GESS 4Exp Pure Chem Prelim 2024 P3

This document consists of 13 printed pages. GAN ENG SENG SCHOOL Preliminary Examination 2024 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY Paper 3 Candidates answer on the Question Paper. No Additional Materials are required. 6092/03 5 August 2024 1 hour 50 minutes READ THESE INSTRUCTIONS FIRST Write your class, index number and name 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, highlighters, glue or correction fluid/tape. 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 page 12. The Periodic Table is provided on page 13. The number of marks is given in brackets [ ] at the end of each questions or part question. Shift Laboratory For Examiner’s Use Q1 17 Q2 18 Q3 5 Total 40

GESS 4EXP Chemistry 6092/03 PRELIM 2024_JAN 2 1 Sodium carbonate reacts with dilute sulfuric acid. This reaction is exothermic. In this experiment you will determine the percentage purity of a sample of impure anhydrous sodium carbonate. You will measure the enthalpy change of reaction when a sample of impure anhydrous sodium carbonate reacts with excess dilute sulfuric acid. Read all the instructions carefully before starting the experiments in Question 1. Instructions X is a sample of the impure anhydrous sodium carbonate. Y is 1.00 mol/dm3 dilute sulfuric acid, H2SO4 (a) • Weigh the container with X. Record this mass. mass of container with X = .................... g • Support the Styrofoam cup in the 250 cm3 beaker. • Use the measuring cylinder to place 25 cm3 of Y into the cup. • Measure the temperature of Y. Record this temperature at time = 0 in the table below. • Start the stopwatch and leave it running for the whole experiment. • Measure the temperature of Y every minute for 2 minutes. Record the temperatures in the table below. • At 3 minutes transfer X from the container into the cup. Stir the contents and continue to measure, and record, the temperature every minute up to 8 minutes. Results time/min 0 1 2 3 4 5 6 7 8 temperature/ºC • Reweigh the container with any residual solid. Record this mass. mass of container with residual X = .................... g [3]

GESS 4EXP Chemistry 6092/03 PRELIM 2024_JAN 3 (b) (i) Plot your results on the grid with temperature on the y-axis and time on the x-axis. Complete the graph by drawing two straight lines of best fit. One line should be drawn using data from 0 to 2 minutes and the other line should be drawn using data from 4 to 8 minutes. Extend both of these lines to 3 minutes. Draw a vertical line at 3 minutes to connect these lines. [4] (ii) Use your lines from (b)(i) to calculate the maximum temperature rise in the experiment. maximum temperature rise = .................... ºC [1]

GESS 4EXP Chemistry 6092/03 PRELIM 2024_JAN 4 (c) (i) The amount of heat released in this experiment can be calculated using the expression shown. heat released (in J) = mass of solution (in g) ´ maximum temperature rise (in ºC) ´ 4.2 Use this expression to calculate, in J, the amount of heat released. Assume that 1.0 cm3 of solution has a mass of 1.0 g. amount of heat released = .................... J [1] (ii) The equation for the reaction between anhydrous sodium carbonate and sulfuric acid is shown. Na2CO3(s) + H2SO4(aq) → Na2SO4(aq) + H2O(l) + CO2(g) A textbook lists the enthalpy change of this reaction as –40.0 kJ/mol. Use this figure, and the value that you found in (c)(i), to find the mass of anhydrous sodium carbonate you used in (a). You should assume that no heat was lost to the surroundings in your experiment. [Mr: Na2CO3, 106] mass of Na2CO3 = .................... g [2] (iii) Calculate the percentage of anhydrous sodium carbonate present in X. percentage Na2CO3 in X = .................... % [2]

GESS 4EXP Chemistry 6092/03 PRELIM 2024_JAN 5 (d) In your calculation in (c), what assumption have you made about the impurity present in X? …………………………………………………………………………………………….. …………………………………………………………………………………………….. [1] (e) Another student carries out the same experiment but only measures the initial temperature of the solution and the highest temperature reached in order to calculate the temperature rise. Suggest why the student who uses a graph to calculate the temperature rise is able to obtain a more accurate value for the amount of heat released. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [1] (f) A student decided to confirm by experiment the literature value for the enthalpy change of the reaction between anhydrous sodium carbonate and sulfuric acid. By mistake the student weighed a sample of hydrated sodium carbonate, Na2CO3•10H2O, instead of anhydrous sodium carbonate, Na2CO3. State what effect this would have on the calculated value of the enthalpy change for the reaction. Explain your answer. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. …………………………………………………………………………………………….. [2] [Total: 17]

GESS 4EXP Chemistry 6092/03 PRELIM 2024_JAN 6 2 You are to determine, by titration, the change in oxidation number of a transition metal ion, M2+, when reacted with acidified potassium manganate(VII). Read all the instructions carefully before starting the experiments in Question 2. Instructions P is 0.0200 mol/dm3 potassium manganate(VII), KMnO4. Q is 0.0795 mol/dm3 transition metal salt, MSO4. Y is 1.00 mol/dm3 sulfuric acid, H2SO4 (a) Put P into the burette. The colour of P makes it difficult to see the bottom of the meniscus so you should take all your readings using the top of the meniscus. Pipette a 25.0 cm3 portion of Q into a conical flask. To the solution in the flask also add about 25 cm3 of Y using a measuring cylinder. Add P from the burette. At first the purple colour disappears quickly but as more P is added the colour disappears less quickly. At the end-point, one drop of P produces a pale pink colour that does not disappear on swirling. Record your results in the table, repeating the titration as many times as you consider necessary to achieve consistent results. Result [5] (b) From your titration results, obtain an average volume of P to be used in your calculations. Show clearly how you obtained this volume. average volume of P = ………………….. [1]

GESS 4EXP Chemistry 6092/03 PRELIM 2024_JAN 7 (c) (i) Calculate the number of moles of potassium manganate(VII) present in the average volume of P required. number of moles of potassium manganate(VII) = ........................... mol [1] (ii) Calculate the number of moles of MSO4 in 25.0 cm3 of Q. number of moles of MSO4 in 25.0 cm3 = ........................... mol [1] (iii) Use your answers to (c)(i) and (c)(ii) to calculate the number of moles of MSO4 that react with 1 mole of KMnO4. number of moles of MSO4 = ........................... mol [2] (iv) Two possible equations for the reaction of acidified KMnO4 with M2+ are shown below. equation 1 2 KMnO4 + 10 MSO4 + 8 H2SO4 → K2SO4 + 2 MnSO4 + 5 M2(SO4)3 + 8 H2O equation 2 2 KMnO4 + 5 MSO4 + 8 H2SO4 → K2SO4 + 2 MnSO4 + 5 M(SO4)2 + 8 H2O State and explain which of these two equations is consistent with your answer to (c)(iii). ……………...……………………………………………………………………… ………..……………………………………………………………………………. [1]

GESS 4EXP Chemistry 6092/03 PRELIM 2024_JAN 8 test observations Test 1 Put about 2 cm depth of R(aq) in a test