2020 SAJC H2 Prelim Paper 4 (QP)

1 [Turn Over NAME Class ST ANDREW’S JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Chemistry (9729) Paper 4 Practical 17 August 2020 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 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 the 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. Shift Laboratory Qualitative Analysis Notes are printed on pages 21 and 22. The number of marks is given in the brackets [ ] at the end of each question or part question. For Examiners’ Use 1 2 3 4 Total This document consists of 22 printed pages.

2 1 Investigation of the concentration of potassium manganate(VII) Potassium manganate(VII) is a very useful oxidising agent. It oxidises iron( II) ions in acidic medium according to equation 1. MnO4 (aq) + 5Fe2+ (aq) + 8H+ (aq)  Mn2+ (aq) + 5Fe3+ (aq) + 4H2O (l) equation 1 In solution, ammonium iron (II) sulfate, (NH 4)2SO4FeSO46H2O, dissociates into iron (II), sulfate and ammonium ions according to equation 2. (NH4)2SO4FeSO46H2O (s)  2NH4+ (aq) + 2SO42 (aq) + Fe2+ (aq) + 6H2O (l) equation 2 You are provided with FA 1, ammonium iron(II) sulfate crystals, FA 2, potassium manganate(VII) solution, 1.0 mol dm3 sulfuric acid. (a) (i) Preparation of standard solution of FA 3 and titration of FA 3 against FA 2 In an appropriate format in the space provided on page 3, prepare tables in which to record results for your experiment in (a):  All weighings to an appropriate level of precision  All burette readings and the volume of FA 2 added 1. Weigh the capped bottle containing FA 1. 2. Dissolve this in about 50 cm3 of 1.0 mol dm-3 sulfuric acid in a 100 cm3 beaker. 3. Transfer the solution and washings into a 250 cm3 standard flask. Make up to the mark with deionised water and shake well to obtain a homogeneous solution, FA 3. 4. Reweigh the emptied bottle and its cap. 5. Fill a burette with FA 2. 6. Pipette 25.0 cm3 of FA 3 prepared into a conical flask. 7. Add about 10 cm3 of aqueous sulfuric acid into the conical flask. 8. Run FA 2 from the burette into the flask. The end -point is reached when the solutions changes from pale green to orange. 9. Repeat steps 6 to 8 until consistent titres are obtained.

3 [Turn Over Results [4] (ii) From your titrations, obtain a suitable volume of FA 2 to be used in your calculations. Show clearly how you obtained this volume. Volume of FA 2 = ………………………………… [1]

4 (b) [Ar of K: 39.1; Mn: 54.9; O: 16.0; N: 14.0; H: 1.0; Fe: 55.8; S:32.1] (i) Calculate the number of moles of FA 1 in the weighed sample and hence calculate the number of moles of Fe2+ ions used during the titration. number of moles of FA 1 = ……………………………… number of moles of Fe2+ ions used = ……………………………… [2] (ii) Calculate the number of moles of MnO 4− reacted during titration and hence the mass of potassium manganate (VII), KMnO4, in 1 dm3 of FA 2. number of moles of MnO4− reacted = …………………………… mass of KMnO4 in FA 2 = …………………………… [3]

5 [Turn Over (c) A student performed the same experiment, using the quantities described earlier, and obtained a mean titre volume of 25.45 cm3. The errors (uncertainties) associated with each reading is shown below: Apparatus Uncertainties volumetric flask ±0.15 cm3 pipette ±0.1 cm3 burette ±0.05 cm3 Calculate the maximum percentage error (uncertainty) of this mean titre volume. maximum percentage error = …………………………..% [2] (d) The teacher informed the student who performed the experiment in (c) that the titre volume should have been 26.00 cm3 instead. (i) Calculate the percentage error in the student’s result. percentage error = ………………………..% [1]

6 (ii) Using your answers in (c) and (d)(i), comment on the accuracy of the student’s result. ……………………………………………………………………………………………. ……………………………………………………………………………………………. [1] [Total: 14]

7 [Turn Over 2 Investigation of the effect of temperature on chemical reactions Potassium manganate(VII) is widely used as a strong oxidising agent. It can oxidise both organic and inorganic compounds. Besides oxidising iron( II) ions, it can also oxidise ethanedioate ions. 2MnO4− (aq) + 5C2O42− (aq) + 16H+ (aq)  2Mn2+ (aq) + 10CO2 (g) + 8H2O (l) equation 3 In this experiment, the effect of temperature on the rate of this reaction is studied by measuring the time taken for the purple solution to turn colourless. You will perform a series of four experiments. Then, you will graphically analyse your results to determine the effect of temperature on the rate of the reaction between potassium manganate(VII) and ethanedioate ions. For each experiment, you will note the time taken, t, for the reaction mixture to become colourless. You will use the same potassium manganate( VII) solution, FA 2, and 1.0 mol dm −3 sulfuric acid, H2SO4, in Question 1. You are also provided with FA 4, 0.1 mol dm−3 sodium ethanedioate, Na2C2O4. (a) Prepare a table in the space provided on page 9 in which to r ecord for each experiment, with the appropriate level of precision:  temperature of the solution inside the boiling tube,  time taken for solution to turn colourless, t, to the nearest second,  calculated values of 1/t and lg (1/t). It is recommended for you to draw this table after step 6 of experiment 1 on page 8. Safety notes Hot water to be collected from the metal dispenser is heated to 90 °C. Use a Styrofoam cup placed in a 250 cm3 beaker to collect the hot water (according to step 4 of experiment 1 on the next page) to prevent your hands from being scalded. Gently pull/push the tap of the dispenser to prevent the hot water from spurting/splashing out.

8 Experiment 1 1. Using a measuring cylinder, add 5.0 cm3 of FA 2 to a boiling tube. 2. Using the same measuring cylinder, add 10.0 cm3 of 1.0 mol dm−3 H2SO4(aq) to the boiling tube. 3. Using another measuring cylinder, measure 5.0 cm3 of FA 4. Do not add it to the boiling tube yet. 4. Place a Styrofoam cup inside a 250 cm 3 beaker. Dispense hot water from the metal dispenser into the Styrofoam cup until it is half-filled. 5. This hot water is reused for subsequent experiments. Do not discard it. 6. Place the thermometer inside the boiling tube containing FA 2 and place the boiling tube inside the Styrofoam cup containing hot water. 7. While waiting for the temperature of the solution inside the boiling tube to increase to around 70 °C, you should prepare the table to record your measurements. 8. It should take about two to three minutes for the temperature to stabilise. Once the temperature reading hardly increases or stays almost constant, note the temperature and remove the boiling tube from the Styrofoam cup and place it in a test tube rack. 9. Pour FA 4 from step 3 rapidly into the boiling tube. Start the stopwatch during this addition. 10. Mix the contents thoroug hly by stirri ng gently with the thermometer but do not read the temperature. Then hold the boiling tube over a while tile. 11. Stop the stopwatch when the solution turns colourless. 12. Record the time taken, t, to the nearest second. 13. Discard the reaction mixture. Wash out the boiling tube and stand it upside down in the test tube rack to drain. 14.