + CO2(g) → NaHCO3(aq) H1 By Hess’s law, 2H1 = H3 − H2 + 2(18.8) H1 = − − + +107 ( 36.9) 2(18.8) 2 = − 53.2 kJ mol−1 [1] (d) Calculate the percentage error in the temperature fall you obtained in 1(a)(i). percentage error =   2(0.1) 100%6.3 = 3.17 % [1] [Total: 11] 2NaOH(aq) + 2CO2(g) 2NaHCO3(aq) Na2SO4(aq) + 2H2O(l) + 2CO2(g) + H2SO4(aq) 2NaHCO3(s) + H2SO4(aq) + H2SO4(aq) H3 2H1 H3 H2 2(+18.8)

4 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2025 2 Investigation of the kinetics of the autocatalytic reaction between manganate(V II) ions and ethanedioate ions. FA 3 is 0.0100 mol dm–3 sodium thiosulfate, Na2S2O3 FA 4 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4 FA 5 is 0.200 mol dm–3 ethanedioic acid, H2C2O4 FA 6 is 0.100 mol dm–3 potassium iodide, KI You are also provided with: • FA 2, 1.0 mol dm–3 sulfuric acid, H2SO4, which you used in Question 1 • Starch Indicator Manganate(VII) ions, MnO4–, react with ethanedioate ions, C2O42−, in acidic medium as shown in equation 4. The Mn 2+ ions produced in equation 4 act as a catalyst for the reaction. This is an example of an autocatalytic reaction. equation 4 2MnO4–(aq) + 5C2O42−(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l) You will prepare a reaction mixture containing FA 4, FA 5 and FA 2. At timed intervals, you will transfer aliquots (portions) of the reaction mixture to excess K I solution which will “quench” the reaction by reacting away manganate(VII) ions as shown in equation 5. equation 5 2MnO4–(aq) + 10I−(aq) + 16H+(aq) → 2Mn2+(aq) + 5I2(aq) + 8H2O(l) You will then titrate the iodine produced in the resulting solution against sodium thiosulfate solution. The reaction is shown in equation 6. equation 6 2S2O32−(aq) + I2(aq) → 2I−(aq) + S4O62−(aq) Your titre values will indicate the concentration of MnO 4− remaining in the reaction mixture at different times. Hence, the rate of reaction between MnO 4− and C2O42− at different times can be determined by graphical analysis of your results. In an appropriate format in the space provided on page 8, prepare a table to record, for each aliquots • the time of transfer, t, in minutes and seconds, • the decimal time, td, in minutes, to 0.1 min, for example, if t = 4 min 33 s then td = 4 min + 33/60 min = 4.6 min, • the burette readings and the volumes of FA 3 added.

5 © Jurong Pioneer Junior College 9729/04/J2 PRELIMINARY EXAM/2025 [Turn Over You are advised to read all the instructions on this page before you start your experiment. Note: You will perform each titration once only. Great care must be taken that you do not overshoot the end-point. Once you have started the stopwatch, it must continue running for the duration of the e