2021 HCI Prelim Paper 4 Questions

HWA CHONG INSTITUTION C2 Preliminary Examination Higher 2 NAME CT GROUP 20S CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper 9729/04 18 August 2021 2 hours 30 minutes READ THESE INSTRUCTIONS FIRST Write your name and class 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 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 17 and 18. 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 2 3 4 Total This document consists of 18 printed pages.

2 © HCI 9729/04/Prelim2021 [Turn over 1 Determination of the values for the enthalpy changes associated with several chemical reactions and concentration of aqueous sodium hydroxide FA 1 is aqueous sodium hydroxide, NaOH FA 2 is 1.00 mol dm–3 sulfuric acid, H2SO4 FA 3 is citric acid crystals, C3H4(OH)(CO2H)3.H2O Sodium hydroxide and sulfuric acid react according to equation 1. The enthalpy change for the reaction is 'H1. equation 1 2NaOH(aq) + H2SO4(aq) o Na2SO4(aq) + 2H2O(l) 'H1 Sodium hydroxide and citric acid crystals react according to equation 2. The enthalpy change for the reaction is 'H2. equation 2 3NaOH(aq) + C3H4(OH)(CO2H)3.H2O(s) o C3H4(OH)(CO2Na)3(aq) + 4H2O(l) 'H2 In this question, you will perform experiments to determine the values for 'H1 and 'H2. You will use your values of 'H1 and 'H2 to calculate a value for the enthalpy change shown in equation 3. equation 3 C3H4(OH)(CO2H)3.H2O(s) o C3H4(OH)(CO2)3(aq) + 3H+(aq) + H2O(l) 'H3 In 1(c), you will use data provided to determine the concentration of sodium hydroxide in FA 1. (a) You will follow the instructions to perform experiments 1 and 2. Record your results in the space provided on page 3. Experiment 1 1. Using a measuring cylinder, transfer 50.0 cm3 of FA 1 into a Styrofoam cup. Place this cup inside a second Styrofoam cup, which is placed in a 250 cm3 glass beaker. 2. Stir and measure the temperature of this FA 1, TFA1. 3. Using another measuring cylinder, measure 20.0 cm3 of FA 2. 4. Stir and measure the temperature of this FA 2, TFA2. 5. Add the FA 2 from the measuring cylinder to the FA 1 in the Styrofoam cup. 6. Using the thermometer, stir the mixture continuously until it reaches its maximum temperature. Record this temperature, Tmax. Experiment 2 1. Weigh the capped bottle containing FA 3. 2. Using a measuring cylinder, transfer 50.0 cm3 of FA 1 into the other clean Styrofoam cup. Place this cup inside the used Styrofoam cup, which is placed in a 250 cm3 glass beaker. 3. Stir and measure the temperature of this FA 1, TFA1. 4. Add all the solid FA 3 to the FA 1 in the Styrofoam cup. 5. Using the thermometer, stir the mixture continuously until it reaches its maximum temperature. Record this temperature, Tmax. 6. Reweigh the empty bottle and its cap.

3 © HCI 9729/04/Prelim2021 [Turn over In an appropriate format in the space below, x record all measurements of temperature, x record all measurements of mass, x calculate the values for the average of TFA1 and TFA2, Tave, and 'Tmax for experiment 1, x calculate the mass of FA 3 added and 'Tmax for experiment 2. Tave =(50 u ்𝑭𝑨𝟏) + (20 u ்𝑭𝑨𝟐) 70 [3] (b) Given that: x sodium hydroxide is used in excess in experiments 1 and 2; x Mr of C3H4(OH)(CO2H)3.H2O is 210.0; x specific heat capacities of the Styrofoam cup, lid and thermometer are negligible; x specific heat capacity of the mixture is 4.18 J g1 K1; x density of the mixture is 1.00 g cm3, (i) calculate the heat change, q1, for Experiment 1 and hence determine a value for the enthalpy change 'H1 shown in equation 1. q1 = ………………………… 'H1 = ………………….…………… [2]

4 © HCI 9729/04/Prelim2021 [Turn over (ii) calculate the heat change, q2, for Experiment 2 and hence determine a value for the enthalpy change 'H2 shown in equation 2. q2 = ………………………… 'H2 = ………………….…………… [2] (iii) Use your values of 'H1 and 'H2 to determine a value for the enthalpy change 'H3 shown in equation 3. Show your working clearly. equation 3 C3H4(OH)(CO2H)3.H2O(s) o C3H4(OH)(CO2)3(aq) + 3H+(aq) + H2O(l) 'H3 'H3 = ………………….…………… [2]

5 © HCI 9729/04/Prelim2021 [Turn over (c) The procedure in experiment 1 can be used to determine the concentration of sodium hydroxide in FA 1. A series of seven experiments were performed using different volumes of FA 1 and FA 2. In each experiment, the total volume of the two solutions was kept at 50 cm3 and the change in temperature, 'T, was determined. The results from the experiments are plotted on the grid in Fig. 1.1. Fig. 1.1 (i) Draw two best-fit straight lines, the first line taking into account the points before the maximum change in temperature and the second line using the remaining data. Extrapolate (extend) both lines until they cross. [1] (ii) Explain, in terms of the chemistry involved, the significance of the point of intersection of the two best-fit lines. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………….. ……………………………………………………………………………………………. [2] 15 20 25 30 35 40 45 50 'T / oC volume of FA 1 / cm3 2 4 6 8 10 12 x x x x x x x

6 © HCI 9729/04/Prelim2021 [Turn over (iii) Using your graph, determine the concentration of sodium hydroxide in FA 1. concentration of sodium hydroxide in FA 1 = ………………….…………… [2] [Total: 14]

7 © HCI 9729/04/Prelim2021 [Turn over 2 Planning The hydrolysis of an ester is catalysed by either an acid or alkali. In the presence of an acid catalyst, the reaction is reversible and reaches equilibrium in about 48 hours. Ethyl ethanoate hydrolyses to give ethanoic acid and ethanol as shown in equation 4. equation 4 CH3CO2CH2CH3(l) + H2O(l) CH3CO2H(l) + CH3CH2OH(l) Kc (a) To determine the equilibrium constant, Kc, it is necessary to determine the amounts of ethyl ethanoate, water, ethanoic acid and ethanol present in the equilibrium mixture. One way of doing this is described below. Known amounts of ethyl ethanoate and an acid catalyst are mixed in a sealed reagent bottle. After sufficient time, the whole equilibrium mixture is diluted with deionised water. Titration with a standard solution of sodium hydroxide determines the total amount of acid in the mixture: both ethanoic acid and the acid catalyst. Since the amount of acid catalyst does not change, it is possible to deduce the amount of ethanoic acid and hence the amounts of each of the other components in the equilibrium mixture. Using these data, a value for Kc may then be determined. Plan a procedure to collect sufficient data to allow you to determine a value for Kc in equation 4 at 40 oC. You should plan to make an initial reaction mixture containing x 5.00 g of ethyl ethanoate, x 5.00 cm3 of 2.00 mol dm3 hydrochloric acid. You may assume that you are also provided with: x 1.50 mol dm3 sodium hydroxide,