2023 VS 6092 P2 MS Chemistry Prelim

1 Victoria School 2022 Secondary 4 Chemistry Prelim Answer Scheme + Markers’ Report Paper 1 1 2 3 4 5 6 7 8 9 10 B D C C C C B A A D 11 12 13 14 15 16 17 18 19 20 D D C A A D B D A D 21 22 23 24 25 26 27 28 29 30 B B D A C A C D D B 31 32 33 34 35 36 37 38 39 40 A D D D A D B A C C Paper 2 Section A Qn Suggested answers Mark A1a CO2 1 A1b K 1 A1c Ni 1 A1d Ca(OH)2 1 A1e K/Ni and Cl2 1 A1f SO2 1 A2ai They dissolve/ionise/dissociate in water to produce H+/hydrogen ions. 1 A2aii Add a 2 cm magnesium strip into 25.0 cm3 of 1 mol/dm3 of both acids. Measure the time taken for the strips to totally react away, T will take a shorter time / Measure volume of gas produced in 5 min, T will produce a larger volume / Measure time taken to collect 10 cm3 of gas, T will take a shorter time REJECT: collect total volume of gas as both will be equal if Mg is limiting. Max 1m: qualitatively determine that T produces a faster speed of reaction (more vigorous effervescence for T) 1 1 A2aiii Brown solution will turn colourless / decolourise. 1 A2bi H+ + OH- → H2O 1 A2bii pH 9.2 Accept range from 9.0 to 9.4 1 499

2 Qn Suggested answers Mark A2biii 13/14 pH 0/1 1m - both correct pHs 1m - 40 cm3 Reject: pH 2 for strong acid 2 A2ci T reacts with calcium carbonate to form an insoluble layer of calcium sulfate that coats on the carbonate, preventing further reaction. 1 A2cii amount of calcium carbonate = 1 100 = 0.0100 mol amount of acid used = 1.2 x 0.050 = 0.0600 mol since 2H+  Ca, calcium carbonate is the limiting agent. since Ca  CO2, volume of CO2 = 0.0100 x 24 = 0.240 dm3 1 1 1 A2d 1m – ester linkage 1m – displayed formula (only if ester linkage present) 2 A3ai Glutamic acid, sodium hydroxide and monosodium glutamate are all soluble in water (to form colourless solution). 1 A3aii 112 dm3 of sodium hydroxide solution. (must state but no mark) Glutamic acid is dibasic/2 mol of sodium hydroxide reacts with 1 mol of glutamic acid (to form a disubstituted salt) (11.20 cm3 will react with 25.0 cm3 aqueous sodium hydroxide to form a monosubstituted salt.) 1 1 A3bi copper(II) oxide/copper(II) hydroxide 1 A3bii Excess copper(II) carbonate can be removed by filtration but not glutamic acid. 1 A4a Total energy absorbed for bond breaking = 610 + 4(414) + 2(463) = 3192 kJ Total energy released from bond forming = 346 + 5(414) + 358 + 463 = 3237 kJ Total energy change = 3192 – 3237 = -45.0 kJ 1 1 1 40 volume of aqueous NaOH/cm3 500

3 Qn Suggested answers Mark A4b 1m – Ea(1) and curve 1m – Ea(2) and curve 1m – energy level and product with state symbol 1m – ΔH 4 A5ai 1 1 A5aii Catalyst / enzyme 1 A5bi Butanal C3H7CHO 1 1 A5bii CnH2n+1CHO / Cn-1H2n-1CHO 1 A5biii Presence of oxygen in the organic compound. Length of carbon chain/number of carbon atoms. ACCEPT percentage by mass of carbon 1 1 A5ci Oxidation 1 A5cii Waxworms can remove non-biodegradable plastic from the environment, reducing pollution. 1 A6a Pigment B has an additional component/ring/circle compared to A. Pigment B is a mixture while A is pure. 1 1 A6bi Ethanol/solvent BOD organic solvent REJECT oil 1 C2H4(g) + H2O(g) C2H5OH(g) ΔH Ea(2) progress of reaction energy / kJ Ea(1) O O H H C H H C H H O O H H C O C O 501

4 Qn Suggested answers Mark A6bii Only a small sample is required. 1 A6ci Phosphoric acid. Experiment 2 uses a higher number of moles of phosphoric acid to produce a higher amount of moles of product. Only the increasing the amount of limiting reactant will affect the amount of product formed. 1 1 A6cii A higher concentration of particles increases the number of moles of H+ ions/particles per unit volume. This results in a higher frequency of effective collisions between reacting particles, resulting in shorter time taken for the reaction to complete. 1 1 Paper 2 Section B Qn Suggested answers Mark B7a largest mass of Mn from pyrolusite = 0.09 x 55 87 x 1 = 0.0569 tonne smallest mass of Mn from hausmannite = 0.08 x 165 229 x 1 = 0.0576 tonne 1 1 1 B7b Mn3O4 + CO → 3MnO + CO2 1 B7c Silica in ores chemically combines with manganese(II) oxide and prevents it from being reduced. Roasting manganese ores with limestone to remove silica will produce more slag which tends to dissolve manganese and lower the yield of the metal. Silica can be reduced to silicon and enter the molten metal. 1 1 1 B7d Calcium carbonate in limestone thermally decompose to form calcium oxide which then react with silica to form slag. 1 B7e Aqueous ammonia is a weak alkali and will not precipitate out manganese(II) hydroxide. 1 B7fi The concentration of manganese(II) ions will decrease resulting in the concentration of hydrogen ions being relatively higher and hence preferentially discharged/reduced. 1 B7fii Hydrogen ions are discharged/reduced at cathode to form hydrogen gas which is trapped in between the metal flakes. 1 B7g Use as a catalyst / added into iron to increase strength 1 B8a The zinc case oxidises to form zinc ions and electrons/ zinc loses electrons At carbon rod, the hydrogen ions react with manganese(IV) oxide and electrons to form manganese(III) oxide and water. 1 1 502

5 Qn Suggested answers Mark The flow of electrons from zinc to carbon rod creates the electricity. 1 B8b Cathode (must state but no mark). Reduction occurred at carbon rod. 1 B8c Zinc case is used up and contents in the cell leaks out. Ammonia gas produced from the dissociation of ammonium ions leaks out. 1 1 B8di Iron is less reactive than zinc 1 B8dii Iron case will rust (in the presence of oxygen and water). 1 EITHER B9a Similarities: Both contain carbon atoms / have covalent bonds / contain unbonded electrons / have three-dimensional structures (Note: carbon nanotubes do not have five-membered rings) Differences: Fullerene has simple molecular structure while carbon nanotube has macromolecular structure / Atoms are arranged like a sphere/soccer ball in fullerene but in a tube in carbon nanotube / Fullerene has both five-membered and six-membered rings but carbon nanotube has only six-membered rings Carbon atom has four outer electrons. However, only three electrons are involved in (covalent) bonding leaving one electron unbonded. This (unbonded) electrons can be delocalised across/down the nanotube /are mobile to conduct electricity. In fullerenes, electrons cannot jump from molecule to molecule, hence they cannot conduct electricity. 1 1 1 1 1 B9bi Fullerenes have simple molecular structure with weak intermolecular forces of attraction. Hence, little energy is required to overcome these forces. 1 1 B9bii More than 600 °C 1 B9c The oxidation state of C increases from 0 in C to +4 in CO2. This is oxidation. The oxidation state of Cl decreases from +1 in NaClO to -1 in NaCl. This is reduction. Since both oxidation and reduction occurred simultaneously, this is a redox reaction. 1 1 OR B9ai White precipitate is formed when aqueous ammonia is added dissolves in excess aqueous ammonia, showing that zinc ions are present. 1 B9aii Aluminium and lead(II) have the same results when tested with aqueous sodium hydroxide and aqueous ammonia. 1 503

6 Qn Suggested answers Mark Add aqueous sodium chloride/dilute