ASR 2020 J2Prelim H2Chem P4 QP

ASRJC JC2 PRELIM 2020 9729/04/H2 [Turn over ANDERSON SERANGOON JUNIOR COLLEGE 2020 JC 2 PRELIMINARY EXAMINATION NAME:______________________________ ( ) CLASS: 20 /____ CHEMISTRY 9729/04 Paper 4 Practical 27 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, class and index number 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 a 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. Quantitative Analysis Notes are printed on pages 19 and 20. Shift Laboratory 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. For Examiner’s Use 1 / 14 2 / 18 3 / 11 4 / 12 Total / 55 This document consists of 20 printed pages.

2 ASRJC JC2 PRELIM 2020 9729/04/H2 Answer all the questions in the spaces provided. 1 Determination of the oxidation number of iodine in a compound In this experiment you will determine the oxidation number of iodine in one of its compounds by titration. You are provided with the following: FA 1 is a 0.0197 mol dm–3 solution of the iodine–containing compound. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous potassium iodide, KI. FA 4 is 0.105 mol dm–3 sodium thiosulfate, Na2S2O3. (This solution is also used in Question 2.) starch indicator FA 1 reacts with excess acidified potassium iodide to produce iodine, I2. This iodine is then titrated with aqueous sodium thiosulfate using starch indicator. (a) Titration of iodine formed against FA 4 1. Fill the burette with FA 4. (You will use the same burette for Question 2.) 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Using a 25 cm3 measuring cylinder, add 10 cm3 of FA 2 to the same conical flask. 4. Using the same measuring cylinder, add 20 cm 3 of FA 3 to the mixture in the conical flask. The mixture will now be a red–brown colour, due to iodine produced. 5. Add FA 4 from the burette until the mixture becomes light brown. 6. Then add 10 drops of starch indicator. The mixture will change to a dark blue colour. 7. Continue titrating until the mixture becomes colourless. This is the end–point. 8. Carry out as many accurate titrations as you think necessary to obtain consistent results. 9. Record in a suitable form below all of your burette readings and the volume of FA 4 added in each accurate titration. (i) Results [5]

3 ASRJC JC2 PRELIM 2020 9729/04/H2 [Turn over (ii) From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. The iodine produced required …………………………….. cm3 of FA 4. [1] (b) Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sodium thiosulfate in the volume of FA 4 calculated in (a)(ii). moles of Na2S2O3 = …………………………….. [1] (ii) The equation for the reaction of iodine with sodium thiosulfate is shown. I2(aq) + 2Na2S2O3(aq)  Na2S4O6(aq) + 2NaI(aq) Calculate the number of moles of iodine that reacted with the sodium thiosulfate calculated in (b)(i). moles of I2 = …………………………….. [1] (iii) Use the information on page 2 to calculate the number of moles of iodine –containing compound in the 25.0 cm3 of FA 1 used in each titration. moles of iodine–containing compound FA 1 = …………………………….. [1]

4 ASRJC JC2 PRELIM 2020 9729/04/H2 (iv) Use your answers to (b)(ii) and (b)(iii) to calculate the number of moles of iodine produced when 1 mole of the iodine–containing compound in FA 1 reacts with excess FA 3. Give your answer as an integer. moles of I2 = …………………………….. [1] (v) The anion in FA 1 is IOx– where x is the number of oxygen atoms present in the formula. Use your answer to (b)(iv) to balance the ionic equation for the reaction between FA 1 and FA 3 under acidic conditions. Hence deduce the value of x in the formula IOx–. IO….– + ...... I– + ...... H+  ...... I2 + ...... H2O x = …………………………….. [2] (vi) Calculate the oxidation state of iodine in FA 1. (If you were unable to calculate x in (b)(v), assume that x = 4.) oxidation state of iodine = …………………………….. [2] [Total: 14]

5 ASRJC JC2 PRELIM 2020 9729/04/H2 [Turn over Question 2 starts on the next page.

6 ASRJC JC2 PRELIM 2020 9729/04/H2 2 To investigate the effect of concentration changes on the rate of a reaction. FA 5 is 0.0500 mol dm–3 acidified iron(III) chloride, FeCl3. FA 6 is 0.0500 mol dm–3 potassium iodide, KI. Iron(III) ions oxidise iodide ions, I–, to iodine, I2 as shown in equation 1. equation 1 2Fe3+(aq) + 2I–(aq)  2Fe2+(aq) + I2(aq) In this experiment you will investigate how the rate of this reaction is affected by the concentration of Fe3+ ions. To do this you will add thiosulfate ions, S2O32–, and starch indicator to a mixture of Fe3+(aq) and I–(aq). The iodine produced by the reaction reacts immediately with the thiosulfate ions and is reduced back to iodide as shown in equation 2. equation 2 I2(aq) + 2S2O32–(aq)  2I–(aq) + S4O62–(aq) When all the thiosulfate has reacted, the iodine remaining in solution turns the starch indicator blue–black. The rate of reaction can be determined by timing how long it takes for the reaction mixture to turn blue–black. (a) Preparation of FA 7 by dilution of FA 4 FA 7 which contains 0.00500 mol dm –3 of sodium thiosulfate, Na2S2O3, can be prepared by diluting FA 4 from Question 1 in a 250 cm3 graduated flask. (i) The concentration of sodium thiosulfate, Na2S2O3, in FA 4 is 0.105 mol dm3. Show, by calculation, that the volume of FA 4 needed to prepare this diluted solution in the 250 cm3 graduated flask is 11.90 cm3. [1] (ii) Using the burette from Question 1, measure 11.90 cm 3 of FA 4 into a 250 cm 3 graduated flask labelled FA 7. Make up to the mark with distilled water. Record the volume of FA 4 added to the flask in the space provided. volume of FA 4 added = …………………………….. cm3

7 ASRJC JC2 PRELIM 2020 9729/04/H2 [Turn over (b) Experiment 1 1. Fill the burette labelled FA 5 with FA 5. 2. Run 20.00 cm3 of FA 5 into a 100 cm3 beaker. 3. Using the same measuring cylinder, add the following to the second 100 cm3 beaker:  10 cm3 of FA 6  20 cm3 of FA 7  10 cm3 of starch solution 4. Add the contents of the first beaker to the second beaker and start timing immediately. 5. Stir the mixture once and place the beaker on a white tile. 6. Stop timing as soon as the solution turns blue–black. Ignore any colour changes that occur before the intense blue–black colouration. 7. Record this reaction time , to the nearest second , in the space provided on page 8. 8. Rinse both beakers and shake dry. Rinse and dry the glass rod. Experiment 2 1. Run 10.00 cm3 of FA 5 into a 100 cm3 b