2024 ANDSS 4E Chem Prelim P2 ANS

EXAM ANSWERS Year: 2024 Exam: Prelim Level/Stream: 4E Subject: Chemistry 6092 P2 Qn Answer Marks Comments 1 a i ii iii iv v chlorine and oxygen iron(II) chloride argon carbon and chlorine iron and iron(II) chloride 5 1m each A1 b Addition reaction with hydrogen ACCEPT: Hydrogenation / Addition with Hydrogen / addition of H2 Esterification Neutralisation ACCEPT: neutralization REJECT: acid-alkali reaction 3 1m each Qn Answer Marks Comments 2a The mass of the mothballs decrease / becomes smaller over time. REJECTED: Mothballs become smaller / reduce in size / shrink. This indicates that the mothball is gradually disappearing, which can be explained by sublimation, where the mothball turns from a solid into a gas without going though the liquid state. ACCEPT: sublimation / solid sublimes to form gas / solid particles gain sufficient energy to overcome forces of attraction and become gas/gaseous particles. REJECT: decomposition 2 1 - trend 1 - reason

Page 2 of 12 2b Diffusion is the movement of gas molecules from a region of higher concentration near the mothballs to a region of lower concentration throughout the cupboard. As the molecules are spread out and mixed with the air molecules, the pungent smell can be observed. / Gas molecules (of mothball) collide/ mix with molecules in the air and spread across the room/space. REJECTED: travel / move 1 1 1m – diffusion definition on solute movement 1m- mixing with air molecules Qn Answer Marks Comments 3a i Fractional distillation REJECTED: distillation, fractionating distillation 1 3a ii In the furnace, the crude oil is heated into a vapour / heated and vapourise before it enters the fractionating column. The vapour contains a mixture of hydrocarbons with different boiling points. As the vapour rises up the column, the fractions with lower boiling points rise further up the column and condense at a lower temperature while the fractions with higher boiling points condense at a higher temperatures and collect at the lower part of the column. / As the vapour rises up the column, the fractions with higher boiling points condense and are collected at lower down the column while the fractions with lower boiling points rise further up the column to be collected. Jet fuel is collected as one of the fractions. 1 1 3b Used cooking oil contains impurities such as water. As the used cooking oil is placed in the separating funnel, the mixture separates into different layers based on density differences. When allowed to settle, the liquid that is denser can be tapped off from the bottom, the remaining layer will be the processed oil. (OWTTE) 1 1 Qn stated water as impurities; water is denser than cooking oil and settles at the bottom layer.

Page 3 of 12 3c Carbon dioxide is released /produced/given out during the burning/combustion of bioethanol, while bioethanol produced by the sugarcane plants which absorb/take in carbon dioxide during photosynthesis. This does not increase the net amount of CO2 in the surroundings. OR The amount of carbon dioxide released/produced/ given out from the burning/combustion of bioethanol is offset by the amount of carbon dioxide absorb/take in carbon dioxide during photosynthesis of the sugar cane plant. However, other processes such clearing of land for sugarcane crops and burning of fossil fuels during farming or transporting of sugarcane would releases even more carbon dioxide , making the use of bioethanol less sustainable. 1 1 OR 2 1 Key processes AND CO2 produced/abs orbed. Carbon neutral Other processes (related to making of bioethanol) that release CO2 3c Overall enthalpy change = Total energy absorbed during bond- breaking in reactants – Total energy released during bond-forming in products = [350 + 5(410) + 358 + 460 + 3(496)] – [2 x 2(799) + 3 x 2(460)] = 4706 – 5956 = –1250 kJ CAP 1 mark for “1250 kJ” 2 1m – for suitable working, correct number of bonds 1m – for correct answer 3d 3 1m – correct shape (exothermic) with reactants and products labelled with (state symbols)

Page 4 of 12 CAP 1 mark for Ea if wrong shape of profile diagram is drawn. ECF wrong ΔH calculated from (c) if ΔH is negative. REJECT: positive ΔH, double-head arrows, ‘floating’ arrows IGNORE: miss enthalpy value (i.e. - 1250 kJ), eqn which is not balance 1m – Ea labelled 1m – ΔH with values labelled (value ECF) Qn Answer Marks Comments 4 a Assume 100g of compound. IGNORE if missing. C H N O Mass 40.8 6.1 9.5 43.6 Ar 12 1 14 16 Mole 40.8 12 = 3.4 6.1 1 = 6.1 9.5 14 = 0.6786 43.6 16 = 2.725 Divide by smallest no 3.4/0.678 6 = 5.0106 6.1/0.678 6 = 8.9895 0.6786/0.678 6 =1 2.725/0.678 6 = 4.0158 Simplest ratio 5 9 1 4 C5H9NO4 ACCEPTED any order of elements. 1 1 Working for calculation of moles correct empirical formula 4b – / -1 / 1- / minus 1 / negative 1 1 C2H5OH(l) + 3O2(g) 2CO2 (g)+ 3H2O(l) Ea ΔH =–1250 kJ

Page 5 of 12 4c i Amide linkage 1 4 c ii 2 Monomer 1 IGNORE unshaded box (marker will annotate for student, teacher please remind) Monomer 2 4 c iii Sulfur REJECT: oxygen (question stated five different elements) 1 4 d C=C bond / carbon-carbon double bond REJECTED: double C=C bond, double carbon bond. 1 Qn Answer Marks Comments 5 a i Reducing agent, hydrogen has removed oxygen atoms from PbO to form Pb. ACCEPTED: Hydrogen takes/took away oxygen; PbO lost oxygen to hydrogen to form Pb; Hydrogen caused PbO to lose oxygen and formed Pb and itself is oxidized. REJECTED: Lead/Pb lost its oxygen from PbO to Pb 1 5 a ii No, aluminium is more reactive than hydrogen / aluminium is a reactive metal that is placed above hydrogen in the reactivity series / hydrogen is less reactive than aluminium and cannot displace aluminium form its ore. It can only be extracted by electrolysis of its molten oxide. REJECTED: Aluminium is more reactive than iron (note that this does not answer to the context). 1

Page 6 of 12 5 a iii After the heating has stopped, the extracted lead metal is still very hot. With the constant flow of hydrogen gas, it prevent the oxygen from the surrounding air form reacting with hot lead metal to form lead(II) oxide. Propose CAP 1 mark for To ensure complete reduction (of lead(II) oxide) / to ensure all lead(II) oxide fully reduced to lead. REJECTED: to prevent hydrogen from reacting with oxygen in the air; to avoid hydrogen from combusting with oxygen/air. 1 1 5 b i H3PO3 is oxidised because the oxidation state of phosphorus increases from +3 in H3PO3 to +5 in H3PO4 . H3PO3 is also reduced because the oxidation state of phosphorus decreases from +3 in H3PO3 −3 in PH3 . CAP 1 mark for correct oxidation states of phosphorus in all compounds but did not specify oxidized / reduced with respect to increase / decrease of oxidation states. 1 1 5b ii Comproposition reaction is a reaction in which the same element in two different reactants is both oxidised and reduced, forming one/a (single) product containing the element. Or OWTTE e.g. Comproposition reaction involves two different reactant containing the same element undergoing redox reaction / reduction and oxidation to form a/one product containing that element. 1 Qn Answer Marks Comments 6 a i By providing an alternative pathway of lower activation energy to speed up / increase / fasten the rate of reaction. 1 6 a ii Comparing experiments 2, 3 and 4. When copper catalyst is used, reaction