HCI Prelim P3_Mark_Scheme

2018 HCI C2 H2 Chemistry Preliminary Exam / Paper 3 Paper 3 1 (a) Aluminium has a giant metallic lattice structure [1] and metallic bonding between Al3+ ions and a sea of delocalised electrons. [1] [1] Number of electrons should be 3 times that of Al3+ ions (b) (i) Oxidising agent [1] (ii) n(S2O32–) = 17.5 × 0.1 / 1000 = 0.00175 mol n(I2) in 25.0 cm3 = 0.00175 / 2 = 0.000875 mol [1] n(I2) in 250.0 cm3 = 0.000875 × 10 = 0.00875 mol [1] ecf n(Cu2+) = 0.00875 × 2 = 0.0175 mol mass of Cu in alloy = 0.0175 × 63.5 = 1.11g percentage of mass of Cu in alloy = 1.11 / 1.20 × 100 = 92.6% [1] ecf (iii) The titre value would be higher. [1] Without step 3, Fe 3+ ion in the solution will undergo reaction with I–, causing more iodine to be produced. [1] (c) (i) V2 = 1830 cm3 [1] (ii) The volume will be smaller as CO 2 has significant dis persion forces between molecules. [1] = ×× =++ 11 2 2 12 281101 1500 30 273 23 273 PV PV TT V HWA CHONG INSTITUTION 2018 C2 H2 CHEMISTRY PRELIMINARY EXAM SUGGESTED SOLUTIONS

1 (d) (i) Cl Cl Cl Cl 6 x 1 = 6 2 x 3.5 = 7 3 x 1 = 3 1 x 5 = 5 [0.5] x 4 = [2] structures, [1] for correct proportions. (ii) Diagram [1] At higher temperatures (such as T 2), the proportion of particles with kinetic energy greater than or equal to the activation energy increases. [1] Thus, the frequency of effective collisions increases (rate constant increases), and reaction rate increases. [1] (iii) C–Cl bond in CFCs can be broken down by UV light to form chlorine radicals, which can react with ozone. [1] 2 (a) (i) Accept any of the following reasons: [1] Wittig reaction allows the formation of an alkene from a carbonyl compound Wittig reaction allows for number of carbon atoms to increase. (ii) [1] (b) Accept any of the following: [1] × 3 (selective) reduction, (acid) hydrolysis, nucleophilic substitution (acid) hydrolysis, nucleophilic substitution, (selective) reduction (acid) hydrolysis, (selective) reduction, nucleophilic substitution (selective) reduction, oxidation, nucleophilic substitution Note: DO NOT allow hydrogenation and DO NOT allow substitution Kinetic energy Ea No. of particles 0 T1 T2 T2 > T1 Proportion of particles with KE ≥ Ea at T1 Proportion of particles with KE ≥ Ea at T2 +

2018 HCI C2 H2 Chemistry Preliminary Exam / Paper 3 (c) [1] x 3 steps (d) [1] condensation [1] (e) Correct reagents and conditions 1: LiAlH4 in dry ether OR NaBH4 in methanol [1] Correct structure of intermediate [1] Correct reagents and conditions 2: excess concentrated H 2SO4, heat OR concentrated H3PO4, heat OR Al2O3, heat [1] (f) D is a constitutional isomer OR structural isomer OR positional isomer of olympicene. [1]

2 (g) (i) Olympicene is not a planar molecule since it contains a sp3 carbon which