ACJC Prelim P4 Answers

3 © ACJC 2018 9729/04/Prelim/2018 [Turn over Answer all the questions in the spaces provided. 1 Determination of the value of n in IOn− IOn− is an oxoanion of iodine. It can react with iodide ions, I−, in an acidic medium, to produce only molecular iodine and water. This amount of iodine can be analysed via titration with sodium thiosulfate, Na 2S2O3. 2S2O32− + I2 S4O62− + 2I− In this question, you will perform an experiment to determine the stoichiometric ratio between IOn− and I2 and the value of n in IOn−. FA 1 is solid hydrated sodium thiosulfate, Na2S2O3•5H2O FA 2 is a solution containing 0.00825 mol dm−3 KIOn FA 3 is 1.0 mol dm−3 sulfuric acid, H2SO4 FA 4 is 0.25 mol dm−3 potassium iodide, KI You are also provided with starch solution. (a) Preparation of a standard solution of 0.0484 mol dm −3 sodium thiosulfate (i) Determine the mass of FA 1 required to prepare 250 cm 3 of sodium thiosulfate solution with a concentration 0.0484 mol dm−3. [Ar: S, 32.1; O, 16.0; Na, 23.0; H, 1.0] Amount of Na2S2O3 = 0.0484 1000 × 250 = 0.0121 mol Mr of Na2S2O3•5H2O = 248.2 Mass of Na2S2O3•5H2O = 0.0121 × 248.2 = 3.003 = 3.00 g mass of FA 1 required = ………………….. g [1] You will follow the instructions to perform the preparation of the standard solution. Record your measurements in Table 1.1. (ii) 1. Using a mass balance, weigh out accurately the mass of FA 1 determined in (a)(i) into a weighing bottle. 2. Dissolve FA 1 in about 50 cm 3 of deionised water in a 100 cm3 beaker. 3. Transfer the solution and washings into a 250 cm 3 volumetric flask, using a filter funnel. 4. Top up to the 250 cm 3 mark with deionised water. 5. Stopper the flask and shake to ensure solution is homogeneous. Label this solution as FA 5. Complete Table 1.1 with your mass measurements and calculate the mass of FA 1 that you used. Table 1.1 Mass of weighing bottle and FA 1 / g Mass of empty weighing bottle / g

4 © ACJC 2018 9729/04/Prelim/2018 [Turn over Mass of FA 1 / g 3.00 [2] Records all masses to same number of decimal places (PDO) Records correctly all masses (PDO) Mass of FA 1 measured is ± 0.05 g of that determined in (a)(i) (MMO) (b) Preparation and titration of the reaction mixture 1. Fill a burette with FA 5. 2. Using a 25.0 cm 3 pipette, add 25 cm3 of FA 2 to a 250 cm3 conical flask. 3. Using a measuring cylinder, add 10.0 cm 3 of FA 3, followed by 10.0cm3 of FA 4 to the same conical flask and swirl quickly. 4. Immediately titrate the I2 in the conical flask with FA 5. When a pale yellow colour is obtained, add 1 cm 3 of starch solution. The end-point is reached when the blue – black colour of the solution decolourises. 5. Repeat steps 2 to 4 until consistent results are obtained and record your results below. Results Final burette reading/cm 3 Initial burette reading/cm3 Volume of FA5/ cm3 23.65 23.65 [5] Records all burette readings to 2 d.p. (MMO) Has at least 2 titre values ± 0.10 cm3 (MMO) Table has appropriate headings and units (PDO) Accuracy (MMO) Obtain, from your titration results, a suitable average titre of FA 5. Show clearly the titres you used in calculating this average. Average volume of FA5 = (23.65 + 23.65 )/ 2 = 23.65 cm 3 Average volume of FA 5 = ……………………….. [1] Selects at least 2 titre values that are ± 0.20 cm 3 and presents average titre to 2 d.p., with appropriate units (PDO) Calculations Show your working and appropriate significant figures in all of your calculations. (c) (i) Calculate the amount, in moles, of Na 2S2O3 present in the volume of FA 5 determined in (b) and hence the amount of iodine produced.

5 © ACJC 2018 9729/04/Prelim/2018 [Turn over Amount of S2O32− = conc × volume Amount of I2 = Amount of S2O32− ÷ 2 Amount of I2 = ………………………..[2] (ii) Calculate the amount of FA 2 used in the titration. Amount of FA 2 = 0.00825 1000 × 25 = 2.0625 × 10−4 = 2.06 × 10−4 mol (ACE) Amount of FA 2 = ……………………….. [1] (d) (i) Construct a balanced half-equation, in terms of n, for the reduction of IOn−. IOn− + (2n−1)e− + 2nH+ ½ I2 + nH2O ……………………………………………………………........................… [1] (ii) Hence construct a balanced equation, in terms of n, for the reaction between IOn− and I− to give I2 and H2O. IOn− + (2n−1)I− + 2nH+ nI2 + nH2O ……………………………………………………………........................… [1] (iii) Determine the amount of I2 produced by one mole of IOn− and determine the value of n in IOn−. Amount of I2 produced per mole of IOn− = n = Answer in (c)(i) Answer in (c)(ii) n =……………………….. [1] (e) Planning The reaction rate between IOn− and I− in an acidic medium can be expressed in the following rate equation: rate = k [IOn−]x [I−]y [H+]z If the concentrations of iodide and hydrogen ions are kept constant, the rate equation becomes rate = k’ [ IOn−]x where k’ = k [I−]y [H+]z

6 © ACJC 2018 9729/04/Prelim/2018 [Turn over Iodine produced by the reaction gives a dark blue colour in the presence of starch solution. A small amount of sodium thiosulfate is measured using a burette and added to the reaction mixture prior to the start of the reaction. The sodium thiosulfate reduces the iodine produced and this can be used to monitor the rate of reaction. By changing the initial concentration of IOn− and studying how this change influences the reaction rate, its order of the reaction with iodide ions in an acidic medium can be determined. (i) Plan an investigation, to determine the order of reaction with respect to IOn−, in its reaction with iodide ions in an acidic medium. You may assume that you are provided with • 100 cm 3 of 0.00825 mol dm−3 KIOn, • 100 cm 3 of 0.25 mol dm−3 potassium iodide, KI, • 100 cm 3 of 0.0484 mol dm−3 sodium thiosulfate, Na2S2O3, • 0.5 mol dm −3 sulfuric acid, H2SO4, • distilled water, • starch solution, • the apparatus normally found in a school or college laboratory. In your plan you should include details of • the reagents and apparatus that you would use, • the procedure that you would follow and the measurements that you would take. …………………………………………………… …………………………………... [4] - Total volume of reaction mixture is constant across different mixtures, with different volume of K IOn used / other method to ensure concentration of IOn− is different in each reaction mixture - Fixed and small (in relation to volume of K IOn used, not marking for this) volume of S2O32− used in each reaction mixture (to monitor a fixed amount o f iodine produced by the reaction) - Volumes of K I, K IOn, H 2SO4, H 2O measured using measuring cylinders o f appropriate capacities - Start taking time when either K I or K IOn is poured into reaction mixture containing the rest of the required reagents and stop time when a blue-black colour is observed

7 © ACJC 2018 9729/04/Prelim/2018 [Turn over (ii) Sketch the graph, with labelled axis, you would expect to obtain from your results in 1(e)(i). Explain your answer. ………………………………………………………………………………......…….[3] Plot lg (rate) = x lg([ IOn−]) + c y – axis is lg (rate) or lg(1/t) - x – axis is lg([IO n−]) or lg (VFA2) - c is lg k’ - straight line with positive gradient - rate ∝ ଵ ௧௜௠௘ since shorter time taken for blue colour to be seen (faster reaction with S2O32−), faster