JJC H2 Chem 2012 Prelim P3 QP

JURONG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS EXAM INDEX NUMBER CHEMISTRY 9647/03 Paper 3 Free Response 27 August 2012 2 hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. 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 your answer to each new question on a fresh sheet of writing paper. At the end of the examination, arrange the answers in numerical order and attach the cover sheet on top. A Data Booklet is provided. Do not write anything on the Data Booklet. 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 work securely together. 1 2 3 4 5 This document consists of 10 printed pages and 0 blank pages. [Turn over

Answer any four questions. 1. Use of the Data Booklet is relevant to the question. (a) Hydrogen can play an important role in the future by replacing the petroleum currently used in automobiles. For example, electric cars can be powered by hydrogen-oxygen fuel cells, with its hydrogen fuel kept at extremely high pressures in thick-walled storage tanks. (i) State the half equations for the reactions occurring at each of the electrodes in a hydrogen-oxygen fuel cell operating under acidic conditions. (ii) Describe one disadvantage of using a hydrogen-oxygen fuel cell over internal combustion engines in automobiles. (iii) A quantity of hydrogen gas is confined in a chamber of constant volume. When the chamber is placed at a temperature of 20°C, the pressure of the gas is 3.29  105 Pa. What will be the pressure of the gas, in Pa, if the chamber is now placed in a water bath of boiling water to simulate the operating temperature of a certain automobile? [5] (b) A student designed an ion-specific probe to determine the concentration of H + ions in solution. He connected a hydrogen electrode to a calomel reference electrode, which comprises a paste of mercury( I) chloride and mercury in a saturated chloride electrolyte solution. The porous glass frit acts as a salt bridge. The hydrogen electrode, sensitive to H+ ion concentration, is the measuring electrode. Therefore, this galvanic cell acts as a pH meter. When both electrodes are in contact with the test solution, an electric current flows through the wire and mercury(I) chloride reacts to form liquid mercury and chloride ion at the reference electrode. Under standard conditions, the Ecell of the pH meter is +0.28 V. (i) Construct the half equation for the reaction occurring at the reference electrode. (ii) Hence write an overall balanced equation for the pH meter. (iii) Explain why the cell potential would increase when the pH meter is placed in a solution of a higher pH. [3] 1. (c) The student used his probe to analyse a sample of 0.104 mol dm 3 solution of mandelic acid, Jurong Junior College 9647/P3/Prelim 2012 [Turn over Measuring electrode Reference electrode Saturated Cl electrolyte Paste of Hg2Cl2 in Hg V Test solution of unknown pHPt electrode coated with finely divided Pt Porous glass frit (acts as salt bridge) 1 atm, H2(g) Pt wire

a bitter almond extract that is used as antibacterial treatment or an oral antibiotic. The student obtained a cell potential reading, Ecell, of +0.41 V. (i) The Nerst equation can be used to calculate the concentration of H + ions detected by the probe: where n is the number of moles of electrons transferred in the overall reaction, [H+] is the concentration of hydrogen ions detected [Cl] is the concentration of Cl ions at the cathode. is the pressure, in atm, of hydrogen gas passed into the probe, Assuming that the concentration of Cl is constant at 1 mol dm 3, use the Nerst equation to calculate the concentration of H+ ions in the solution and the pH of the solution. (ii) Hence calculate the value of acid dissociation constant, Ka, of mandelic acid. (iii) How would you expect the value of Ka of mandelic acid to compare with that of phenylacetic acid? Explain your answer. (iv) Suggest a chemical test (stating reagents and observations) that would distinguish mandelic acid from phenylacetic acid. (v) Suggest a synthetic route to form mandelic acid from starting material phenylmethanol. [12] [Total: 20] Jurong Junior College 9647/P3/Prelim 2012 [Turn over

2. (a) Peppermint oil, also known as brandy mint and balm mint, is extracted from mentha piperita of the Labiatae family. Menthol and menthone are the two main components of peppermint oil. Menthone can be reduced to menthol, which can be easily dehydrated to give 3-menthene as the major product. Both menthone and menthol exhibit stereoisomerism but 3-menthene has no stereoisomer. (i) Which compound, menthone or menthol, is expected to be more volatile? Give your reasoning. (ii) Menthol is insoluble in water but is freely soluble in ethers and alcohols. Explain why menthol is insoluble in water. (iii) Suggest the reagents and conditions required for step 1 and step 2. (iv) An alkene M is also formed in step 2 in a much lower proportion than 3-menthene. Suggest the structure for M and give the organic product when M is heated with concentrated acidified KMnO4(aq). (v) When HCl(g) is bubbled through a solution of 3-menthene, an addition reaction occurs via a 2-stage mechanism. Suggest the mechanism of this reaction by means of a diagram. Include all whole or partial charges, and represent the movements of electron pairs by curly arrows. (vi) What do you understand by the term stereoisomerism? Explain why 3-menthene does not exhibit any stereoisomerism. [12] (b) Magnesium nitrate is a hygroscopic salt. In air, it readily forms a hydrate with the formula Mg(NO3)2.6H2O. Barium nitrate, however, does not form any hydrate in air. Both nitrates are soluble in water. Like all soluble barium compounds, barium nitrate is toxic by ingestion. Solutions of sulfate salts such as Epsom salt may be given as first aid for barium poisoning. (i) Suggest reason(s) why magnesium nitrate readily forms a hydrate whereas barium nitrate does not. (ii) Describe what you would see when a sample of white crystals of Mg(NO 3)2.6H2O is heated in an open test-tube. Write a balanced equation for the reaction. (iii) Explain why barium nitrate decomposes at a higher temperature than anhydrous magnesium nitrate. Jurong Junior College 9647/P3/Prelim 2012 [Turn over menthol OHO menthone 3-menthene step 1 step 2

2. (b) (iv) When water is added to the solid residue after barium nitrate has been heated, a colourless solution is obtained. When water is added to the solid residue after magnesium nitrate has been heated, a white suspension is obtained instead. Explain these observations. (v) Suggest how administering of Epsom salt solution, which contains magnesium sulfate, helps to treat barium poisoning. [8] [Total: 20] Jurong Junior College 9647/P3/Prelim 2012 [Turn over

7.4 pH 80x39x X X Volume of NaOH(aq) / cm3 pHequivalence 3. (a) Amino acids, such as alanine and phenylalanine, are essential building blocks for making proteins. They can be synthesised by a general 2-step reaction as exemplified below: (i) Suggest the structure of the intermediate organic compound E. (ii) What type of reaction is occurring in step I? (iii) Phenylalanine is found naturally in the breast milk of mammals. It is used in the manufacture of food and drink products and is sold as a nutritional supplement. Suggest the structural formula of the starting ma