NYJC H2 CHEM P3 QP Prelim

[Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response 12 September 2011 2 hours Candidates answer Section A on the Question Paper Students answer on separate paper. Additional Materials: Answer Paper Graph 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. 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 together. For Examiner’s Use Section A 1 2 3 4 5 Section B Answer all questions on separate answer paper. Total This document consists of 13 printed pages and 1 blank page.

2 NYJC 2011 H2 Chemistry 9647/03 Prelim [Turn Over Answer any four questions. 1 This question is about the reactions of compounds containing the cyanide group. (a) Cyanogen, (CN) 2 is called a pseudohalo gen because several of its reactions are similar to those shown by halogens Cl 2, Br2 and I2. For example, it can be reduced to hydrogen cyanide, which is a weak acid. ½ ( C N ) 2 + H+ + e HCN E Ө = +0.37 V (i) By analogy with the reactions of halogens or halides ions and by making use of the above data and information in the Data B ooklet, predict the reactions, if any, that might occur when the following reagents are mixed. I. Cl 2 + HCN II. Na2S2O3 + (CN)2 III. K2Cr2O7 + (CN)2 (ii) Draw a dot-and-cross diagr am of the cyanogen molecule, suggesting its shape. [8] (b) (i) When chiral 2-chloroheptane, C 7H15Cl reacts with ethanolic sodium cyanide, the optical isomerism of the product is reversed. Explain, with the aid of structural formu la, why the optical isomerism is reversed. (ii) Another compound A with the same molecular formula C 7H15Cl also reacts with ethanolic sodium cyanide. A, which is optically active, eventually loses its optical activity after reaction wit h ethanolic sodium cyanide. Give the structural formula of A and describe the mechanism of its reaction with ethanolic sodium cyanide. (iii) Predict which reaction b(i) or b(ii) will proceed at a faster rate when the concentration of ethanolic sodium cyani de is increased. Explain your answer. [8]

3 NYJC 2011 H2 Chemistry 9647/03 Prelim [Turn Over (c) Suggest the reagents and condit ions required for stage I and the structural formula for B, C and D for the following scheme. OH OH O O OH B C I HCN NaCN Na in ethanol D Heat with concentrated H2SO4 (does not give CO2(g) with Na2CO3) [4] [Total: 20]

4 NYJC 2011 H2 Chemistry 9647/03 Prelim [Turn Over 2(a) The term “octane rating” is often used in the petroleum industry for rating the ability of octane’s various branched isomers in reducing engine knock in vehicles. Compounds A, B, C and D are isomers of the hydrocarbon octane, C8H18. The table below shows the boiling points and data relating to the optical activity of the compounds. Structure Boiling Points / oC Number of chiral centres Optical Activity? Isomer A 119 1 Yes CH2 CH CH CH 2 CH3CH3 CH3 CH3 Isomer B 118 2 No Isomer C 110 1 Yes Isomer D 107 0 No (i) Suggest why Isomer B has no optical activity even though it has 2 chiral centres. (ii) Draw the structural formula of each of the compounds A, C and D. (iii) Explain in terms of structure and bonding, why isomers A to D have different boiling points. (iv) Controlled chlorination of Isomer B in the presence of UV light pr oduces 4 different isomers of formula C 8H17Cl. Draw the structural formula of each of the 4 isomers and give the mole ratio in which they are formed. ( v ) Monochlorination of Isomer B in part (iv) produces many by-products, one of which has the formula C 16H34. Assuming that the carbon atoms in Isomer B do not rearrange themselves during the reacti on, draw the structur al formula of the isomer of C16H34 that has the lowest boiling point. [11]

5 NYJC 2011 H2 Chemistry 9647/03 Prelim [Turn Over (b) Studies have shown that drinking water t hat contains high levels of dissolved barium compounds results in health effects such as difficulties in breathing, changes in heart rhythm, stomach irritation and even brain swelling. The Environmental Protection Agency has decreed that a concent ration of more than 1.45 × 10 -5 moldm-3 of barium in drinking water is considered harmful to humans. A sample of water was obtained from a lake that is located near a paint manufacturing plant where bar ium sulfate which is the main component for white paint is discharged into the lake. The water sample obtained is also saturated with other compounds such as lead(II) hydroxide and lead(II) sulfate. The lake water has a pH of 7.4. Calculate the concentration of Ba 2+ in the lake water which is saturated with lead(II) sulfate, lead(II) hydroxide and bariu m sulfate and concl ude whether the Ba 2+ concentration lies within the safety limits. [K sp of Pb(OH)2 = 1.43 × 10-20 mol3dm-9; Ksp of PbSO4 = 2.53 × 10-8 mol2dm-6; Ksp of BaSO4 = 1.08 × 10-10 mol2dm-6] [3] (c) The following statements invo lve the chemistry of Group I elements. (i) “Lithium is only slowly attacked by water at 25 oC, whereas, with water, sodium reacts vigorously, potassium inflames, and rubidium and caesium reacts explosively”. Explain the difference in reactivity of the Group I metals with water. (ii) “Lithium and sodium metals are obtai ned by electrolysis of their molten salts. However this method is not suitable for obtaining potassium, rubidium and caesium”. Explain this observation in te rms of the structure and bonding of the different metals. (iii) Lithium is the only Group I metal which produces the oxide, Li 2O, when heated in oxygen whereas sodium, potassium, and the other Group I metals produces peroxides as in Na2O2 and the superoxides such as KO2 when heated in oxygen. Explain the difference in products formed. [6] [Total 20]

6 NYJC 2011 H2 Chemistry 9647/03 Prelim [Turn Over 3 Bad news always seems to get more att ention than good news. Newspaper headlines dwell on atmospheric pollut ants, not the components of the atmosphere that are essential to life. One of the major pollutants fr om the internal combustion engine of cars are the oxides of nitrogen. Many are highly toxic, cont ribute to acid rain and react with other compounds to form even more toxic pollutant s. Consider the following questions on some of the oxides of nitrogen. (a) The chemistry of nitrogen oxides is very versatile. Given the following reactions and their standard enthalpy changes: Δ H rxn / kJ (1) NO(g) + NO2(g) Æ N2O3(g) ─ 39.8 (2) NO(g) + NO2(g) + O2(g) Æ N2O5(g) ─112.5 (3) 2NO2(g) Æ N2O4(g) ─ 57.2 (4) 2NO(g) + O2(g) Æ 2NO2(g) ─ 114.2 (5) N2O5(g) Æ N2O5(s) ─ 54.1 Calculate by means of an energy cycle or otherwise, the heat of reaction for N2O3(g) + N2O5(s) Æ 2N2O4(g) [5] (b) The decomposition of gaseous dinitrogen pentoxide is represented by 2N2O5(g) Æ 4NO2(g) + O2(g) The data for the decomposition of gaseous dinitrogen pentoxide at 333 K are shown below. Pressure of N2O5(g) remaining / Pa Rate of reactio