SJI 2023 Y4 Prelim Chemistry Paper 1 & 2 Answers

1 St. Joseph’s Institution 2023 Year 4 OP Preliminary Exam Answer Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 D D A D B A D B A C 11 12 13 14 15 16 17 18 19 20 B D C C B C D B B D 21 22 23 24 25 26 27 28 29 30 C C D A D C C C D C 31 32 33 34 35 36 37 38 39 40 C B D D B D C A A C Paper 2 Section A Qn Suggested answers Mark A1a sodium iodide 1 A1b silver nitrate and sodium iodide 1 A1c aluminium sulfate and zinc nitrate 1 A1d ammonium chloride, silver nitrate and zinc nitrate 1 TOTAL 4 A2a There is more oxygen (from air) mixed with petrol, hence there is less incomplete combustion of petrol, thus producing lower amounts of carbon monoxide. 1 A2bi Lightning activity 1 A2bii Lean burn engine has a lower operating temperature. Less nitrogen gas and oxygen gas will react to form nitrogen oxides, as this reaction only occurs at high temperatures. 1 1 TOTAL 4 A3a There is no trend in the melting point, as it decreases from 2306 °C in boron to 30 °C in gallium then increases to 304 °C in thallium. 1 A3b Aluminium has a higher electrical conductivity than sodium as it has more mobile electrons than sodium. 1 A3ci [1m] – bonding electrons [1m] – non-bonding electrons 2 A3cii Boron has a giant molecular/covalent structure, while boron trichloride has a simple molecular/covalent structure. A lot of energy is needed to overcome the extensive network of strong covalent bonds between the atoms in boron. Little energy is needed to overcome the weak intermolecular forces of attraction between the molecules in boron trichloride. 1 1 1 TOTAL 7 Cl Cl Cl

2 Qn Suggested answers Mark A4a 2H+ + 2e- → H2 Reduction occurs at the silver electrode. The hydrogen ions gain electrons to form hydrogen gas. 1 1 A4b Solution / electrolyte turns from colourless to blue. 1 A4ci The difference in reactivity between lead and silver is greater than the difference in reactivity between copper and silver, thus resulting in a higher voltmeter reading. 1 A4cii The lead anode oxidises / dissolves to form lead(II) ions, which react with the sulfate ions in the electrolyte to form insoluble lead(II) sulfate. The concentration of mobile ions in the solution decreases, thus the electrical conductivity decreases over time. 1 1 TOTAL 6 A5ai sodium, magnesium, X, lead, silver 1 A5aii aluminium / zinc / iron / tin 1 A5aiii 2Al + 3CuO → Al2O3 + 3Cu / Zn + CuO → ZnO + Cu / Fe + CuO → FeO + Cu / Sn + CuO → SnO + Cu 1 A5aiv Aluminium / zinc / iron / tin is more reactive than copper and displaces copper from copper(II) oxide. 1 A5bi Difference (any one of the below): • Iron has a high density but potassium has a low density. • Iron is hard but potassium is soft. • Iron has high melting and boiling points but potassium has low melting and boiling points. 1 A5bii Galvanising with zinc forms a protective barrier and prevents oxygen a