YJC_H1_CHEM_ANS

YISHUN JUNIOR COLLEGE 2009 JC2 PRELIMINARY EXAMINATIONS H1 CHEMISTRY MARK SCHEME Paper1 1 2 3 4 5 6 7 8 9 10 C B B A C D C C B C 11 12 13 14 15 16 17 18 19 20 A D D C B C A A B C 21 22 23 24 25 26 27 28 29 30 D A A B D C C A A D Paper 2 Section A 1 (a) (i) Electrons [1] (ii) Electrostatic forces of attractions between the electrons and protons [1] (iii) Protons and neutrons [1] (b) (i) species protons neutrons electrons 11 12 11 8 8 8 8 10 10 [1] [1] [1] (ii) Na2O + 2HNO3 → 2NaNO3 + H2O [1] (iii) Let x % = abundance of oxygen-16 = 75% = 25% [1] (c) (i) Nitrogen: 1s2 2s2 2p3 (electrons have to be unpaired) Oxygen: 1s2 2s2 2p4 [1] [1]

(ii) 1st I.E. of N = 1400 kJ mol-1 1st I.E. of O = 1310 kJ mol-1 The 1st I.E. of O is lower than that of N. For O, electron is removed from a 2p orbital that is occupied by another electron. Thus, electron experiences inter-electronic repulsion and less energy is required to remove it. [1] [1] (iii) N3- [1] (iv) A lot of energy is required to remove all 5 electrons in nitrogen and 6 electrons in oxygen to form a stable octet. [1] 2 (a) (i) [1] (ii) bent, 104.5º [1] (b) [1] (c) Hydrogen peroxide is simultaneously oxidized and reduced in this reaction. Oxidation state of oxygen atom in hydrogen peroxide increase from -1 to 0 to oxygen and decrease from -1 to -2 in water. [1] (d) (i) 5.50 x 10-4 mol [1] (ii) 1 mol of vanadium donates 2 mol of electrons to hydrogen peroxide. Original oxidation state of vanadium is +3 [1] [1]

3 (a) (i) Na Mg Al P formula NaCl MgCl2 AlCl3 PCl3 PCl5 pH 7 6.5 2 1 [1] [1] (ii) PCl3 + 3H2O → H3PO3 + 3HCl or PCl5 + 4H2O → H3PO4 + 5HCl [1] (b) The lattice energy of MgO is more exothermic than NaCl. Mg2+ and O2- have higher charge than Na+ and Cl-. Hence the attraction between Mg2+ and O2- is stronger than Na+ and Cl-. [1] [1] (c) (i) AlF3 is an ionic compound and a lot of energy is required to overcome the strong electrostatic forces of attractions between the Al3+ and F- ions. AlCl3 is a simple covalent molecule and less energy is required to overcome the weak van der Waals forces of attraction between the AlCl3 molecules. [1] [1] (ii) [1] 4 (a) C H O % 69.8 11.6 18.6 No. of moles 5.81 11.6 1.16 5 10 1 Empirical formula = C5H10O Mr = 86 = 5 × 12.0 + 10 × 1.0 + 1 × 16.0 Molecular formula = C5H10O [1] [1] (b) (i) A contains a C=C double bond [1] (ii) A contains a –OH group / is an alcohol [1]

(iii) A has the structure [1] (c) (i) Orange precipitate [1] (ii) Secondary alcohol (do not accept alcohol) [1] (d) 1. Presence of restricted rotation about the C – C bond due to the presence of π bond; 2. No two similar groups is bonded to the C with the double bond [1] [1] (e) cis trans [2]

Section B 5 (a) [1] (b) (i) order of reaction w.r.t PABA is 1 order of reaction w.r.