03 2024 CCKSS 4E Prelim CHEM MS

2023 / S4 / SCI CHEM / PRELIM / MS Secondary 4E Chemistry Preliminary Examination 2024 Mark Scheme Paper 1 (40 marks) 1 B 11 D 21 A 31 D 2 D 12 D 22 C 32 A 3 B 13 D 23 D 33 C 4 A 14 B 24 C 34 A 5 B 15 A 25 C 35 D 6 B 16 C 26 D 36 D 7 B 17 A 27 A 37 D 8 C 18 B 28 B 38 A 9 B 19 C 29 C 39 B 10 C 20 A 30 C 40 C A – 9, B – 10 , C – 11 , D – 10 Paper 2 Section A (70 marks) 1(a) Potassium carbonate 1 1(b) Potassium iodide 1 1(c) Ammonium chloride 1 1(d) Potassium iodide 1 1(e) Lead(II) fluoride, sodium chloride, ammonium chloride 1 1(f) Barium sulfate, lead(II) fluoride 1 Total: 6 2(a) Element K Cu Cl Mass 31.5 25.6 100-31.5-25.6 = 42.9; Ar 39 64 35.5 Mol 31.5/39 = 0.80769 25.6/64 = 0.4 42.9/35.5 = 1.2084; ;implest ratio 0.80769/0.4 = 2.0192 = 2 0.4/0.4 = 1 1.2084/0.4 = 3.02 = 3 Empirical formula is K2CuCl3 1 1 1 2(bi) Protons: 29; Neutrons: 37; Electrons: 27; 1 2(bii) Add aqueous sodium hydroxide / add aqueous ammonia 1 Blue ppt insoluble in excess / blue ppt soluble in excess to form a dark blue solution 1

2023 / S4 / SCI CHEM / PRELIM / MS Total: 6 3(a) Calcium oxide is an ionic compound with giant ionic lattice structure. 1 At high temperatures, sufficient/large amount of energy is provided to 1 to overcome strong electrostatic force of attraction between oppositely charged ions 1 3(b) CaO(s) + 3C(S) à CaC2(s) + CO(g) 1m balanced chemical equation 1m state symbols 2 3(c) Number of moles of CO present = 2400 / 24000 = 0.1 mol 1 Mole ratio, CaO/CO = 1/1 Number of moles of CO = 0.1 mol 1 Mass of CaO = 0.1 x 56 = 5.6g Percentage purity = 5.6 / 8 X 100% = 70% 1 Total: 8 4(a) Reaction is endothermic/takes in heat from the surroundings/water. 1 Causing water to freeze/cool to below 0 °C OR temperature of water to decrease below melting/freezing point (of 0 °C). 1 4(b) Single-head arrow ΔH – 1m Single-head arrow Ea – 1m Correct energy levels + reactants and products (accept names or formula, no need state symbols) – 1m 3 Total: 5 5(a) No. of moles of MgO: 1 / 40 = 0.025mol No of moles of HCl = [25/1000] x 0.1 = 0.0025 mol MgO: HCl 1: 2 Therefore, HCl is the LR 1 No of moles of MgCl2= 0.0025/2 = 0.00125 mol 1 Mass of MgCl2 = 0.00125 x [24 + 2(35.5)]= 0.119g 1

2023 / S4 / SCI CHEM / PRELIM / MS 5(b) A: Before adding, pH= 1 thus presence of strong acid As MgO is added, pH increases gradually as H+ ions is removed by OH-/ HCl reacting with MgO B: At pH=4, the reaction is complete/neutralisation C: pH remains at 7 as excess MgO is insoluble in water/ no increase in concentration of OH- ions 6pts – 4m 4-5pts – 3m 2-3 pts – 2m 1pt – 1m 4 Total:7 6(a)(i) The high temperature of the engine causes the nitrogen and oxygen in air to react and form nitrogen oxides. 1 Incomplete combustion of fuels 1 6(a)(ii) Eye/ lung irritation, causes breathing difficulties 1 Prevents transport of oxygen to the body, leading to breathing difficulties/death 1 6(b) Carbon dioxide, a greenhouse gas, is being released excessively when catalytic converters are used, causing global warming. 1 melting of polar ice causes rising sea levels causing floods worldwide OR extreme weather events such as increase/decrease rainfall in some areas, causing disruption of food crops leading to possible famine. 1 Total: 6 7(a)(i) 3 1 7(a)(ii) Disagree, acid solutions speed up rusting OR metal will react with citric acid. 1 7(b) No, Citric acid is a weak acid that dissociates partially to form lower concentration of hydrogen ions. 1 While sulfuric acid is a strong dibasic acid that dissociates fully to form higher concentration of hydrogen ions, thus will remove rust faster. 1 7(c) No, because calcium carbonate is also an insoluble solid. 1 Calcium citrate formed cannot be separated from the unreacted calcium carbonate. 1 Total: 6 8(a)(i) The relative molecular mass/ molar mass of each metal carbonate is different. 1 Hence, the same mass of carbonate will produce different number of moles of carbon dioxide, result in different mass of carbon dioxide which escapes and thus varying decrease in mass. 1

2023 / S4 / SCI CHEM / PRELIM / MS 8(b) 2.00g 1 Potassium is a highly reactive metal, thus potassium carbonate is very thermally stable and hence does not decompose upon heating. 1 1 8(c) copper 1 Total: 6 9(a) 𝐻!, 𝑂𝐻" , 𝐶𝐻#𝐶𝑂𝑂" 1-2 ions – 1m 3 ions- 2m 2 9(b)(i) Fe (s) → Fe2+ (aq) + 2𝑒" 1 9(b)(ii) Change the iron to a more reactive metal (zinc, magnesium) / Change the copper to a less reactive metal (silver, gold) 1 9(c)(i) 1 9(c)(ii) Positive electrode: becomes smaller/loses mass Negative electrode: increases in size Electrolyte: remains colourless/ no change in concentration 3pts – 2m 1-2 pts – 1m 10(c) 1. Silver anode oxidises to form silver ions which goes into the solution. The anode loses mass. 2. Silver ions are selectively discharged over hydrogen ions because they receive ions more readily. The cathode gains mass. 3. The concentration of the electrolyte remains the same as the silver ions that are selectively discharged at the cathode is replenished by the silver ions produced at the anode. 1 1 1 [Total: 10] 10(a)(i) 2 battery silver metal metal object silver nitrate solution

2023 / S4 / SCI CHEM / PRELIM / MS 10(a)(ii) Diazomethane is a covalent compound, all the electrons are involved in sharing between the atoms/ exist as neutral molecules and there are no free mobile charged particles. 1 1 10(b)(i) All points plotted correctly. [1] Smooth curve starting from origin [1] 10(b)(ii) When partial pressure is 5.4 Pa, half-life is 1800s, When partial pressure is 2.7 Pa, half-life is 1700 – 1900 s half-life is constant (1800s) , this is a first-order reaction 1 1 10(b)(iii) When initial pressure is increased, the number of particles per unit volume increases. The collisions between the particles of gaseous diazomethane become more frequent. As a result, the frequency of effective collisions increases and the rate of reaction increases. 1 1 Total: 10

2023 / S4 / SCI CHEM / PRELIM / MS Section B (2C1, 10 marks) 11(a)(i) 1 1 11(a)(ii) Mr of 1 repeat unit = 14(12) + 2(16) + 2(14) + 10(1) = 238 1 Min no. of repeating units = 12000/238 = 50.42 = 51 Max no. of repeating units = 20000/238 = 84.03 = 84 1 11(b) Advantage : more durable / stronger / waterproof 1 Disadvantage : non-biodegradable, hence accumulate in landfills 1 11(c)(i) propene 1 11(c)(ii) • Polymer M is formed from unsaturated monomers with carbon-to-carbon double bonds but Kevlar is formed from monomers with two reactive functional groups in each molecule i.e diamines and dicarboxylic acids. • Polymer M is formed by without the removal of a small molecule hence only one product, the polymer is formed but Kevlar is formed with the removal of small molecules such as H2O. • Formation of Polymer M occurs at high temperature and pressure and in the presence of a catalyst while formation of Kevlar can take place at room conditions. 1 point – 1m 3 Total: 10 12(a)(i) Since ethanol made from glucose comes from crops like sugarcane, And as sugarcane can be grown and replaced in a short period of time. Therefore ethanol made from glucose is considered a renewable resource. 1 12(a)(ii) Bioethanol is made from plants (corn, sugarcane, potato) that take in carbon dioxide for photosynthesis. This offsets the carbon dioxide given out during combustion of bioethanol. Therefore the net carbon dioxide output is lower than burning crude oil 1 1 12(b)(i) oxidation 1

2023 / S4 / SCI CHEM / PRELIM / MS 12(b)(ii) 1 12(b)(iii) Addition of bromine takes place. reddish-brown solution turns colourless / decolourises. 1 1 12(b)(iv) 2 12(b)(v) 1 Total: 10