2019 NYJC Prelim 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 19 August 2019 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 18 and 19. 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 / 28 2 / 11 3 / 16 Total / 55 This document consists of 19 printed pages.

2 H2 Chemistry 9729/04 NYJC J2/2019 PRELIM 1 Determination of the percentage by mass of ethanedioic acid FA 1 is a solution made by dissolving an unknown mass of a mixture of ethanedioic acid, H2C2O4, and sodium ethanedioate, Na2C2O4. In aqueous solution, both ethanedioic acid and sodium ethanedioate release all their ethanedioate ions, C2O42ꟷ. These ions react with manganate(VII) ions as shown. 2MnO4ꟷ(aq) + 16H+(aq) + 5C2O42ꟷ(aq)  10CO2(g) + 2Mn2+(aq) + 8H2O(l) You will carry out titrations to find the percentage by mass of ethanedioic acid in the mixture. FA 2 is 0.0200 mol dm–3 potassium manganate(VII), KMnO4. FA 3 is 1.00 mol dm–3 sulfuric acid, H2SO4. (a) (i) Titration of FA 1 against FA 2 1. Fill a burette with FA 2. 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Use the measuring cylinder to add 30.0 cm3 of FA 3 to the same conical flask. 4. Place the conical flask on the tripod and gauze and heat until the temperature of the solution is approximately 70 °C. 5. Carefully remove the flask from the tripod. Add a few drops of FA 2 from the burette and swirl the mixture until it turns colourless. Note: If the reaction mixture does not decolourise or turns brown, reheat it to about 70 °C. Swirl the mixture till it decolourises. If the brown colour remains, discard the contents of the flask and begin a new titration. 6. Continue the titration until a permanent pale pink is obtained. 7. Record your titration results, to an appropriate level of precision, in the space provided on page 3. 8. Repeat points 2 to 7 until consistent results are obtained.

3 H2 Chemistry 9729/04 NYJC J2/2019 PRELIM [Turn Over Titration results [2] (ii) From your titration results, obtain a suitable volume of FA 2, VFA 2, to be used in your calculations. Show clearly how you obtained this volume. VFA 2 = cm3 [3] (iii) Calculate the total number of moles of ethanedioate ions present in 25.0 cm3 of FA 1. moles of ethandioate ions present = [1] (b) Titration of FA 1 against FA 4 A separate experiment was carried out by titrating FA 1 against FA 4 containing 0.0400 mol dm–3 sodium hydroxide from a burette. It was found that 25.0 cm3 of FA 1 required 26.75 cm3 of FA 4 for complete reaction. Calculate the number of moles of ethanedioic acid present in 25.0 cm3 of FA 1. moles of ethandioic acid present = [1]

4 H2 Chemistry 9729/04 NYJC J2/2019 PRELIM (c) (i) Calculate the mass of sodium ethanedioate, Na2C2O4, present in 25.0 cm3 of FA 1. [Ar: C, 12.0; O, 16.0; Na, 23.0] mass of sodium ethanedioate present = [1] (ii) Calculate the mass of ethanedioic acid, H 2C2O4, present in 25.0 cm3 of FA 1. Hence calculate the percentage by mass of ethanedioic acid in the solid mixture used to prepare FA 1. [Ar: C, 12.0; O, 16.0; H, 1.0] percentage by mass of H2C2O4 = [2] (d) Explain why the solution in the conical flask was heated to approximatly 70 oC before titration. [1] (e) Student B suggested improving the accuracy of the titration results by decreasing the concentration of FA 2 to 0.0150 mol dm–3. State and explain if the student’s suggestion would make the titration results more accurate. [1]

5 H2 Chemistry 9729/04 NYJC J2/2019 PRELIM [Turn Over (f) The FA 2 that was initially added into the conical flask took some time to decolourise. As the titration continued, the rate of decolourisation changed due to the products formed. To investigate the kinetics of the reaction in part (a), a separate experiment was carried out to observe the rate of carbon dioxide produced over a period of time. 30.0 cm3 of FA 2 was added from a measuring cylinder to a conical flask containing 20.0 cm 3 of FA 1 and 50.0 cm3 of FA 3 and the stop watch was started. The volume of carbon dioxide was collected by downward displacement of water using a measuring cylinder and the volume of gas was recorded at 30 s intervals. (i) Sketch the graph, in Fig 1.1, you would expect to obtain for this experiment and use it to explain why FA 2 decolourises at different rates throughout the titration. [3] Fig 1.1 Time / min Vol of CO2 / cm3

6 H2 Chemistry 9729/04 NYJC J2/2019 PRELIM (ii) A similar experiment described in (f)(i) was carried out using 2 0.0 cm3 of FA 1 with 30.0 cm3 of another oxidising agent, aqueous potassium persulfate, K 2S2O8, in the presence of Fe 3+(aq) catalyst. The volume of carbon dioxide produced was similarly recorded at 30 s intervals. Sketch in Fig 1.2, a graph you would expect for this new experiment [1] Fig 1.2 Time / min Vol of CO2 / cm3

7 H2 Chemistry 9729/04 NYJC J2/2019 PRELIM [Turn Over (g) Planning The kinetics of a chemical reaction can be studied by using the initial rates method. In this method, we note the time taken for a reaction to reach an identification point early in the reaction. Reaction between iodide ions, I–, and persulfate ions, S2O82– is an example where its initial rate can be determined. 2I–(aq) + S2O82–(aq)  I2(aq) + 2SO42–(aq) If a small and same amount of aqueous sodium thiosulfate is added to every reaction mixture together with starch before the start of the reaction, the iodine produced during the reaction will react immediately with the thiosulfate ions present. When the small amount of thiosulfate ions have reacted with the iodine produced, a blue-black iodine-starch complex will appear, indicating the experiment has reached the identification point. By keeping the amount of thiosulfate ions small, t he time taken for the reaction to reach the identification point is therefore used to determine the initial rate of that reaction. The same procedure is repeated with different volumes of reactants and deionised water while keeping the total volume of the reaction mixture constant. In this question, you are to investigate how the rate of reaction between potassium persulfate and potassium iodide depends on the concentration of potassium persulfate. (i) You are to p lan a series of experiments to determine the time taken for the reaction between potassium persulfate and potassium iodide to reach a same identification point for all the experiment