Bowen 5105 Sci Chemistry ANSWER KEY

Marking Scheme for Sec 4N Science (Chemistry) Preliminary Exam 2023 Paper 3 & 4 Paper 3 1 B 2 C 3 D 4 C 5 B 6 A 7 D 8 C 9 A 10 B 11 C 12 C 13 D 14 B 15 A 16 B 17 A 18 C 19 B 20 D Paper 4 Section A [MAX:14M] Question No Answers Comments A1 Substance Diagram(s) (a) Water C [1] (b) Oxygen gas E [1] (c) Mercury B [1] (d) Mixture F [1] A2 (a) (i) Low air-fuel ratio produces higher concentration of CO emitted/ High air- fuel ratio produces lower concentration of CO emitted [1] (ii) Accelerating mode generates higher temperature that allows more nitrogen and oxygen in the air to react to produce NO. [1] (b) Incomplete combustion [1] (c) NO causes breathing difficulty/ eye and lung irritation/ inflammation of the

lungs. [1] Reject: Acid rain CO combines with haemoglobin and obstruct oxygen uptake/ headaches and fatigue/ heart damage and loss of consciousness/ death as body tissues do not receive oxygen for respiration [1] Reject: if only write death A3 (a) Cracking [1] (b) Number of mol of pentadecane = 5.30/ ((15x12) + 32) = 0.025 mol [2] [1] for correct Mr; [1] for correct final answer (c) Correct drawing of C2H4 Correct bonding – [1] Correct number of valence electrons for all atoms – [1]

Section B [Max:16] Any 2 questions out of 3 Question No Answers Comments B4 (a) 4M + O2 → 2M2O [1] (b) The reactivity of the alkali metals towards oxygen increases down the group. [1] (c) It reacts very violently/ explosively with water/ Effervescence observed./ Flame observed [1] (d) (i) Correct plotting for all points – [1] (ii) Correct drawing of the curve of best fit, joining the points together– [1] (iii) 71°C [1] (+/- 2) [1] (e) (i) To conserve natural resources/ To save cost of extracting metals/ helps to reduce environmental problems caused by extraction of metals. [1] (ii) There should be 2 different sizes of atoms – bigger atom to be labelled as iron, smaller atom as carbon [1] No label – [0] B5 (a) Correct plotting for all points – [1] (b) Correct drawing of graph, joining the points together using straight lines – [1] (c) 9cm3 [1] (d) Violet [1] Reject: blue (e) (i) H2SO4 + CaCO3 → CaSO4 + CO2 + H2O [1] (ii) Acidifies lakes and streams, causing aquatic plants and animals die [1] Corrode marble building and statues [1] (iii) Slaked lime/ quicklime/ Calcium oxide/ Calcium hydroxide [1] Iron Carbon

B6 (a) Diesel oil and petrol [1] (b) There is high number of vehicles/ transportation that requires the use of the fractions. [1] (c) (i) [1] (ii) name of process: Hydrogenation/ Addition [1] conditions: 200°C, Nickel catalyst [1] (iv) Chemical Test: Add aqueous bromine to each respectively [1] Observation with propane: No decolorization of bromine/ Solution remains reddish brown [1] Observation with propene: decolourisation of bromine/ Solution turns from reddish-brown to colourless [1]