CJC_ H2_CHEM_P1 P2 P3 ANS Prelim

CJC: H2 CHEMISTRY 2009 Preliminary Exam ANSWERS Paper 1 MCQ Answers: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 C B C D A C A C A D C C A C A D A B D B 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 C B C D C C B A B A D B B C B A B A A C Paper 2 1(a) (i) Mg+(g)  Mg2+(g) + e- (ii) The second ionisation involves removal of an electron from the 3p subshell from Si +, and the 3s subshell from A l+. Since the 3p subshell is on average further from the nucleus / at a higher energy level / experiences more shielding than the 3s subshell , the electron removed from Si+ is less tightly held and requires less energy to remove. (b) Neon, being already in the gaseous state, is ionised first and emits light, whereas sodium is in the solid state and takes time to vaporise before it can be ionised. OR Neon has a higher first ionisation energy than sodium. The initial power surge when the light is turned on is able to ionize neon, giving rise to the red light, after which the power drops and is only enough to ionise sodium, giving rise to the orange light. (c) (i) Al 2O3 has a giant ionic lattice structure with strong ionic bonds / high lattice energy / partial covalent character, hence it is insoluble. SiO2 has a giant molecular structure with strong covalent bonds requiring large amounts of energy to break, hence it is insoluble. (ii) AlCl3 dissolves to give [Al(H2O)6]3+(aq) which undergoes hydrolysis due to the high charge density on Al3+: [A l(H2O)6]3+  [Al(H2O)5(OH)]2+ + H+ OR 2AlCl3 + 6H2O ⇌ Al2O3 + 6HCl OR AlCl3 + 3H2O ⇌ Al(OH)3 + 3HCl SiCl4 hydrolyses in water to give HCl, a strong acid / H+ is produced: SiC l4 + 2H2O  SiO2 + 4HCl OR SiCl4 + 4H2O  Si(OH)4 + 4HCl (d) (i) Solubility of Li2O is lower than that of BaO. Therefore, the basicity of Li2O is also lower than that of BaO. (ii) Percentage change in mass = % 100)] 0 . 16 ( 2 ) 9 . 6 ( 2 [ )] 0 . 16 ( 2 ) 9 . 6 ( 2 [ )] 0 . 16 ( 3 0 . 12) 9 . 6 ( 2 [     = 61.1% [Total: 12] 2(a) x NH4NO3(l) + aq  NH4NO3(aq) 71 -50 NH3(g)+ HNO3(g) -31 + 37 NH 3(aq) + HNO3(aq)

CJC: H2 CHEMISTRY 2009 Preliminary Exam ANSWERS From energy cycle, show x = 71 -31 + 37 -50 = +27 kJ mol-1 (b) The reaction is endothermic as the temperature of the reaction mixture drops. S is positive as the number of gaseous molecules increase from 0 to 1. Since reaction is spontaneous, G < 0 . G = H - TS, -TS must be more negative than H.