HCI 2018 H2 CHEM P3 QP Prelim

This document consists of 11 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 17S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 3 Free Response Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet 9729/03 18 September 2018 2 hours READ THESE INSTRUCTIONS FIRST Write your name, CT group, centre number and index number clearly in the spaces above. Write in dark blue or black pen. You may use a HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Section A Answer all questions. Section B Answer one question. Begin each question on a new sheet of writing 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. Circle the question number for the question that you have attempted for Section B on the cover page provided.

2 © Hwa Chong Institution 2018 9729 / 03 / C2 Prelim 2018 Section A Answer all the questions in this section. 1 (a) Aluminium is a major component used in the manufacture of various parts of airplanes. With the aid of a labelled diagram, describe the structure and bonding in aluminium. [3] (b) Pure aluminium is too soft to be used directly for aircraft applications. An alloy of aluminium is used instead to improve its tensile strength and corrosion resistance. One such alloy is aluminium bronze, which contains aluminium, copper and iron. To determine the percentage by mass of copper in the alloy, an experiment is performed with the following procedure: step 1: Concentrated HNO 3 is added to a 1.20 g sample of the alloy. The mixture is then heated until all the solid dissolves. The resulting solution, which contains Cu 2+, Fe2+ and Al3+ ions, is cooled and diluted. step 2: H 2O2(aq) is added dropwise until no further change is observed. step 3: Excess concentrated NH 3 is added to the solution. The resulting mixture is filtered and a dark blue solution is obtained as filtrate. step 4: HNO 3(aq) is added to the filtrate until no more traces of precipitate is observed in the solution. step 5: Excess K I(aq) is added to the resulting solution. The reaction produces brown iodine and a white precipitate of copper(I) iodide, CuI. 2Cu2+(aq) + 4I−(aq) → I2(aq) + 2CuI(s) A filtration is performed again, and the filtrate is made up to 250 cm 3 in a volumetric flask. step 6: 25.0 cm 3 of this solution is titrated with aqueous sodium thiosulfate, Na2S2O3(aq), in the presence of starch indicator. I2(aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq) (i) State the chemical role of H2O2(aq) in step 2. [1] (ii) It is found that 17.50 cm 3 of 0.100 mol dm –3 Na2S2O3(aq) is required for complete reaction in step 6. Calculate the percentage by mass of copper in the alloy. [3] (iii) Suggest how the volume of Na 2S2O3(aq) needed in step 6 would be affected if step 3 was not carried out in the procedure. Explain your answer. [2]

3 © Hwa Chong Institution 2018 9729 / 03 / C2 Prelim 2018 (c) When an airplane flies at very high altitude, the atmospheric pressure outside the airplane becomes very low. For the comfort of passengers, the airplane cabin is typically pressurised to 81.0 kPa and the temperature is maintained at 23.0 C at cruising altitude. (i) A passenger brought a balloon filled with 1500 cm3 of carbon dioxide gas on board the airplane at sea level, which has an atmospheric pressure of 101 kPa, and a temperature of 30.0 C. Assuming that carbon dioxide gas behaves ideally, calculate the final volume of the balloon when the airplane is at cruising altitude. [1] (ii) Would the actual volume of the balloon at cruising altitude be larger or smaller than the volume you calculated in (c)(i)? Justify your answer. [1] (d) Swift Enterprises, a chemical company, has proposed 2-methylbutane as a fuel for light airplanes. Its structure is shown in Fig. 1.1. Fig. 1.1 2-methylbutane can react with chlorine in the presence of UV light to give substitution products. The following table shows the relative reactivity of different types of hydrogen for this reaction. Table 1.1 types of hydrogen relative reactivity primary 1 secondary 3.5 tertiary 5 (i) Draw the structures of all the monochlorinated products for 2-methylbutane (ignoring stereoisomers), and give the relative proportions of all the products obtained using the data in Table 1.1. [3] (ii) With an appropriate sketch of the Boltzmann distribution, explain how an increase in temperature affects the rate of reaction between 2-methylbutane and chlorine. [3] (iii) The monochlorinated products serve as raw materials in the manufacture of chlorofluoroalkanes (CFCs), which have been banned in a number of countries due to their effect on the ozone layer in the upper atmosphere. Explain how CFCs destroy ozone. [1] [Total: 18]

4 © Hwa Chong Institution 2018 9729 / 03 / C2 Prelim 2018 2 Olympicene, first synthesised to celebrate the 2012 London Olympics, is made up of five rings like the Olympic logo. The scheme in Fig. 2.1 shows a synthesis of olympicene. Fig. 2.1 (a) In stage I, A is formed from 1-pyrenecarboxaldehyde via a Wittig reaction. In a Wittig reaction, an aldehyde or ketone reacts wi th a triphenyl phosphonium ylide to give an alkene and triphenylphosphine oxide, as shown in Fig. 2.2. Fig. 2.2 (i) Suggest a reason why Wittig reaction is useful in organic synthesis. [1] (ii) By considering the structure of A, suggest the structure of the triphenyl phosphonium ylide used in stage I. [1] (b) State the types of reaction that occur during each of the three steps in stage II. [3] (c) The formation of C from B in stage III follows a three-step mechanism:

5 © Hwa Chong Institution 2018 9729 / 03 / C2 Prelim 2018 1. C l on B is first abstracted by A lCl3 catalyst to form a carbocation and [AlCl4]–. 2. Electrons from a neighbouring π bond attack the carbocation to form a second carbocation which has five rings. 3. [A lCl4]− deprotonates the second carbocation, producing C and a by-product, and regenerating the catalyst. Outline this mechanism, showing clearly any intermediates that are formed and use curly arrows to indicate the movement of electron pairs. You may represent B as shown below. [3] (d) Draw the structure of the organic product formed when C reacts with 2,4-dinitrophenylhydrazine and state the type of reaction that occurs. [2] (e) Suggest a 2-step synthesis of D from C in stage IV. You should state the reagents and conditions needed for each step, and show clearly the structure of the intermediate compound. [3] (f) State the isomeric relationship between D and olympicene. [1] (g) (i) Deduce whether olympicene is a planar molecule. [1] (ii) State the number of π electrons in one molecule of olympicene. [1] (iii) Explain why olympicene has a higher boiling point than benzene. [2] (h) Under suitable conditions, olympicene can also be formed from E, along with another by-product, F. State the type of reaction shown above. [1] [Total: 19] 3 (a) Magnesium oxide, MgO, can be obtained from sea water which contains significant amounts of Mg 2+ and Ca2+ ions.

6 © Hwa Chong Institution 2018 9729 / 03 / C2 Prelim 2018 The steps involved