2022 HYSS Chem Prelims Ans

Marking scheme Chemistry Prelims 2022 Paper 1: 1 2 3 4 5 6 7 8 9 10 D A C A A B A B C A 11 12 13 14 15 16 17 18 19 20 A B D B C B C D C A 21 22 23 24 25 26 27 28 29 30 D D C A B C D D B B 31 32 33 34 35 36 37 38 39 40 C C B B D D A C D C

Marking scheme Chemistry Prelims 2022 Paper 1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Paper 2: A1 (a) Fe (b) Pt (c) Zn (d) C (e) C / N (f) K / Ca 1m each A2(a) The molecules can roll and glide over one another and can move throughout the liquid. They are arranged closed together in a disorderly arrangement. 1m 1m A2(b) 1m A2(c) Bromine is more reactive than iodine and gains electrons more readily. Hence it will displace iodine from KI to give KBr and I2. Br2 (l) + 2 I- (aq) → 2 Br- (aq) + I2 (s/aq) 1m 1m 1m A3(a) (i) no change/(solution remains pink) (ii) grey solid; forms in green solution 1m 1/2m, 1/2m A3(b) CuO + 2 E → E2O + Cu 1m formulae 1m balanced A3(c) Electrolysis 1m A4(a) Total amount of energy absorbed during bonding breaking = (413 × 8) + (390 × 6) + (3 × 498) = 7138 kJ Total amount of energy released during bonding forming = (413 × 2) + (891 × 2) + (464 × 12) = 8176 kJ Total amount of energy released is greater than total amount of energy absorbed OR Enthalpy change of energy is negative 1m 1m 1m A4(b) Platinum is a catalyst. It provides an alternative pathway with a lower activation energy. The proportion of particles with sufficient energy to overcome the activation energy increases. As a result, the reacting molecules/ particles (reject ions) collide with each other more often causing the frequency of effective collisions to increase and therefore the rate of reaction. 1m 1m 1m Br Br

A4(c) The reaction is reversible so hydrogen cyanide can decompose back to its reactants as it is formed. 1m A5(a) F: ammonium sulfate G: barium sulfate L: calcium carbonate 1m 1m 1m A5(b) Blue precipitate formed which dissolves in excess ammonia solution to give a dark blue solution 1m 1m A6(a) 1. Purple solution turned colourless. 2. Effervescence observed. 1m 1m A6(b) The oxidation state of C increases from +3 in C2O42- to +4 in CO2. C2O42- is oxidised to form CO2. C2O42- is the reducing agent. OR The oxidation state of Mn decreases from +7 in MnO4- to +2 in Mn2+. MnO4- is reduced to Mn2+. C2O42- is the reducing agent. 1m 1m A6(c) No. of moles of KMnO4 used = 250/1000 x 0.5 = 0.125 mol No. of moles of CO2 produced = 0.125 x (10/2) = 0.625 mol Volume of CO2 produced = 0.625 x 24 = 15.0 dm3 1m 1m 1m A7(a) (i) electrons (ii) ions 1m 1m A7(b) Apparatus A: A1 Apparatus B: B1 1/2m 1/2m A7(c) Copper(II) ions (Cu2+) gain electrons more easily than hydrogen ions (H+) to form copper solid. OR Copper(II) ions (Cu2+) are preferentially discharged to form copper solid. Cu2+ (aq) + 2e → Cu (s) 1m 1m A7(d) Electrode A2: 4 OH– (aq) → 2 H2O (l) + O2 (g) + 4e Electrode B2: Cu (s) → Cu2+ (aq) + 2e 1m 1m A8(a) Carboxyl, Amine, Amide, Ester (any two answers) 2m A8(b) 1m A8(ci) Methyl propanoate 1m A8(cii) concentrated sulfuric acid 1m A8(di) A macromolecule is a very large molecule that is made up of many small molecules. 1m

A8(dii) 1m 1m circle linkage B9(a) Coagulation increases size of particles, allow it to be trapped by the filters/cannot pass through the pores. Coagulation increases mass of particles , hence is heavier and settles to the bottom of the tank for removal. 1m 1m B9(b) Filtration removes undissolved, insoluble particles while BAC removes dissolved, soluble particles. Filtration separates by size, while BAC specifically attracts organic compounds and not other compounds. 1m 1m B9(c) Calcium oxide is a basic oxide that can neutralise acids in water to increase pH. 1m B9(d) Error 1: quote any 2 ions in Table B9.4 which will form precipitate; state they will form precipitate / insoluble salt OR Error 2: Precipitate of lead(II) hydroxide and aluminium hydroxide will react away / dissolve in excess (sodium) hydroxide solution 1m 1m OR 1m 1m B9(e) one spot for copper(II) at 4.1 cm one spot for iron(III) at 2.4 cm (-1m for no labelling) 1m 1m B9(f) No. The components would have different solubilities in a different solvent, hence the Rf values would be different, resulting in a different chromatogram. 1m 1/2m 1/2m B10(a) They have the same functional group carbon-carbon triple bond; C{C They have the same general formula CnH2n-2. The two members differ by a -CH2 unit. 1m each (Any two) B10(bi) Any value within the range of 36-43. 1m B10(bii) As the molecular sizes increase, there are more intermolecular forces of attraction to overcome. More energy is required to overcomes these forces of attraction during boiling. 1m 1m B10(ci) 1m B10(cii) Isomers. OR They have the same molecular formula but different structural formulae. 1m

B10(d) 2 C4H6 + 11 O2 → 8 CO2 + 6 H2O 2m B11E (a) CuCO3 → CuO + CO2 1m B11E (b) Down Group II, the reactivity of the metal increases. The more reactive the metal, the greater the thermal stability of the metal carbonate, hence the temperature for decomposition of the metal carbonate increases. 1m 1m 1m B11E (ci) When sulfuric acid is added to CaCO3, insoluble CaSO4, CO2 and water is produced. The CO2 produced gives the effervescence. The insoluble CaSO4 formed forms a protective layer over the CaCO3, preventing further reaction. 1m 1m B11E (cii) No of moles of sulfuric acid = 0.200 x 25/1000 = 0.00500 mol No of moles of MgCO3 = 1/ [24 + 12 + (16 x3)] = 0.0119 mol Mole ratio H2SO4 : MgCO3 is 1:1 0.005 mol of H2SO4 requires 0.005 mol of MgCO3 but there is 0.0119 mol, hence MgCO3 is in excess. Since there is excess MgCO3, there should be solid MgCO3 left behind and not disappear. 1m 1m 1m for explanation 1m for observation B11O (ai) Gas syringe 1m B11O (aii) Insert a lighted splint into the gas. The gas will extinguish the lighted splint with a pop sound. 1m 1m B11O (aiii) Observation 1 – sulfuric acid is strong acid and hence ionises completely to produce a higher concentration of H+ ions. Observation 2 – Citric acid is a tribasic acid while sulfuric acid is dibasic. The number of mole of H+ ions produced is greater with citric acid. 1m 1m B11O (bi) Ionic AND Covalent 1m B11O (bii) It is formed when the hydrogen ion in citric acid is replaced by the potassium ion. 1m B11O (biii) Heat the solution until it is saturated and allow it to cool for crystals to form. Filter to collect crystals as residue. Wash the crystals with a little cold deionised water and dry by pressing between sheets of filter paper. 1m 1m 1m