TJC Prelim P3 Solutions

1 O O •• Answers to 2008 TJC Prelim Paper 9746/3 Free Response 1. (a) (i) O N O N O N ClO [4] (ii) · Both have simple molecular structures and boiling involves breaking of intermolecular forces of attraction. · HNO3 has intermolecular hydrogen bonding which are stronger than the Van der Waals forces exist between NOCl molecules, hence more energy required to overcome, so higher boiling point. [2] (b) (i) PV = nRT n = RT PV = 40031.8 102010101 33 ´ ´´´ - · n = 0.608 mol [1] (ii) Let be degree of dissociation of NOCl. 2NOCl (g) 2NO (g) + Cl 2(g) Initial no of mol 0.5 0 0 Change in no of mol - (0.5) +(0.5) + 2 a (0.5) Equil no of mol 0.5(1-) +0.5 + 2 5.0 a · Total equilibrium number of mol = 0.5 – 0.5 + 2 5.0 a + 0.5 = 0.608 = 0.432 · % of NOCl dissociated = 10010 32.4 ´ % = 43.2 % [2] (iii) · Kp = 2 2 2 NOCl ClNO P PP NOClP = 2002.03.03.0 3.0 ´++ kPa = 75 kPa NOP = 8.0 3.0 x 200 kPa = 75 kPa 2ClP = 2008.0 2.0 ´ kPa = 50 kPa •• < 120o 120o <109.5o ·

2 10 20 25 30 0.5 0.3 0.2 time 0 NOCl NO Cl2 40 Number of moles · Kp = 2 2 75 5075 ´ = 50 kPa at 400 K [3] (iv) · When temp is increased, by Le Chatelier’s Principle, since reaction is endothermic, position of equilibrium shifts to the right to reduce temp. At the new equilibrium, lesser number of moles of NOCl will be present. of NOCl [3] (c) (i) · Increasing pKa: C < A < B · Alcohol B is least acidic as the negative charge of the alkoxide ion is localized on the O atom while for phenol A and C, the negative charge on O atom can be delocalized onto the benzene ring, giving a more stable phenoxide anion. · C is more acidic than A as C has an electron-withdrawing Cl atom which can further disperse the negative charge on O atom of the phenoxide ion, thus stabilizing the anion. For A, the – CH3 group is electron releasing and this will intensify the negative charge on the O atom, so destabilizing the phenoxide ion. [3] (ii) · Compound C is insoluble in water due to the hydrophobic benzene ring which forms Van der Waals forces with water, releasing insufficient energy to overcome intermolecular hydrogen bonds in water and in compound C. · Compound C is acidic and forms a soluble salt sodium phenoxide with aqueous NaOH. Sodium phenoxide is ionic and forms ion-dipole interactions