2021 NYJC Prelim 9729 P4 (QP)

[Turn Over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME TUTOR’S NAME CLASS CHEMISTRY 9729/04 Paper 4 Practical 23 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 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, 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 16 and 17. 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 / 13 2 / 15 3 / 9 4 / 18 Total / 55 This document consists of 17 printed pages..

2 H2 Chemistry 9729/04 NYJC J2/2021 PRELIM. 1 Determination of the concentration of hydrogen peroxide FA 1 is a solution of hydrogen peroxide, H2O2, of unknown concentration. In this experiment, you will react hydrogen peroxide with excess potassium iodide solution in an acidic condition to produce iodine as shown in equation 1. H2O2 + 2I– + 2H+ J I2 + 2H2O equation 1 The iodine produced is then titrated with a standard solution of sodium thiosulfate as shown in equation 2. I2 + 2S2O32– J 2I– + S4O62– equation 2 FA 2 is 0.50 mol dm–3 sulfuric acid, H2SO4. FA 3 is 0.500 mol dm–3 potassium iodide, KI. FA 4 is 0.200 mol dm–3 sodium thiosulfate, Na2S2O3. Starch solution. You will determine the concentration of hydrogen peroxide by first reacting a fixed amount of hydrogen peroxide with excess potassium iodide and subsequently titrating the iodine produced with sodium thiosulfate. (a) In a preliminary experiment, a student pipetted 25.0 cm3 of FA 1 into a conical flask containing FA 2 and FA 3. The resulting solution was titrated against FA 4 from a burette and a very large titre value was obtained. He deduced the concentration of FA 1 was too high and required dilution. (i) Procedure 1. Using the burette labelled FA 1, run between 24.00 cm3 and 24.50 cm3 of FA 1 into a clean 250 cm3 graduated flask. Set aside this burette labelled FA 1 and its contents for Question 2. 2. Record your burette readings to an appropriate level of precision in the space provided below. 3. Top up the flask to mark with deionised water. Stopper the flask and mix the contents thoroughly to obtain a homogenous solution. Label this solution as FA 5. Dilution of FA 1

3 H2 Chemistry 9729/04 NYJC J2/2021 PRELIM. [Turn Over 4. Fill another clean burette with FA 4. 5. Pipette 25.0 cm3 of FA 5 into a 250 cm3 conical flask. 6. Using separate measuring cylinders, transfer 50.0 cm3 of FA 2 and 20.0 cm3 of FA 3 into the same conical flask. Leave the solution to stand for 7 minutes. While you are waiting, prepare another of this solution by repeating steps 5 and 6. 7. At the 7th minute, titrate the contents in the conical flask against FA 4 from the burette until a pale-yellow solution is obtained. 8. Add about 5 drops of starch solution and continue titrating until the solution just turns colourless. Ignore any subsequent return of the colour. 9. Repeat the titration until consistent results are obtained and record your readings in the space below. Make certain that your results show the precision of your working. Results [4] (ii) From your titrations, obtain a suitable volume of FA 4, VFA 4, to be used in your calculations. Show clearly how you obtained this volume. VFA 4 = cm3 [1] (b) (i) Use your answer from 1(a)(ii) to calculate the concentration of hydrogen peroxide in FA 5. [H2O2] in FA 5 = mol dm–3 [2]

4 H2 Chemistry 9729/04 NYJC J2/2021 PRELIM. (ii) Hence, calculate the concentration of hydrogen peroxide in FA 1. [H2O2] in FA 1= mol dm–3 [1] (c) Besides expressing concentration of H2O2 in moles per cubic decimetre, mol dm–3, it can also be expressed in volume strength. Volume strength refers to the volume of oxygen formed when 1.00 cm3 of hydrogen peroxide solution completely decompose at room temperature and pressure. When 1.00 cm3 of a 100 volume hydrogen peroxide sample is allowed to completely decompose at room temperature and pressure, 100 cm3 of oxygen gas is produced. Calculate the volume strength of the hydrogen peroxide in FA 1 at room temperature and pressure that you have calculated in 1(b)(ii). Volume strength of FA 1 = volume [2]

5 H2 Chemistry 9729/04 NYJC J2/2021 PRELIM. [Turn Over (d) State and explain how the use of a measuring cylinder to measure the volume of FA 3 in step 6 will affect the accuracy of the titration result. [1] (e) Based on the procedure in 1(a)(i), the value you obtained in 1(b)(ii) for concentration of H2O2 in FA 1 differs from the actual concentration. Explain with evidence how the procedure in 1(a)(i) will impact the calculated concentration of H2O2 in FA 1. Impact: [1] Evidence: [1] [Total: 13]

6 H2 Chemistry 9729/04 NYJC J2/2021 PRELIM. 2 Determination of the enthalpy change of decomposition of hydrogen peroxide Decomposition of hydrogen peroxide occurs spontaneously but slowly at room temperature. The heat evolved from the decomposition is not significant hence is not measurable. By increasing the rate of decomposition, temperature changes of the reaction become more measurable and the enthalpy change of decomposition can be determined experimentally. FA 6 is 0.250 mol dm–3 sodium hydroxide, NaOH. You will determine the enthalpy change for the decomposition of hydrogen peroxide by mixing FA 1 and FA 3 from Question 1 together with FA 6 and measuring the temperature of the solution over a period of time. (a) Procedure 1. Using a clean 50 cm3 measuring cylinder, measure 48.0 cm3 of FA 3 followed by 2.0 cm3 of FA 6. Stir the solution with a glass rod. 2. Using the burette labelled FA 1 from Question 1, transfer 50.00 cm3 of FA 1 into a polystyrene cup. Place the cup inside a 250 cm3 beaker. 3. Stir the solution in the cup with a thermometer and measure its temperature, T, and start the stopwatch. This is the temperature when time, t = 0 min. Record this temperature in a suitable format in the space below. Repeat the measurement at one-minute interval for the next three minutes. 4. At the 4th minute, pour the contents of the measuring cylinder into the polystyrene cup and stir the solution. Do not read the temperature at the 4th minute. 5. Stir the solution continuously and record the temperature at the 5th minute and every half–minute interval till the 12th minute. Record all the temperatures measured. Temperature readings [2]

7 H2 Chemistry 9729/04 NYJC J2/2021 PRELIM. [Turn Over (b) Plot a graph of temperature, T, on the y-axis, against time, t, on the x-axis on the grid. Draw a best–fit straight line taking into account all of the points before t = 4.0 min. Draw another best–fit straight line taking into account all of the points after the temperature of the mixture has started to fall steadily. Extrapolate both lines to t = 4.0 min.