SAJC H2 Chem 2012 Prelim P3 QP

1 ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Chemistry Higher 2 9647/03 Paper 3 18 September 2012 2 hours Candidates answer on separate paper. Additional Materials: Writing paper, Data Booklet READ THESE INSTRUCTIONS FIRST: Write your name and civics group on all the work you hand in. Write in dark blue or black pen. 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. 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. This document consists of 12 printed pages including this page. [Turn Over

2 [Turn Over Answer any 4 questions. 1. Paracetamol and aspirin are well known drugs to treat pain and to reduce fever. (a) The following are structures of paracetamol and aspirin: N H C O CH3 HO O C O CH3 COOH Paracetamol Aspirin pKa = 9.5 pKa = 3.5 Mr = 151.0 Mr = 180.0 (i) Explain the difference in the pKa values of aspirin and paracetamol. (ii) Phenacetin is an analgesic whose use has largely declined due to its carcinogenic properties. Starting from paracetamol, show the steps involved in the synthesis of phenacetin. In your answer, include the reagents and conditions required for each step, as well as the structure of the intermediate involved. N C CH3 O H O Phenacetin CH3CH2 (iii) Salicin is an anti-inflammatory agent that is closely related in chemical make -up to aspirin. Suggest a chemical test that could be used to distinguish salicin from aspirin. Write a balanced equation for the reaction that occurred. Salicin O O CH2OH OH OH OHCH2OH

3 [Turn Over 1. (a) (iv) Aspirin is supplied as a crystalline solid. A standard solution of aspir in may be made by dissolving aspirin in an organic solvent. Explain why water is not used as a suitable solvent. [9] (b) (i) Write an expression for the acid dissociation constant, Ka, of aspirin. (ii) A tablet of aspirin is swallowed into the stomach. Given that the pH of the stomach is 1, calculate the percentage of aspirin molecules that remain undissociated in the stomach. [3] (c) 0.270 g of aspirin and 0.121 g of paracetamol are dissolved in 30 cm 3 of ethanol and titrated against 0.100 mol dm-3 aqueous sodium hydroxide, using 1 to 2 drops of two indicators, methyl red and phenolphthalein. x cm3 of aqueous sodium hydroxide is required to change the colour of the methyl red and a further y cm3 of aqueous sodium hydroxide is needed to change the colour of phenolphthalein. (i) Explain why both indicators are added in small amounts. (ii) Calculate the initial pH of the mixture, assuming the initial pH is from aspirin only. (iii) Calculate the values of x and y. (iv) Sketch the shape of the pH curve during this titration. [8] [Total 20]

4 [Turn Over 2. (a) In an effort to provide a sustainable system for waste treatment and energy production, scientists are currently working in the area of microbial fuel cell (MFC) technology. MFC is a device that converts chemical energy to electrical energy by the catalytic reaction of microorganisms such as bacteria. A typical microbial fuel cell consists of an anode and a cathode separated by a cation exchange membrane which only allows cations to pass through. Copper has been used extensively as electrodes in MFCs as shown in the following diagram. Sucrose Air Copper anode Bacteria cation exchange membrane Copper cathodeelectron flow On the anode side, bacteria grow and proliferate, forming a dense cell aggregate known as a biofilm that adheres to the MFC’s anode. In the course of their microbial metabolism, the bacteria act as catalysts for converting the organic substrate such as sucrose, C 12H22O11, into carbon dioxide. At the cathode, in the presence of air, steam is produced. (i) Write balanced half -equations, occurring at each electrode, when the MFC discharges. (ii) Given the electromotive force of the MFC is 1.25 V, c alculate the reduction potential of the carbon dioxide / sucrose half-cell. (iii) Suggest with a reason, the effect on the standard cell potential if the pH of the electrolyte at the cathode is increased. [5]

5 [Turn Over 2. (b) Copper reacts with 50% nitric (V) acid, HNO 3, to give a blue solution, containing complex ion A and a brown gas B. If the solution containing A is diluted and sodium hydroxide solution added cautiously, a gelatinous blue precipitate C is obtained, which if heated, forms a black solid D. Addition of concentrated aqueous ammonia solution to A gives a deep blue solution that contains the ion E. Addition of concentrated hydrochloric acid to E g ives a yellow solution of the ion F, which on dilution with water gives a solution containing the same ion A. If the brown gas B is passed into water, a mixture of acids G and H is formed in a disproportionation reaction. (i) Identify A – H. Construct balanced equations , with state symbols, for the formation of C, D, E and F. (ii) Draw a dot-and-cross diagram of nitric (V) acid, HNO 3. With reference to the Valence Shell Electron Repulsion theory, explain the shape about the nitrogen atom. [15] [Total 20]

6 [Turn Over 3. Hydrazine, N2H4, is a colourless, volatile and corrosive liquid with an ammonia -like odour. It is well known for its use in various rocket fuels and to prepare gas precursors used in air bags. (a) Hydrazine decomposes thermally, liberating a large amount of heat and a large volume of gaseous products in the following ways: 3 N2H4 (l) à 4NH3 (g) + N2 (g) ∆Hr = +111.8 kJ mol-1 N2H4 (l) à N2 (g) + 2H2 (g) ∆Hr = z kJ mol-1 (i) Write the equation that corresponds to the enthalpy change of formation of ammonia. (ii) Hence, using your answer to (a)(i), the above reactions and relevant data from the Data Booklet, construct an energy cycle to calculate z. [3] (b) Hydrazine can be synthesis ed by the Raschig process, where C l2 gas and NH 3 gas are heated under certain conditions in the presence of sodium hydroxide. (i) State the change in oxid ation number of nitrogen atom during the Raschig process. (ii) Predict the product of Cl2 in the process. (iii) Construct an ionic equation for the Raschig process. [3]

7 [Turn Over 3. (c) The Wolff–Kishner reduction is a chemical reaction that reduces a carbonyl compound (ketone or aldehyde) to its corresponding alkane. Condensation of the carbonyl compound with hydrazine forms a hydrazone intermediate, and treatment with a base induces the formation of the corresponding alkane. Below shows the synthesis of propane from propanone vi a the Wolff-Kishner reduction. CH3 C CH3 O hydrazone intermediate CH3CH2CH3 H2NNH2 KOH Carbonyl compounds A and B both undergo the Wolff-Kishner reduction separately forming ethylbenzene. A is found to give a silver mirror with Tollen’s reagent but B does not. Draw the displayed formulae of A, B and the hydrazone intermediate formed with A. [3]

8 [Turn Over 3. (d) Ethylbenzene undergoes the following reaction scheme producing C. CH2CH3 CH2CH3 CH2CH3 Step I Step II Step III HNO3 + BF3 NO2 NH2 NH C For Step I, (i) outline the mechanism of the reaction. (ii) with reference to the structure of BF 3, explain how BF 3 helps in the mechanism. For Step II, (iii) state the reagents and conditions required. For Step III, (iv) an intermediate is involved. Draw the stru