EJC Prelim P3 with Answer

© EJC [Turn Over EUNOIA JUNIOR COLLEGE JC2 Preliminary Examination 2018 General Certificate of Education Advanced Level Higher 2 CHEMISTRY Paper 3 Free Response 9729/03 19 September 2018 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, civics group and registration number on the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use paper clips, highlighters, glue or correction fluid. Section A Answer all questions. Section B Answer one question. Begin each question on a fresh piece of answer paper. A Data Booklet is provided. The use of an approved scientific calculator is expected, where appropriate. 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. This document consists of 13 printed pages and 1 blank page.

2 © EJC 9729/03/J2PE/18 Section A Answer all the questions in this section. 1 This question concerns acid-base properties of organic compounds and compounds of some elements in the Periodic Table. (a) The behaviour of a molecular specie as an acid or base is dependent of the surrounding medium and its interaction with another reacting specie. Using the symbol HZ to represent a Brønsted-Lowry acid and the symbol B – to represent a Brønsted-Lowry base, write equations to show the following: (i) CH 3OH acting as a Brønsted-Lowry base. [1] (ii) NH 3 acting as a Brønsted-Lowry acid. [1] (b) In an acid-base reaction, conjugate acid-base pairs can be determined. Copy the table below onto your writing paper and complete it by writing the formula of the conjugate acid-base pairs of the species listed, assuming the acids/bases are monobasic/monoacidic in nature respectively. acid base I H2O II H2 [1] (c) A particular weak monobasic acid HA has a dissociation constant, Ka, of 1.0×10–5 mol dm–3. The un-ionised acid molecules are blue and the anions are yellow. (i) Calculate the pH at which a solution containing HA molecules and A – ions is green, i.e. midway between blue and yellow. [1] (ii) Describe qualitatively how the answer in (c)(i) would be affected by the addition of solid sodium salt of the acid, NaA. [2] A solution mixture is prepared by adding 50 cm 3 of 0.10 mol dm –3 of the acid HA to 50 cm3 of 0.10 mol dm–3 of the sodium salt NaA. (iii) Calculate the final pH of the above solution mixture if 0.04 g of NaOH(s) was added. [3]

3 © EJC 9729/03/J2PE/18 [Turn Over (d) Elements and their compounds of the Periodic Table display varied chemical and physical properties. (i) The peroxide ion has the formula 2 2O − . Group 2 peroxides, on heating, decompose to the oxides and oxygen as exemplified by barium peroxide below: () () ()1 22 2BaO s BaO s O g⎯⎯→+ Predict how the temperature of decompos ition of calcium peroxide would differ from that of barium peroxide. Support your prediction with a brief explanation. [2] (ii) Explain why the melting point of A l 2O3, is higher than that of P4O10, but lower than that of SiO2. [3] (e) Bromine occurs as bromide ion in low concentration in sea-water. It is extracted commercially by the following series of steps. I Chlorine is passed into acidified sea-water and bromine is then removed in a stream of air. The concentration of bromine in the gas phase is too low at this stage for it to be condensed efficiently. II Therefore it is converted to hydrogen bromide by reaction with added sulfur dioxide and a small excess of water vapour: Br2(g) + SO2(g) + 2H2O(g) → 2HBr(g) + H2SO4(g) A moderately concentrated, aqueous mixture of hydrobromic and sulfuric acids is obtained when the vapour is cooled and condensed. III The concentrated mixture of acids is treated with chlorine and steam, producing a bromine vapour from which liquid bromine is obtained through condensation on cooling. The sulfuric acid remains in the solution. IV The crude bromine is purified by fractional distillation. (i) Explain in terms of the relevant standard electrode potentials the feasibility of the reactions taking place in Steps I and II. [3] (ii) Discuss the difference between the polarity of SO 2 and Br2. [2] (iii) Explain in terms of bonding, why, in Step III, the sulfuric acid remains in the solution. [1] [Total: 20]

4 © EJC 9729/03/J2PE/18 2 (a) Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that is commonly used as a painkiller and for fever reduction. The structure of ibuprofen is as shown below. (i) Describe what is a chiral carbon. [1] (ii) Draw the pair of enantiomers of ibuprofen. You may represent the aryl substituent using Ar. [2] In the laboratory, the enantiomers of ibuprofen have been separated into their enantiomerically pure forms. However, the bottles containing these pure enantiomers were not labelled. (iii) State a suitable physical property that you could use to identify the two different enantiomers. Explain your answer. [2] There are a few routes that can be used to synthesis ibuprofen. Below is a reaction scheme proposed by a student to synthesise ibuprofen from the hydrocarbon A, via two intermediates X and B. (iv) Suggest the identity of intermediate X. [1] (v) Name and outline the mechanism for Step I, including curly arrows, showing the movement of electrons, and all charges. [3] (vi) State the reagents and conditions for Step II and Step III. [2]

5 © EJC 9729/03/J2PE/18 [Turn Over (b) Methylbenzene, an important starting material in synthetic chemistry, can be manufactured from heptane via catalytic reforming: () () ( )71 6 78 2CH CH 4 H g→+ll At T = 27 ºC, the enthalpy of reaction, HΔ d , is +211 kJ mol–1, and the change in Gibbs free energy of reaction, GΔ d , is +110 kJ mol–1. (i) Using the information above, calculate for SΔ d in J mol–1 K–1. [2] (ii) Calculate for GΔ d at T = 627 ºC, in kJ mol–1. [1] (iii) Hence, comment on the effect of temperature on the spontaneity of the reaction. State any assumption that must be made for your comment to be valid. [2] (c) The position of an incoming electrophile in electrophilic substitution is dependent upon nature and position of the substituent(s) already bonded to the benzene ring. Ethylbenzene undergoes a three-step reaction as shown below. (i) Suggest the structures of intermediate Y and Z. [2] (ii) With reference to the substituents on the benzene ring, explain the rationale behind the order of substitution in determining the identity of Y and Z. [2] [Total: 20]

6 © EJC 9729/03/J2PE/18 3 Ethanal used to be manufactured from ethyne, HC≡CH. This process is now being replaced by a method based on ethene. Widely known as the Wacker Oxidation, the overall process amounts to oxidation. Ethene and oxygen are bubbled together through an aqueous solution containing CuCl2 and PdCl2 catalysts: 1 22 3 2CH =CH CH CHO+⎯ ⎯ ⎯ ⎯ →l l 2 22 PdC CuC O The laboratory scale modification – the Wacker-Tsuji Oxidation – is useful for the synthesis of various ketone: 1 23 2RCH=CH RCOCH+⎯ ⎯ ⎯ ⎯ →l l 2 22 PdC CuC O (a) (i) Draw the structure of the starting material to manufacture the following ketones using the Wacker-Tsuji Oxidation: (I) acetophenone, (II) cyclohexanone, [2] (ii) Describe one simple chemical test to distinguish between the above two ketones. You should state the reagents, conditions and the expected observations with each compound. [2]

7 © EJC 9729/03/J2PE/18 [Turn Over (b) Ethanal is often used in the manufacture of 2-chloroethanoyl chloride, ClCH2COCl. (i) Propose a 3-step synth