NJC H2 Chem 2013 Prelim P3 QP

NATIONAL JUNIOR COLLEGE PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME SUBJECT CLASS REGISTRATION NUMBER CHEMISTRY Paper 3 Free response 9647/03 Tues 17 Sep 2013 2 hours READ THESE INSTRUCTIONS FIRST Answer any four questions. Start your answer to each question on a fresh piece of paper. A Data Booklet is provided. 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 behind the cover page. This paper consists of 11 printed pages and 0 blank page. 133

NJC/H2 Chem/03/2013 [Turn over 2 Answer any four questions. 1(a) Dinitrogen difluoride is a chemical compound with the formula N 2F2. It is a gas at room temperature. N 2F2 was first identified as the thermal decomposition product of fluorine azide, N3F, in 1952. (i) Draw a dot- and-cross diagram for N2F2. Predict the shape around the central atom using the Valence Shell Electron Pair Repulsion theory. (ii) N 2F2 is able to exist as a pair of isomers at low temperature. State the type of isomerism exhibited by N 2F2. Hence, draw the structure of the more stable isomer. [4] (b) Nitrogen reacts with fluorine to give the compound N 2F2 in the following equation: N2 (g) + F2 (g) N2F2 (g) A 1 : 2 mixture of N 2 and F 2 is allowed to reach dynamic equilibrium at 500 K at an equilibrium pressure of 1.12 × 105 Pa in a 500 cm 3 flask. The mass of the equilibrium mixture is found to be 0.578 g. (i) Explain what is meant by the term dynamic equilibrium and write the expression for the equilibrium constant, Kp , for this reaction, stating its units. (ii) Calculate the average M r of the gaseous mixture. Hence, calculate the mole fraction of N2F2 in the equilibrium mixture. (iii) Using your answer to (b)(ii), calculate the partial pressure of N2F2. (iv) Predict the sign of ΔS for the reaction, showing your reasoning. (v) The value of ΔG for the above reaction is negative. Use this information, together with your answer to (b)(iv), to predict the sign of ΔH for the reaction. Explain your reasoning. [10] 134

NJC/H2 Chem/03/2013 [Turn over 3 (c) When phosphorus( III) oxide is heated in air, another oxide A which contains 49% of phosphorus by mass is obtained. On hydrolysis, 1 mole of A forms a mixture containing 1 mole each of H 3PO3 and H3PO4. Deduce the identity of A. [2] (d) Industrially, 1,2dichloroethane, CH 2ClCH2Cl is used to make chloroethene, CH2=CHCl, the monomer which the polymer PVC is produced from. When heated with aqueous silver nitrate solution, a white precipitate is observed for 1,2dichloroethane within 10 minutes but not for chloroethene even after prolonged period. (i) Why does the reactivity of chloroethene differ from that of 1,2dichloroethane? (ii) When aqueous silver nitrate is added to benzoyl chloride, a white precipitate is formed immediately even without heating. Account for this observation. [4] [Total: 20] 135

NJC/H2 Chem/03/2013 [Turn over 4 2 This question concerns potassium manganate(VII). (a) Potassium manganate( VII) can be used as a disinfectant. Over time, the concentration of potassium manganate( VII) in aqueous solution decreases due to a redox reaction taking place in the sealed bottles. Oxygen is also formed in this reaction. Suggest the reducing agent responsible for the decrease in concentration of potassium manganate( VII) solutions on storage in sealed bottles. Support your answer by quoting relevant data from the Data Booklet. [2] (b) Compounds containing manganese oxo-anions often disproportionate in aqueous solution. However, it is found that in neutral or alkaline solution, potassium manganate( VII) reacts with sodium sulfite, Na 2SO3, in a 1:1 mole ratio to give a bright blue solution. Suggest a formula for the species responsible for the blue colouration and write an ionic equation for the reaction. [2] (c) Solutions of potassium manganate( VII) can be standardised against a measured volume of a standard solution of sodium ethanedioate, Na 2C2O4, under acidic conditions. The contents of the titration flask have to be warmed to 60 °C before commencing the titration. Explain, as fully as you can, why warming is necessary before commencing the titration. [2] 136

NJC/H2 Chem/03/2013 [Turn over 5 (d) A student wanted to investigate the rate of reaction between MnO 4 − and C2O4 2− ions in an acid medium. The rate of reaction was followed by mixing the following two solutions, and measuring the concentration of remaining MnO 4  ions at fixed time intervals. Solution A: 100 cm 3 of 0.20 mol dm 3 ethanedioic acid, 5 cm 3 of 2 mol dm 3 sulfuric acid, 15 cm 3 of 0.20 mol dm 3 copper(II) sulfate and 80 cm 3 of water Solution B: 50 cm3 of 0.020 mol dm3 potassium manganate(VII) The following results were obtained. Time/s [MnO4 ] / mol dm3 0 0.0040 10 0.0026 20 0.0016 30 0.00096 50 0.00040 75 0.00016 (i) By means of a graphical method, determine the relevant rate equation under the conditions whereby the reaction was conducted. (ii) If the rate of consumption of MnO 4 (aq) at a particular time is 1.2 × 104 mol dm3 s1, what will be the rate of consumption of ethanedioate ions at that time? (iii) What is the role of copper(II) sulfate in the reaction? (iv) The experiment is repeated at room conditions in the absence of aqueous copper(II) sulfate. Sketch a graph of the volume of carbon dioxide produced in this reaction against time, and explain the shape of your graph. [8] (e) Reagents like potassium manganate( VII) and sodium borohydride, NaBH 4, are of great help in elucidating the structure of organic compounds. Compound C of molecular formula, C 6H10O2, produces a yellow precipitate when treated with an alkaline solution of iodine. C does not react with an ammoniacal solution of silver nitrate. Treatment of C with excess hot acidified concentrated potassium manganate(VII) gives compound D, C4H6O3, as the only organic product. Compound C produces compound E, C6H12O2, when reacted with NaBH4. Suggest possible structural formulae of C, D and E, explaining your reasoning. [6] [Total: 20] 137

NJC/H2 Chem/03/2013 [Turn over 6 3 Acetolactic acid, C 5H8O4, is a precursor in the biosynthesis of the branched chain amino acids, valine and leucine. (a) Acetolactic acid can be synthesis ed from 2 -chlorobut-2-ene by the following reactions. Cl NaOH(aq) heat Qstep 1 step 2 R C4H8O2 step 3 OH O OH O acetolactic acid HCN Cl Cl Cl step 4 H2SO4(aq) heat P KMnO4 C5H9NO2 (i) Name the reagent and conditions used in step 1 and describe the mechanism for this reaction. In your answer, you should include curly arrows showing the movement of electrons and any relevant charges and dipoles. (ii) Compound P contains a chiral carbon. Explain why compound P obtained from step 1 does not rotate th