ASRJC 2021 J2Prelims H2Chem P4 QP

ASRJC JC2 PRELIM 2021 9729/04/H2 [Turn Over ANDERSON SERANGOON JUNIOR COLLEGE 2021 JC 2 PRELIMINARY EXAMINATION NAME:________________________________ ( ) CLASS: 21 /____ CHEMISTRY 9729/04 Paper 4 Practical 26 August 2021 2 hours 30 minutes Candidates answer on the Question Paper. Additional Materials: As listed in the Confidential Instructions READ THESE INSTRUCTIONS FIRST Write your name, class and index number on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a 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. Quantitative Analysis Notes are printed on pages 27 and 28. Shift Laboratory 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 / 13 2 / 10 3 / 18 4 / 14 Total / 55 This document consists of 26 printed pages and 2 blank pages.

2 ASRJC JC2 PRELIM 2021 9729/04/H2 Answer all the questions in the spaces provided. 1 Determination of enthalpy change of reaction FA 1 is solid potassium hydrogencarbonate, KHCO3 FA 2 is 1.50 mol dm–3 sulfuric acid, H2SO4 (I) KHCO3(s) + aq ® K+(aq) + HCO3–(aq) DHsol(KHCO3) The molar enthalpy change of solution of solid potassium hydrogencarbonate is the enthalpy change when one mole of solid potassium hydrogencarbonate dissolves in sufficient water such that the ions are well separated as shown in (I). When added to water, solid potassium hydrogencarbonate quickly dissolves and the temperature of the mixture falls. (II) KHCO3(aq) + ½H2SO4(aq) ® ½K2SO4(aq) + H2O(l) + CO2(g) DHr(KHCO3(aq)) The molar enthalpy change of reaction of aqueous potassium hydrogencarbonate with sulfuric acid in (II) is the enthalpy change when one mole of aqueous potassium hydrogencarbonate reacts with excess sulfuric acid. You are to perform experiments by which you will determine the enthalpy change DHsol(KHCO3). You will also determine the enthalpy change DHr(KHCO3(aq)) using the data provided before using your results in a Hess’s Law calculation.

3 ASRJC JC2 PRELIM 2021 9729/04/H2 [Turn Over (a) Follow the instructions below to determine the maximum temperature change when a known mass of solid potassium hydrogencarbonate, FA 1, dissolves completely in water. In an appropriate format in the space provided below, record • all weighings to an appropriate level of precision, • all values of temperature to an appropriate level of precision. 1. Weigh the capped bottle containing FA 1. 2. Place one polystyrene cup inside a second polystyrene cup. Place these in a glass beaker to prevent them from tipping over. 3. Use a measuring cylinder to transfer 50 cm3 of deionised water into the first polystyrene cup. 4. Stir the water in the cup with the thermometer. Read and record its temperature. 5. Transfer all the FA 1 to the polystyrene cup. Stir the mixture. 6. Continue to stir the mixture. Observe the temperature and record the value that shows the maximum change from the initial temperature. 7. Reweigh the empty bottle and its cap. Determine the maximum temperature change and the mass of FA 1 used. Results [5]

4 ASRJC JC2 PRELIM 2021 9729/04/H2 (b) In the following calculations, you should assume that the specific heat capacity of the solution is 4.18 J g–1 K–1, and the density of the solution is 1.00 g cm–3. (i) Use your results from 1(a) to calculate the heat change for your experiment. heat change = ………………………………………. [1] (ii) Hence, determine a value for DHsol(KHCO3). Include the sign of DHsol in your answer. [Ar: H, 1.0; C, 12.0; O, 16.0; K, 39.1] DHsol(KHCO3) = ………………………………………. [1]

5 ASRJC JC2 PRELIM 2021 9729/04/H2 [Turn Over The results of an experiment where a solution of aqueous potassium hydrogencarbonate, KHCO3(aq), similar to the one you have prepared in 1(a), was reacted completely with an excess of dilute sulfuric acid, FA 2, are shown in Table 1.1. Table 1.1 mass of KHCO3(s) used / g 3.450 volume of KHCO3(aq) used / cm3 50.0 initial temperature of KHCO3(aq) / oC 27.4 volume of FA 2 used / cm3 25.0 initial temperature of FA 2 / oC 31.0 minimum temperature reached / oC 28.2 (iii) Use the results given in Table 1.1 and the formula below to calculate the weighted average initial temperature, Tav, of the reaction mixture. The formula for Tav is given as Tav = (vol. of FA 2 × initial temp. of FA 2) + (vol. of KHCO3 × initial temp. of KHCO3)total volume of reaction mixture Tav = ………………………………………. [1] (iv) Hence, calculate a value for DHr(KHCO3(aq)). DHr(KHCO3(aq)) = ………………………………………. [3]

6 ASRJC JC2 PRELIM 2021 9729/04/H2 (c) Molar enthalpy change for the reaction of dilute sulfuric acid with solid potassium hydrogencarbonate is as shown. 2KHCO3(s) + H2SO4(aq) ® K2SO4(aq) + 2H2O(l) + 2CO2(g) DHr(KHCO3(s)) Use your answers from 1(b)(ii) and 1(b)(iv) to determine a value for DHr(KHCO3(s)). If you are not able to determine a value for 1(b)(ii) and/or 1(b)(iv), you may use x and y to represent the respective enthalpy changes and proceed with this part of the question. DHr(KHCO3(s)) = ………………………………………. [2] [Total: 13]

7 ASRJC JC2 PRELIM 2021 9729/04/H2 [Turn Over 2 Planning Magnesium sulfate, MgSO4 is commonly used in instant hot packs. The hot pack is made up of water surrounding a pouch containing the salt. When the pack is squeezed, this inner pouch is broken, releasing the salt, which quickly dissolves and increases the pack’s temperature. The solubility of magnesium sulfate at 20 oC is about 0.292 mol per 100 cm3. A student decided to conduct an experiment to find out the molar enthalpy change of solution of magnesium sulfate by adding a known quantity of solid magnesium sulfate to water. The maximum temperature change occurring during this reaction may be determined graphically. The maximum temperature change, DTmax, obtained from the graph can be used to calculate the heat change, q, for this experiment. Using q, a value for the molar enthalpy change of solution, DHsol, for magnesium sulfate may be determined. In this question, you are to plan a procedure that would provide sufficient data to allow you to determine an accurate and reliable value for the molar enthalpy change of solution, DHsol, for magnesium sulfate. (a) The literature value for the molar enthalpy change of solution of magnesium sulfate is found to be approximately –78.9 kJ mol–1. You may assume that 4.3 J are required to raise the temperature of 1.0 cm3 of any solution by 1 °C. Suggest a suitable volume of water you would use in this experiment and hence, calculate the minimum mass of solid magnesium sulfate required to ensure a DTmax of at least 5 oC. [Mr MgSO4: 120.4] volume of water used = ………………………………………. cm3 minimum mass of solid MgSO4 = ………………………………………. g [2]

8 ASRJC JC2 PRELIM 2021 9729/04/H2 (b) Plan an investigation to determine the maximum temperature change, DTmax, graphically for the dissolution of magnesium sulfate. Measurements should be taken: • before the reaction starts, • during the reaction, • for some time after the reaction is complete. You may assume that you are provided with: • 50 g of magnesium sulfate, • the equipment normally found in a school or college laboratory. In your plan, you should include bri