SJI 2024 Y4 Prelim Chemistry Paper 1 & 2 Answers

1 St. Joseph’s Institution 2024 Year 4 OP Preliminary Exam Answer Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 D B A C C A A C B B 11 12 13 14 15 16 17 18 19 20 D A B B B B C D C C 21 22 23 24 25 26 27 28 29 30 B B A C B B D C B C 31 32 33 34 35 36 37 38 39 40 C A B D A A C B C C Paper 2 Section A Qn Suggested answers Mark 1a hydrogen 1 1b argon 1 1c chlorine 1 1d sodium and chlorine 1 1ei lithium and sodium 1 1eii 2Li + 2H2O → 2LiOH + H2 OR 2Na + 2H2O → 2NaOH + H2 1 TOTAL 6 2a Chlorine is oxidised as the oxidation number of chlorine increases from 0 in Cl2 to +5 in ClO3-. Chlorine is reduced as the oxidation number of chlorine decreases from 0 in Cl2 to -1 in Cl-. As the same element, chlorine, is both oxidised and reduced, this is a disproportionation reaction. [must state but no mark] 1 1 2b No. of moles of 1.0 g of Si2OCl6 = 1.0 / [28(2) + 16 + 35.5(6)] = 0.0035088 mol No. of moles of chloride ions in 1.0 g of Si2OCl6 = 0.0035087 x 6 = 0.0210526 mol ≈ 0.0211 mol (to 3 s.f.) No. of moles of 1.0 g of Si3O2Cl8 = 1.0 / [28(3) + 16(2) + 35.5(8)] = 0.00250 mol No. of moles of chloride ions in 1.0 g of Si3O2Cl8 = 0.00250 x 8 = 0.0200 mol (to 3 s.f.) Reasoning: The 1.0 g sample of Si2OCl6 contains a larger amount / no. of moles of chloride ions → larger amount of AgCl(s) ppt formed → larger mass of AgCl(s) ppt formed 2 1 TOTAL 5

2 Qn Suggested answers Mark 3a Diesel engine [must state but no mark] The carbon dioxide and carbon monoxide emissions are the highest at 160 and 408 respectively. 1 3bi incomplete combustion of carbon-containing fuels 1 3bii 2CO + 2NO → CO2 + N2 2CO + O2 → 2CO2 Carbon monoxide is oxidised by nitrogen monoxide to form carbon dioxide and nitrogen which are harmless. Carbon monoxide is also oxidised by oxygen to form carbon dioxide which is harmless. 1 1 1 3c Carbon dioxide is a greenhouse gas which traps excess heat in the atmosphere, causing global warming. This causes negative effects on the environment such as ice bergs melting, seas levels rising and flooding in lowland areas. 1 1 TOTAL 7 4ai High temperature and high pressure and acid catalyst / phosphoric acid 1 4aii Ethene is obtained from crude oil / petroleum / fossil fuels, which is a finite resource that will run out. In contrast, glucose is obtained from plants , of which there is a continuous supply / will not run out 1 4b (Mr of propan-1-ol = 12(3) + 1(8) + 16(1) = 60) Enthalpy change of combustion of propan-1-ol = -(134/4) x 60 = -2010 kJ/mol OR No of mol of 4.0g of propan-1-ol = 4.0 / 60 = 0.0667 mol Enthalpy change of combustion of propan-1-ol = -134 / 0.0667 = -2010 kJ/mol 1 4c In these combustion reactions, more energy is given out from bond forming in the products (CO2 and H2O), than absorbed / taken in for bond breaking in the reactants (alcohol and O2); Butan-1-ol contains the most