PJC_H1 Chemistry_2008 JC2 H1 Chemistry Prelim Paper 2 Suggested Answers

1 Pioneer Junior College JC2 Preliminary Examination 2008 H1 Chemistry Paper 2 Suggested Answers Section A 1 (a) Number of Particle Electric charge Mass number Protons Electrons Neutrons X 0 32 16 16 16 Y 1- 81 35 36 46 Z 3+ 70 31 28 39 (b) Source beams of particle Y and particle Z - + Particle Y Particle Z 2 (a) Electrons repel each other and the electron pairs would arrange themselves as far apart as possible. Or lone pair-lone pair repulsion > bond pair - lone pair> bond pair - bond pair (b) . . O C O x . x . x x . . . . x .x . . . . . O ... . 2- . . O S O x . x . x x . . . . x .x . . . . . O . .. . 2- x x

2 For CO 3 2-, there are 3 bond pairs and 0 lone pair. By VSEPR, the shape is trigonal planar. For SO 3 2-, there are 3 bond pairs and 1 lone pair. By VESPR, the shape is trigonal pyramidal. 3 (a) Phosphorus burns with a blue flame. P4 + 5O2 → P4O10 Magnesium burns readily in oxygen wi th a bright white flame, leaving a white residue of MgO. 2Mg + O 2 → 2MgO (b) Calcium has one more quantum shell of electrons than magnesium. Due to more effective shielding by inner she ll electrons, the valence electrons are less attracted to the nucleus than m agnesium and thus it would lose the electrons more readily to form ions. Or As calcium has a larger atomic radius than magnesium, the valence electrons of calcium are further away from the nucleus. Thus, calcium will lose electrons more readily to form ions than magnesium. 4 (a) (i) Standard enthalpy change of formation of a compound is the heat change when one mole of compound is formed from its elements in their standard states under standard conditions of 298 K and 1 atm. (ii) Enthalpy / kJ mol-1 0 BF3(g) ΔHo at(B) B(s) + 3/2F2(g) B(g) + 3/2F2(g) B(g) + 3F(g) ΔHo f(B) 3/2 B.E. (F-F) 3 B.E. (B-F)

3 (iii) By Hess’ Law, 3B.E.(B-F) = -(-1137) + (573) + 3/2(+158) B.E. (B-F) = +649 kJ mol -1 4 (b) (i) Number of moles of propan-1-ol burnt = 0.60 / [3(12.0) + 8(1.0) + 16.0)] = 0.010 mol (ii) Let the final temperature be T°C Amount of heat liberated from combustion = 0.010 x 2021 = 20.21 kJ 1000 ) 0 . 21 T ( 18 . 4 200− × × = 20.21 T = 45.1°C (iii) Heat loss to surroundings or incomp lete combustion of propan-1-ol resulting in less heat produced. 5 (a) CH2CN OH CH2COOCH3 O A B (b) Step 1: KCN, ethanol, heat Step 2: HCl, heat Step 3:KMnO 4, H2SO4 (aq), heat (c) Step 1: substitution Step 2: acid hydrolysis Step 3: oxidation

4 6 (a) Molar mass of C15H15NO2 = (12.0 x 15) + (1.0 x 15) + 14.0 + (16.0 x 2) = 241 g mol-1