RI H2 Chem P3 Prelim

RAFFLES INSTITUTION 2014 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 23 September 2014 Candidates answer on separate paper. 2 hours Additional Materials: Writing Paper Data Booklet READ THESE INSTRUCTIONS FIRST DO NOT open this question booklet until you are told to do so. Write your name, class and index number in the spaces provided on the cover page. Write in dark blue or black pen on both sides of the paper. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer any four questions. Begin each question on a fresh sheet of paper. A Data Booklet is provided. Do not write anything on it. You are reminded of the need for good English and clear presentation in your answers. 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 wo rk securely together, with the cover page on top. ______________________________________________________________________________ This document consists of 14 printed pages.

2 Raffles Institution 2014 9647/03/S/14 [Turn Over Answer any four questions. Begin each question on a fresh sheet of paper. 1 (a) Nitrobenzene is formed when benzene is treated with concentrated nitric acid and concentrated sulfuric acid. The reaction proceeds via the formation of an electrophile, a nitronium ion, NO2 +. (i) Draw a dot-and-cross diagram of NO2 +. Nitric acid accepts a proton from sulfuric acid to form H 2NO3 + and HSO 4 –. This is followed by the loss of water from H 2NO3 + to produce the nitronium ion. The mechanism is initiated by the donation of a lone pair of electrons from the oxygen atom of the O─H bond in nitric acid to a hydrogen atom in sulfuric acid. (ii) Use the information given above to draw out the full “curly arrows” mechanism for the formation of the nitronium ion. You are advised to use structural formulae to show clearly the charges on all the species and which bonds are broken and formed. (iii) With the aid of an equation, show how H 2SO4 behaves as a catalyst in the nitration reaction. (iv) Explain why the nitration of phenol  occurs more readily than the nitration of benzene,  gives two products, 2-nitrophenol and 4-nitrophenol. [6] (b) Chlorobenzene can be converted to phenylamine at low temperature using the amide ion (NH 2 –) as a nucleophile. This reaction involves the formation of a very reactive intermediate called benzyne, C 6H4. Benzyne contains a carbon -carbon triple bond. (i) Draw the structural formula of benzyne and explain why this intermediate is highly reactive. 4-Chloromethylbenzene reacts with sodium amide in liquid ammonia to produce two isomers J and K, with the molecular formula C 7H9N. Both compounds decolourise Br2(aq) with the formation of white precipitates. Compound J reacts with Br 2(aq) to give a compound L, C7H7NBr2, whereas c ompound K reacts with Br2(aq) to give a compound M, C7H6NBr3. (ii) Suggest structures for compounds J, K, L and M. Write a balanced equation for the reaction of compound J with Br2(aq). [7]

3 Raffles Institution 2014 9647/03/S/14 [Turn Over Cl2 (c) A graphene sheet is a layer of graphite. A recent reaction scheme of the synthesis of graphene is shown below. When monomer A is polymerised to make polymer B, an inorganic by-product X is formed. In the transformation of polymer B into polymer C, another inorganic by-product, Y, is produced. (i) Give the molecular formulae of X and Y. Another synthesis was developed in an effort to prepare graphene sheet in which one of the C=C bonds was replaced with a B-N unit. (ii) Name the reaction mechanism for the formation of compound D in reaction II and suggest the role of AlCl3. (iii) Explain why compound D is non-planar. (iv) Compare and explain the changes in the electrical conductivity and softness of graphene sheet with the newly synthesised BN embedded sheet in which one of the C=C bonds was replaced with a B-N unit. [7] [Total: 20] BN embedded graphene sheet I II AlCl3 D

4 Raffles Institution 2014 9647/03/S/14 [Turn Over 2 Barium oxide is used to replace lead oxide for the development of lead -free glasses. Barium oxide can be prepared by the thermal decomposition of barium nitrate. (a) Write a balanced equation for the thermal decomposition of barium nitrate. [1] (b) Other oxides of magnesium, calcium and strontium may also be added to the formulations of lead-free glasses. Likewise, these oxides can be produced from the decomposition of their respective nitrates. The variation in the decomposition temperatures can be explained in terms of enthalpy changes between the two ionic compounds involved in the reaction. The calculated lattice energies of the nitrates and oxides of magnesium, calcium and strontium are shown in Table 1 below. Compound Lattice energy/ kJ mol–1 Compound Lattice energy/ kJ mol–1 Mg(NO3)2 –2481 MgO –3889 Ca(NO3)2 –2268 CaO –3513 Sr(NO3)2 –2176 SrO –3310 Table 1 (i) What do you understand by the term lattice energy? (ii) Explain the following trends observed in Table 1: (I) The decrease in the magnitude of the lattice energies of both the nitrates and the oxides. (II) The decrease in the magnitude of the lattice energy of oxides is greater than that of nitrates. (iii) The enthalpy change of thermal decomposition of the Group II nitrates is found to be endothermic and can be represented by the following equation: ΔHthermal decomposition = LE of XO – LE of X(NO3)2 + constant where LE: lattice energy, and X: Group II metal Deduce, in terms of enthalpy changes , the variation in the decomposition temperatures down the group. [5]

5 Raffles Institution 2014 9647/03/S/14 [Turn Over (c) "Barium swallow" consists of a white fluid of barium sulfate, often given to patients to help diagnose problems that affect the large intestine. Soluble barium salts, on the other hand, are highly poisonous. The numerical value of the solubility product , K sp, for BaSO4 is 1.30 × 10–10 mol2 dm–6. (i) Calculate [Ba2+(aq)] in a saturated solution of BaSO4. (ii) Suggest why barium carbonate is unsuitable to be administered a s “barium swallow”. [3] (d) Barium, which is a soft silvery metallic alkaline earth m etal, is never found in nature in its pure form, due to its reactivity with air or water. (i) Describe the react ions of barium with air and water , and give the relevant equations, with state symbols. (ii) Describe and explain how the reactivity of Group II metals towards air and water varies down the group. [6] (e) When solid NH 4Cl is mixed with solid Ba(OH) 2•8H2O, the beaker freezes to the wooden table almost immediately. Diagram I illustrates the observation and the equation for the reaction is given below. Ba(OH)2•8H2O(s) + 2NH4Cl(s) 2NH3(g) + 10H2O(l) + BaCl2(s) Comment on why the reaction is thermodynamically feasible. [2] ice Diagram I

6 Raffles Institution 2014 9647/03/S/14 [Turn Over (f) Grignard reagents are organometallic reagents derived from an alkyl halide and magnesium. The use of a Grignard reagent in the conversion of bromoethane to 2-methylbutan-2-ol is