ACJC Prelim P2 Answers

3 © ACJC2018 9729/02/Prelim Exam/2018 [Turn over Index No. Name Form Class Tutorial Class Subject Tutor ANGLO-CHINESE JUNIOR COLLEGE DEPARTMENT OF CHEMISTRY Preliminary Examination CHEMISTRY 9729/02 Higher 2 Paper 2 Structured Questions 16 August 2018 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, index number, form class, tutorial class and subject tutor’s name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Answer all questions. A Data Booklet is provided. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. For Examiner's Use Question no. Marks 1 / 14 2 / 6 3 / 9 4 / 13 5 / 12 6 / 9 7 / 12 TOTAL / 75 This document consists of 21 printed pages, including this cover page. 9729/02/Prelim/18 ANGLO-CHINESE JUNIOR COLLEGE © ACJC 2018 Department of Chemistry [Turn over

4 © ACJC2018 9729/02/Prelim Exam/2018 [Turn over Answer all the questions in the spaces provided. 1 A mixture contains two white solids XCO3 and YSO4 where X and Y could be either magnesium, zinc or barium. The following investigations were carried out to determine the identity of X and Y. (a) When the mixture of solids was treated with excess dilute hydrochloric acid, a colourless gas was evolved and some, but not all, of the mixture dissolved. The resulting mixture was filtered. To the filtrate, dilute aqueous ammonia was added dropwise till in excess. The white precipitate formed was insoluble in excess aqueous ammonia. Deduce the identity of elements X and Y, showing your reasoning clearly. [3] • X- Mg Y- Ba • BaSO 4 insoluble in dil HCl , • filtrate contains Mg2+ which ppt to form Mg(OH)2 white ppt insol in excess aq ammonia OR • White ppt contains Mg2+ as Zn2+ forms ppt but it is soluble in excess NH3 (b) Group 2 carbonates decompose according to the following equation: ZCO3 (s) ZO (s) + CO2 (g) element magnesium calcium barium enthalpy change for the reaction, ∆Hr/ kJ mol−1 +101 +178 +269 (i) Use the data above to state and explain the trend in the thermal stability of the Group 2 carbonates. [3] • ∆Hr become more endo, thermal stability increases • increasing ionic radii leading to decreasing charge density/ polarising power and • less distortion of electron cloud of large anion (ii) Lithium carbonate, a Group 1 carbonate, decomposes on heating. Use relevant data from the Data booklet to calculate relative charge densities of the above cations and hence deduce which of the above ∆H r values in the table is likely to be the nearest to that of lithium carbonate. [2]

5 © ACJC2018 9729/02/Prelim Exam/2018 [Turn over Mg 2+ Ca 2+ Ba 2+ Li + Relative charge density 2/0.065 =30.8 2/0.099 =20.2 2/0.135 =14.8 1/0.06 =16.7 ∆Hr of Li2CO3 resembles that of BaCO3 (c) Patients with digestive tract problems are sometimes asked to take an x-ray after they have swallowed a “barium meal”, consisting of a suspension of BaSO4 in water. (i) Write an expression for the solubility product, Ksp, for BaSO4, including its units. K sp = [Ba2+][SO42–] units: mol2 dm–6 (ii) The numerical value of Ksp is 1.30 × 10–10. Calculate [Ba 2+(aq)] in a saturated solution of BaSO4. In saturated solution, IP = Ksp [Ba2+] = √1.30 x 10–10 = 1.14 x 10–5 mol dm–3 (iii) Although Ba2+ ions are toxic, the [Ba 2+(aq)] in a saturated solution of BaSO 4 is too low to cause any problems of toxicity. The numerical value of Ksp for BaCO3 (5 × 10–10) is not significantly higher than that of BaSO4, but BaCO3 is very poisonous if ingested. Suggest a reason why this might be so. BaCO3 reacts with the acid in the stomach to release Ba2+ ions. (iv) BaSO4 is commonly prepared in the laboratory by combining solutions containing Ba2+ and SO42–. Determine if BaSO 4 will form when equal volumes of 2.00 × 10–2 mol dm –3 Ba(NO3)2 and 4.00 × 10–2 mol dm–3 Na2SO4 are mixed. [Ba2+] after mixing = 1.00 x 10–2 mol dm–3 [SO42-] after mixing = 2.00 x 10–2 mol dm–3 IP on mixing = 2.00 x 10–4 > Ksp (1.14 x 10–10 ) Hence precipitation will take place. [2] [Total: 14]

6 © ACJC2018 9729/02/Prelim Exam/2018 [Turn over 2 A student conducted two separate electrolysis using inert electrodes under room temperature and pressure. electrolysis reaction electrolyte volume of gas collected at anode / cm3 duration 1 concentrated hydrochloric acid 150 10 minutes 2 aqueous sodium sulfate 37.5 (a) (i) Write the half-equation with state symbols for the reaction occurring at the anode for electrolysis reaction 1. [1] 2 C l– (aq)  Cl2 (g)+ 2e– (ii) Calculate the current that was used in electrolysis reaction 1. [2] Amt of Cl2 collected = 150/24000 = 6.25 x 10–3 mol Amt of electrons passed through = 6.25 x 10–3 x 2 = 0.0125 mol Quantity of charge passed through = 0.0125 x 96500 = 1206.25 C Current used = 1206.25 / (10 x 60) = 2.01 A (b) (i) Write the half-equation with state symbols for the reaction occurring at the anode for electrolysis reaction 2. [1] 2 H 2O (l)  O2 (g) + 4H+ (aq) + 4e– (ii) Determine the current used in electrolysis reaction 2. [2] If the current used was the same, the volume of gas obtained in reaction should be halved based on the number of e- required. Since the volume of gas is four times less than that in reaction 1, the current used must be halved. Current used = ½ x 2.01 =1.005 = 1.01A [Total: 6]

7 © ACJC2018 9729/02/Prelim Exam/2018 [Turn over 3 Sertraline is an antidepressant used to treat depression, obsessive compulsive disorder, post-traumatic stress disorder and related c onditions. This medication can help to improve mood, mental alertness, energy level and sleep pattern for the patients. Sertraline works by helping to restore the balance of serotonin, a neurotransmitter in the brain which regulates anxiety, mood and happiness. A proposed synthesis route of sertraline is shown below. (a) State the reagents and conditions needed for step 1 and draw the structure of compound V in the box above. K 2Cr2O7, dilute H2SO