2019 HCI Prelim H2 Chem P4 QP

This document consists of 18 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examination Higher 2 NAME CT GROUP 18S CHEMISTRY Paper 4 Practical Candidates answer on the Question Paper 9729/04 28 August 2019 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

2 © Hwa Chong Institution 2019 9729 / 04 / C2 Prelim 2019 Answer all the questions in the spaces provided. 1 Determination of the enthalpy change of neutralisation, Hneut, of a strong acid by a strong base The enthalpy change of neutralisation, Hneut, is the enthalpy change when one mole of water is formed during a neutralisation reaction as shown in equation 1. equation 1 H+(aq) + OH−(aq)  H2O(l) FA 1 is a solution of sulfuric acid, H2SO4 FA 2 is 1.50 mol dm–3 sodium hydroxide, NaOH You will perform a series of experiments using different volumes of FA 1 and FA 2 which together give a total volume of 50 cm3. The change in temperature, T, for each experiment will be determined and used to plot a graph of T against volume of FA 1 used. You will then use data from the graph to determine the concentration of sulfuric acid in FA 1, and a value for the enthalpy change of neutralisation, Hneut. (a) (i) Determining the change in temperature for a series of reactions between FA 1 and FA 2 1. Place the Styrofoam cup in a 250 cm3 beaker to prevent it from tipping over. Use a measuring cylinder to transfer 10.0 cm3 of FA 1 into the cup. 2 Use a measuring cylinder to measure 40.0 cm3 of FA 2. 3. Measure the temperature of the FA 1 solution using the thermometer. Record the initial temperature of FA 1 as TFA1. 4. Add FA 2 to FA 1 in the Styrofoam cup. Stir the mixture using the thermometer and record the maximum temperature, Tmax, reached. 5. Wash and dry the Styrofoam cup. 6. Repeat steps 1 to 5 using 20.0 cm3, 25.0 cm3, 30.0 cm3, 35.0 cm3 and 40.0 cm3 of FA 1 and appropriate volumes of FA 2 each time such that the total volume of the reacting mixture is 50.0 cm3. Keep the remaining FA 1 and FA 2 solutions for use in questions 2 and 4. In an appropriate format in the space provided on page 3, record:  all measurements of volumes used,  all temperatures measured and the change in temperature, T.    

3 © Hwa Chong Institution 2019 9729 / 04 / C2 Prelim 2019 Results [4] (ii) On the grid provided, plot a graph of T (y-axis) against volume of FA 1 (x-axis) using the data you obtained in 1(a)(i).

4 © Hwa Chong Institution 2019 9729 / 04 / C2 Prelim 2019 Draw two lines of best fit.  The first best-fit line should be drawn using the plotted points before the maximum change in temperature.  The second best-fit line should be drawn using the plotted points after the maximum change in temperature. Extrapolate these lines until they cross. [3] (iii) Determine from your graph, the maximum change in temperature, Tmax, and the volume, Vmax, of FA 1 required to obtain this value. Tmax = …………………………… Vmax = …………………………… [1] (b) Using your answers in 1(a)(iii), calculate (i) the concentration, in mol dm–3, of H2SO4 in FA 1. concentration of H2SO4 in FA 1 = ............................................. [1] (ii) the heat change for the neutralisation reaction at Tmax. You should assume that the specific heat capacity of the final solution is 4.18 J g–1 K–1, and the density of the final solution is 1.00 g cm–3. heat change = ........................................ [1]

5 © Hwa Chong Institution 2019 9729 / 04 / C2 Prelim 2019 (c) Using your answers from 1(b)(i) and 1(b)(ii), calculate a value for the enthalpy change of neutralisation, Hneut. Hneut =........................................ [1] (d) Predict the effect on Hneut if the experiment was repeated with malonic acid, HO2CCH2CO2H, of the same concentration instead of sulfuric acid. Explain your answer. ………………………………………………………….………………………………………………. ………………………………………………………………….………………………………………. ……………………………………………………………………………………………………….[1] (e) State one significant source of error in the experiment and suggest an improvement that can be made to reduce this error. ………………………………………………………….………………………………………………. ………………………………………………………………….………………………………………. ……………………………………………………………………………………………………….[1] [Total: 13]

6 © Hwa Chong Institution 2019 9729 / 04 / C2 Prelim 2019 2 Determination of water of crystallisation in a hydrated iron(III) salt FA 3 is hydrated iron(III) sulfate with formula Fe 2(SO4)3.nH2O. The addition of excess zinc to a solution of FA 3 reduces the Fe3+ ions to Fe2+ ions. The amount of Fe2+ ions can be determined quantitatively by titration against a standard solution of potassium manganate(VII), KMnO4. The reaction is shown in equation 2. equation 2 5Fe2+ + MnO4 + 8H+  5Fe3+ + Mn2+ + 4H2O In this experiment, you will prepare a standard solution using FA 3 and perform titrations to determine the value of n, the water of crystallisation in FA 3. You are also provided with FA 4, 0.0200 mol dm3 potassium manganate(VII), KMnO4, zinc powder. You will also need access to the FA 1 solution you used earlier. (a) Preparation of standard solution of hydrated iron(III) salt 1. Weigh accurately the sample of FA 3 provided with the weighing bottle. Transfer all the solid into a 250 cm 3 beaker. Determine the mass of solid FA 3 used and record all your weighings, to an appropriate level of precision, in the space below. [1] 2. Use a measuring cylinder to add about 100 cm3 of FA 1 to the beaker. Stir the mixture with a glass rod for 3 minutes. Ignore any cloudiness that remains. 3. Transfer the solution into a 250 cm 3 volumetric flask. Rinse the beaker with deionised water and pour the washings into the volumetric flask. 4. Make up to the 250 cm 3 mark with deionised water, stopper and mi x thoroughly by inverting the flask a number of times. 5. Label this solution FA 5.

7 © Hwa Chong Institution 2019 9729 / 04 / C2 Prelim 2019 (b) Preparation of Fe2+ solution from FA 5 6. Use a measuring cylinder to transfer 100 cm 3 of FA 5 into a dry 250 cm 3 beaker. 7. Add all the zinc powder provided in the bottle into the beaker. Cover the beaker with a watch glass. 8. Allow the reaction to take place for about 5 minutes, stirr ing the reaction mixture from time to time. Record your observations in the space below. Observations in step 8 [1] 9. Filter the mixture into the dry conical flask provided using dry filter paper and filter