Compassvale Sec 4 Exp Prelim Chem P2 2025 MS

COMPASSVALE SECONDARY SCHOOL PRELIMINARY EXAMINATION 2025 SECONDARY FOUR EXPRESS CHEMISTRY 6092 Page 1 of 7 Mark Scheme Paper 2 Theory Section A 1 salt barium sulfate copper(II) nitrate reagent 1 aqueous barium nitrate dilute nitric acid reagent 2 aqueous sodium sulfate – 1m or any solution containing SO42– copper(II) oxide/hydroxide/carbonate – 1m description on how to obtain solid product from reaction mixture Filter the mixture to collect barium sulfate as residue. – 1m Filter to collect copper(II) nitrate as filtrate. – 1m Heat to evaporate the mixture until saturated. – 1m Cool for crystals to form. Filter the mixture to collect copper(II) nitrate crystals as residue. –1m accept any other suitable reagents [6] 2 (a) (i) Cotton wool changes colour from colourless to red-brown/brown. [1] (ii) Chlorine, being more reactive than bromine , can displace bromine from sodium bromide to form red-brown bromine and sodium chloride. Cl2 + 2Br– → Br2 + 2Cl– [1] [1] (b) sodium iodide [1] 3 (a) BO33– [1] (b) For the same concentration of acids used, the pH of hydrochloric acid is 1.30, which is lower than that of boric acid at 5.25. As a weak acid, boric acid only ionises partially in water to form H+, producing a lower concentration of H+, hence resulting in higher pH. [1] [1] (c) (i) Insert lighted splint to the gas. Lighted splint extinguishes with a ‘pop’ sound if gas is hydrogen. [1] [1] (ii) The final volume of gas produced will be the same. For both reactions, the zinc to gas mole ratio is the same . Since the zinc is the limiting reagent/same number of moles of zinc, the final volume of gas produced will be the same. [1] [1]

COMPASSVALE SECONDARY SCHOOL PRELIMINARY EXAMINATION 2025 SECONDARY FOUR EXPRESS CHEMISTRY 6092 Page 2 of 7 4 (a) locating agent is added to react with the amino acids to form coloured spots. [1] (b) S is the most soluble as its Rf value is the highest with both solvents. Since Rf is the ratio between distance travelled by spot to distance travelled by solvent, highest Rf means it travelled the furthest from the start line. [1] [1] (c) Solvent 2 as all the amino acids are soluble in solvent 2. Rf value for Q in solvent 1 is 0.00, which means it is insoluble in the solvent. [1] [1] 5 (a) Mass of 1 mol of octane = 12 x 8 + 18 x 1 = 114 g Vol of 1 mol of octane = mass/density = 114 / 703 = 0.162 dm3 No. of mol of 1 kg of ethanol = 1000/(12 x 2 + 6 x 1 + 16) = 21.739 mol ΔH when 1 kg of ethanol is completely burnt = –1367 x 21.739 = –29717 kJ [1] [1] [1] (b) Propane exists as a gas at room temperature and pressure, takes up more volume per mole/a lot of it is required to be stored to be effectively used as a fuel. Ethanol exists as a liquid at room temperature and can be stored more easily/more closely packed (24.3dm3 compared to 0.0583dm3) Propane produces more energy per kilogram compared to ethanol when completely burnt (–50 455 kJ/kg compared to –29 717 kJ/kg). Propane is produced from non-renewable crude oil while ethanol can be produced from renewable sugarcane/corn. [1] [1] [1] (c) [3] (d) During photosynthesis, plants take in carbon dioxide and water to make glucose and oxygen in the presence of sunlight. [1] energy progress of reaction C2H5OH + 3O2 2CO2 + 3H2O Ea ΔH = -1367 kJ/mol 1m – correct formulae and balanced eqn 1m – correct shape of graph 1m – label Ea and ΔH with correct sign and value with units

COMPASSVALE SECONDARY SCHOOL PRELIMINARY EXAMINATION 2025 SECONDARY FOUR EXPRESS CHEMISTRY 6092 Page 3 of 7 6 (a) Zinc sulfide has a giant molecular structure while disulfur dichloride has a simple molecular structure. Large amount of energy is needed to break the strong covalent bonds between the zinc and sulfur atoms. Little energy is required to overcome weak intermolecular forces of attraction between disulfur dichloride molecules. Hence zinc sulfide it has a high melting point while disulfur dichloride has a low boiling point. [1] [1] [1] [1] (b) poor electrical conductivity/insoluble in water [1] (c) bonding electrons – 1m valence electrons – 1m [2] 7 (a) experiment 1: copper experiment 2: platinum/graphite [1] [1] (b) Mass of both A1 and B1 increases: Cu is less reactive than H hence Cu2+ is preferentially discharged over H+, gaining electrons and is reduced to form copper solid. Cu2+ (aq) + 2e → Cu (s) Mass of A2 has decreased: Since electrode is reactive, copper electrode loses electrons and gets oxidised at the anode to form copper ions. Cu (s) → Cu2+ (aq) + 2e Effervescence observed at B2: Since electrode at B is inert, m ass of B 2 remained unchanged. OH– is preferentially discharged over SO42-, loses electrons and gets oxidised to form oxygen gas, hence effervescence observed. 4OH– (aq) → 2H2O (l) + O2 (g) + 4e [1] [1] [1] [1] [1] [1] (c) Copper(II) sulfate remains blue in experiment 1 because copper electrode/anode is oxidised to form Cu 2+ which replenishes the Cu 2+ ions from the electrolyte is used for reduction at A1. Overall concentration of Cu2+ ions remains unchanged. Copper(II) sulfate turns pale blue/colourless in experiment 2 because Cu2+ ions from the electrolyte is reduced to form Cu at B1, decreasing overall concentration of Cu2+ ions in the electrolyte. both ob – 1m; both reasoning – 1m; concentration – 1m or both ob – 1m, reasoning + conc for each – 1m each [1] [1] [1]

COMPASSVALE SECONDARY SCHOOL PRELIMINARY EXAMINATION 2025 SECONDARY FOUR EXPRESS CHEMISTRY 6092 Page 4 of 7 8 (a) (i) high carbon steel [1] (ii) Presence of different sized carbon atoms disrupt the orderly arrangement of iron atoms, making it difficult for the layers of atoms to slide over each other when a force is applied. 1m for any 2 points With more carbon atoms, there is greater disruption of the orderly arrangement of iron atoms in high carbon steel compared to low carbon steel and medium carbon steel hence making it harder than the other carbon steels. [2] [1] (b) Low carbon steel is used to make car bodies as it has the highest ductility hence it can be easily moulded into shapes without facturing. High carbon steel is used to make wire ropes for suspension bridges as it has the highest tensile strength hence it can support the weight of the suspension bridge without breaking. [1] [1] (c) (i) Fe2O3 loses oxygen to form Fe hence it is reduced. CO gains oxygen to form CO2 hence it is oxidised. since both oxidation and reduction occurs at the same time in the same reaction, it is a redox reaction. [1] [1] (ii) When carbon reacts with oxygen, reaction is exothermic/gives out heat. [1] (iii) 4Fe + 3O2 → 2Fe2O3 [1] (d) Use of scrap steel instead of solely pure iron reduces the need for extraction of new iron, which produces carbon dioxide as by-product, which is a greenhouse gas that causes global warming. Addition of scrap steel absorbs excess heat and decreases the temperature , hence reduce possibility of production of oxides of nitrogen , an air pollutant that is produced due to N2 and O2 reacting under high temperature. [1] [1] 9 (a) A: neither Pentane has the formula C5H12 while the product has the formula C5H10. B: isomerisation Hexane has the molecular formula of C6H14. The product formed also has the formula C6H14. Since both have the same molecular formula but different structural formula/arrangement of atoms in each molecule, isomerisation has occurred. C: cracking Heptane has the molecular formula C7H16 while the product has