SJI 2022 Y4 Prelim Chemistry 6092 P1 P2 AS

1 St. Joseph’s Institution 2022 Year 4 OP Preliminary Exam Answer Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 B D B C C C C D B B 11 12 13 14 15 16 17 18 19 20 C D C A A C C B A D 21 22 23 24 25 26 27 28 29 30 D A C D A B B C B B 31 32 33 34 35 36 37 38 39 40 A B A A D A A B B C Paper 2 Section A Qn Suggested answers Mark A1a W: iron / Fe X: zinc sulfate / ZnSO4 Y: barium sulfate / BaSO4 1 1 1 A1b Lighted splint extinguishes with a pop sound. 1 TOTAL 4 A2ai The rate of reaction increases. When temperature increases, the reacting particles have higher kinetic energy, move faster and collide more often. More particles have energy equal to or higher than the activation energy. Thus, the frequency of effective collisions between reacting particles increases. [every 2 points – 1m] 2 A2aii No. Increasing the pressure has no effect on liquids as the particles in liquids are closely packed. 1 A2bi No. of moles of Si2OCl6 = 1 / 285 = 0.0035088 mol No. of moles of Cl- ions = 0.0035088 x 6 = 0.021053 = 0.0211 mol (to 3 s.f.) 1 A2bii No. of moles of Si3O2Cl8 = 1 / 400 = 0.00250 mol No. of moles of Cl- ions = 0.00250 x 8 = 0.0200 mol 1 A2ci Ag+ + Cl- → AgCl 1 A2cii No. of moles of AgCl = 3.03 / 143.5 = 0.0211 mol (to 3 s.f.) Since 1 mole of AgCl forms 1 mole of C l- ions, the oxochloride is Si2OCl6 since the number of moles of Cl- ions are the same at 0.0211 mol. 1 1 TOTAL 8

2 Qn Suggested answers Mark A3ai Nitrogen in the air reacts with oxygen in the air at high temperatures to form nitrogen dioxide gas. 1 A3aii Nitrogen dioxide reacts with oxygen and moisture in the air to form nitric acid. The nitric acid dissolves in rainwater to form acid rain, which falls to the soil. The nitrate ions are formed from the dissociation of nitric acid, causing the soil to have a high nitrate content. 1 1 A3aiii CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2 1 A3bi Carbon dioxide is a greenhouse gas which traps excess heat, causing global warming. Global warming causes ice bergs at the poles to melt, sea levels to rise and leading to floods in lowlands / causes droughts and crop yield to decrease (or any other possible answer). 1 1 A3bii More people should take public transport instead of driving personal cars to decrease carbon dioxide emissions. This is because each car generates much more carbon dioxide than the bus, i.e. 5750 kg of carbon dioxide per year as compared to 3200 kg of carbon dioxide per year. 1 1 A3ci 450 °C, 250 atm and finely divided iron catalyst 1 A3cii The reaction between nitrogen and hydrogen is reversible. 1 TOTAL 10 A4a The oxidation state of sulfur increases from -2 in ZnS to +4 in SO2. ZnS has been oxidised / This is oxidation. The oxidation state of oxygen decreases from 0 in O2 to -2 in ZnO. O2 has been reduced / This is reduction. Since oxidation and reduction occurs simultaneously, this is a redox reaction. 1 1 A4b ZnO(s) + CO(g) → Zn(s) + CO2(g) [1m – chemical equation, 1m – state symbols] 2 A4ci Reddish brown solid will be formed on the copper-coated rod No visible reaction on the surface of the zinc-coated rod. 1 1 A4cii Copper is less reactive than iron, iron will corrode faster and iron will rust. Zinc is more reactive than iron, so zinc will corrode in place of iron and iron will not rust. 1 1 TOTAL 8 A5a Transition metals form coloured compounds. / Transition metals and their compounds are good catalysts. 1 A5bi scandium and zinc 1 A5bii Both scandium and zinc have only one oxidation state of +3 and +2 respectively instead of having variable oxidation states. Scandium has a much lower density of 2.99 g/dm3 as compared to the rest of the transition metals with higher densities between 4.50 g/dm3 to 8.92 g/dm3. Zinc has a much lower melting point of 420 °C as compared to the rest of the transition metals with higher melting points between 1083 °C to 1890 °C. 1 1 1 TOTAL 5 A6a Experiment 1: copper metal Experiment 2: hydrogen gas, zinc electrode 1 2 A6b • Effervescence / bubbling of a colourless gas at the anode. • Reddish brown / pink solid formed at the cathode. • Size of cathode increases • Blue solution turns colourless. [any two of the above] 2 A6c 4OH-(aq) → 2H2O(l) + O2(g) + 4e [1m – ionic equation, 1m – state symbols] 2

3 Qn Suggested answers Mark A6d The gases can be dried by passing through drying agents such as concentrated sulfuric acid / calcium oxide / calcium chloride. The volumes of the gases can be determined by collecting them in a calibrated gas syringe. 1 1 TOTAL 9 A7ai A unsaturated hydrocarbon contains at least one carbon-carbon (C=C) double bond. 1 A7aii Bubble propene gas into aqueous bromine. Aqueous bromine turns from reddish brown to colourless / decolourises rapidly. 1 A7bi Addition of steam to propene 300 °C, 60 atm, phosphoric(V) acid catalyst 1 1 A7bii propyl ethanoate 1 1 TOTAL 6 Paper 2 Section B Qn Suggested answers Mark B8a Butan-2-ol. There are two alkyl groups attached to the carbon atom with hydroxyl group. 1 B8b 1 B8ci As the number of alkyl groups attached to the carbon atom with hydroxyl group increases, the boiling point decreases. 1 B8cii The intermolecular forces of attraction between the molecules become weaker, so less energy required to overcome the weaker intermolecular forces of attraction. 1 B8d Similarity: Hydrogen is produced in both reactions. Difference: An alkoxide is produced in the reaction between an alcohol and sodium but a salt is produced in the reaction between a carboxylic acid and sodium. 1 1 B8e As the number of alkyl groups attached to the carbon atom with hydroxyl group increases, the acidity decreases. 1 B8f Ethanoic acid. It has the smallest pKa value of 4.77, so it is the strongest acid. 1 1 B8g 2C4H9OH + 2Na → 2C4H9O-Na+ + H2 1 B8hi acidified potassium manganate(VII) / acidified potassium dichromate(VI) 1 B8hii Acidified potassium manganate(VII) turns from purple to colourless / Acidified potassium dichromate(VI) turns from orange to green. 1 TOTAL 12 B9a Base. Methylamine accepts a proton / gains a hydrogen ion. 1 B9bi Any pH above 7 and lower than 13 1 B9bii Methylamine is a weak alkali. Hence methylamine dissolves and dissociates partially in water to form a low concentration of hydroxide ions, resulting in a lower pH than sodium hydroxide. 1 1 H – C – C – O – H H O C – C – C – H H H H H H H H – C – C – OH CH3 H H C2H5

4 Qn Suggested answers Mark B9ci Titration 1 B9cii • Heat the solution to saturation. • Leave to cool and for crystals to form. • Filter the mixture to obtain the crystals. • Wash the crystals with a little cold distilled water and then dry between sheets of filter paper. [every 2 points – 1m] 2 B9ciii (CH3NH3)2SO4 1 TOTAL 8 Either B10a The particles are far apart and disorderly / irregularly arranged , and moving randomly at high speeds. 1 1 B10b Fluorine / F2. F2 gains hydrogen atoms to form HF. Since F2 is reduced, it is the oxidising agent. / The oxidation state of fluorine decreases from 0 in F2 to -1 in HF. Since F2 is reduced, it is the oxidising agent. 1 1 B10ci The energy absorbed to break bonds in Cl2 is more than that for F2, as the bond energy of Cl-Cl is higher than that of the F-F bond. 1 1 B10cii Energy released in Reaction 1 = 679 + 185 = 864 kJ Energy released in Reaction 2 = 595 + 543 = 1138 kJ 1 1 B10ciii hydrogen fluoride / HF 1 B10civ Dotted line to be drawn below the product (HF) 1 TOTAL 10 Or B10a aluminium chloride, phosphorus trichloride and phosphorus trifluoride They have low melting points. 1 1 B10bi [1m] – cation, [1m] – anion 2 B10bii Molten an