CJC_ H1_CHEM_P1 P2 ANS Prelim

Paper 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 C A C C D B A B B D B A C A A 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 C C C A C A C D B B B B D C A Paper 2 1(a) (i) (ii) H = 180 – 282 = -102 kJ mol-1 (iii) The activation energy will decrease. (iv) H = 150 – ½ (496) = -98 kJ mol-1 (v) Bond energy is an estimated value / average value only. (b) (i) Amount of unreacted iodine = 2 102 . 01000 60 . 23   = 2.36 x 10 -4 mol (ii) Amount of iodine reacted with sulfur dioxide = 410 36 . 2 01 . 01000 40    = 1.64 x 10-4 mol (iii) I2 +2e  2I- SO2 + 2H2O  SO4 2- + 4H+ + 2e Balanced equation: SO2 + I2 + 2H2O  2I- + SO4 2- + 4H+ Amount of SO2 present in wine = 1.64 x 10-4 mol (iv) Concentration of SO 2 = 1000 50 10 64 . 14 = 3.28 x 10-3 mol dm-3 [Total: 12] 8872 / CJC JC 2 CHEMISTRY Promotional Exam 09 ANSWERS

2(a) (i) The first ionisation energy of an element is defined as the amount of energy required to remove one electron from each atom in a mole of gaseous atoms producing one mole of gaseous ions with one positive charge. (ii) Be(g)  Be+( g ) + e (b) N 1s 2 2s2 2px 12py 12pz 1 O 1s 2 2s2 2px 22py 12pz 1 Less energy is required to remove a paired 2p electron in O since repulsion is experienced between the paired electrons. (c) Ionisation energy / arbitrary units Ionisatio n number 1 2 3 4 5 6 (d) (i) SiO2 is giant covalent that requires a lot of energy to break the strong covalent bonds between atoms during melting. SiCl4 is simple covalent that requires little energy to break the weak Van der Waals’ forces between molecules during melting. (ii) SiCl 4 + 2H2O  SiO2 + 4 H C l pH = 2 (3 is also accepted) [Total: 10] 8872 / CJC JC 2 CHEMISTRY Promotional Exam 09 ANSWERS

3(a) CH2Cl O CH2OHO H (ecf from B) E: HCN, trace amount of NaOH, (10 – 20 oC ) F: H C l ( a q ) , r e f l u x (b) Test: Add Tollen’s reagent to J, K and L and warm. Observation: Only Y gives silver precipitate, the others will not. OR Add 2,4-DNPH and warm, only Y gives orange precipitate, the others will not. Test: Add Br 2 in CCl4 or aqueous to J and L. Observation: Only J decolourises the reddish-brown Br2. L will not. Test: Add I2, NaOH (aq) and warm L. Observation: L gives yellow precipitate OR Add PCl 5 to L, white fumes (HCl) is evolved. Answers of the correct logic or sequence will be given marks. (c) [Total: 12] 4(a) The sailors ate fruits / vegetables. (b) OR (c) S t a r c h (d) Am