DHS H2 CHEM P3 Prelim

This question paper consists of 9 printed pages and 1 blank page. © DHS 2011 [Turn over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination Year 6 H2 CHEMISTRY 9647/03 Paper 3 Free–Response 12 September 2011 2 hours Additional Materials: Cover Sheet Writing Papers Data Booklet INSTRUCTIONS TO CANDIDATES 1 Write your name, register number and class on this question paper and on the Cover Sheet provided. 2 Write in dark blue or black pen on both sides of the paper. 3 You may use a soft pencil for any diagrams, graphs or rough working. 4 Do not use staples, paper clips, highlighters, glue or correction fluid. 5 Answer any four questions. 6 Start each question on a fresh sheet of paper. 7 A Data Booklet is provided. 8 You are reminded of the need for good English and clear presentation in your answers. 9 The number of marks is given in brackets [ ] at the end of each question or part question. 10 At the end of the examination: 11 Fasten all work securely together with the Cover Sheet on top. 12 Hand in the question paper separately. 13 The total marks for this paper is 80 marks.

2 © DHS 2011 9647/03 [Turn over Answer any four questions. 1 Iron is the cheapest and one of the most abundant of all metals, comprising nearly 5.6% of the earth's crust and nearly the earth’s entire core. It exists in a wide range of oxidation states, from −2 to +6, although ferrous (Fe 2+) and ferric (Fe 3+) compounds are more common. (a) Acidic solutions containing ferrous ions are oxidised to ferric ions in air, with no precipitation seen. On the other hand, ferrous ions give a precipitate in alkaline solutions and the precipitate turns reddish–brown in air. With reference to the Data Booklet , explain the two reactions using relevant E  values, writing equations where appropriate. [4] (b) Ferric ions can catalyse the reaction between I–(aq) and S2O8 2–(aq). By considering relevant E values, describe and explain the role of the ferric ions in this reaction, writing equations where appropriate. [3] (c) Ferric chloride is an industrial scale commodity inorganic compound which is often used as catalyst in organic synthesis. One example is its use as a Lewis acid for catalysing the alkylation reaction of benzene by chlor oethane to form ethylbenzene. This reaction is similar to the reaction between benzene and chlorine. (i) (ii) (iii) (iv) Write a balanced equation for the overall reaction of chloroethane and benzene. State and outline the mechanism, with equations only, for the above reaction using ferric chloride as a catalyst. Hence, or otherwise, suggest how you would synthesise the following alcohol, 1–(3–nitrophenyl)propan–2–ol, starting from a chloroalkene and benzene as your only organic reagents. Your synthesis route should be no more than three steps. 1–(3–nitrophenyl)propan–2–ol Explain why 1–(3–nitrophenyl)propan–2–ol exis ts as enantiomers, and describe how pure samples of the enantiomers can be distinguished by a physical method. [8] (d) Consider this iron compound, NH 4[Fe(SCN)x(NH3)y], with the following composition by mass: Fe 16.4% S 37.7% N 28.8% (i) Define the term ligand, and identify the ligands in this complex. (ii) Calculate the values of x and y in the formula. (iii) Determine the oxidation number of iron in the compound. [5] [Total: 20]

3 © DHS 2011 9647/03 [Turn over 2(a) The Van Slyke’s method, named after an Amercian biochemist Van Slyke Donald Dexter, refers to a method to test for primary amino groups. Amino acids can be determined by measuring the volume of nitrogen released from their reaction with nitrous acid. For example, alanine can be reacted as shown in the equation below. CH 3CH(NH2)COOH + HNO2  CH3CH(OH)COOH + N2 + H2O Another method to test for primary amino groups consists of reacting amino acids with a volumetric solution of perchloric acid, for example: CH 3CH(NH2)COOH + HClO4 CH3CH(N+H3)COOH + ClO4 –  The reaction above is a weak base and strong acid neutralisation reaction. The excess perchloric acid is then determined by titration with aqueous sodium hydroxide. (i) Explain, with the aid of an equation, why alanine is a weaker base than propylamine. The above method is used to test for primary amino groups in lysine. 50.0 cm 3 of 0.100 mol dm–3 perchloric acid is added to a sample of lysine. The amount of excess perchloric acid is determined by titration with 0.150 mol dm –3 of sodium hydroxide solution. 16.0 cm 3 of sodium hydroxide is needed to complete the titration. (ii) (iii) Write an equation for each of the following reaction between: (I) nitrous acid and lysine (II) perchloric acid and lysine Calculate the volume of the nitrogen released at a pressure of 103 kPa and a temperature of 20 C by the same lysine sample. [6] (b) Amino acids like lysine show both acidic and basi c properties. In acidic solution, lysine is completely protonated and exists as the conjugate acid. 10.0 cm3 of completely protonated lysine is titrated with 0.10 mol dm –3 sodium hydroxide solution. Its titration curve is shown below. 2.20 pH C Y B X A

4 © DHS 2011 9647/03 [Turn over (i) Calculate the first dissociation constant Ka1 of lysine. (ii) Identify the species present at point X. (iii) Calculate the concentration and initial pH of lysine at A. (iv) Explain why the predominant species at point C has a high melting point. [6] (c) Equimolar amount of R and S are mixed and dissolved in a 1 dm 3 of distilled water. The pH of this mixture is then adjusted by adding a suitable amount of solid NaOH. By means of a balanced equation, show how the mixture above can act as a buffer when a few drops of aqueous sodium hydroxide are added to it. [1] (d) When 30 cm 3 of 1.0 mol dm –3 S in its neutral form, is added to 25 cm 3 of 1.0 mol dm –3 sulfuric acid in a plastic container, the temperature rose by 6.3 C. The process efficiency is expected to be 90%. (i) Calculate the standard enthalpy change of the reaction assuming that it takes 4.18 J to increase the temperature of 1 cm3 of the solution by 1.0 C. (ii) Predict if the enthalpy change of the reaction will be higher, the same or lower than that calculated in (i) if sulfuric acid is replaced by a weak acid. Explain. [4] (e) One mole of solid R in (c) dissolves in water to release 930 kJ of energy. Given the lattice energy of R is –234 kJ mol –1 and the following enthalpy changes of hydration, draw an energy cycle to determine the enthalpy change of hydration of the –O2CCH(NH3 +)(CH2)4NH3 + ion. ions Hθ hyd /kJ mol–1 Na+ –499 ? Cl – –381 [3] [Total:20]

5 © DHS 2011 9647/03 [Turn over 3(a) An optically active ester, L, (11.6 g) with the molecular formula of C 6H12O2, is hydrolysed by heating with an excess of aqueous sodium hydroxide solution. After terminating the hydrolysis, the alkaline reaction mixture is extracted several times with an organic solvent. The aqueous and organic layers are then collected separately. The aqueous solution is found to be optically inactive. Anhydrous magnesium sulfate is added to the organic layer. The mixture is filtered and the filtrate distilled. 7.4 g of a liquid J is obtained (assume 100 % purity and yield). (i) Suggest the role of anhydrous magnesium sulfate in the procedure above. (ii) Calculate the number of moles of ester used and hence calculate the M r of liquid J. (iii) Propose a possible structural form