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 carbon (and hydrogen) atoms (of the four alcohols shown). Burning butan-1-ol would form more moles of CO2 and H2O and give out more energy than burning the same amount of methanol / ethanol / propan-1-ol OR Number of carbon (and hydrogen) atoms increase, going down the homologous series; Increase in energy given out from bond forming in products (CO2 and H2O) is even larger than increase in energy absorbed / taken in for bond breaking in reactants (alcohol and O2) 1 1 (1) (1) 4di C4H10O 1 4dii 1 4ei The total energy absorbed to break bonds is different, as butan -1-ol and diethylether contain different numbers / types of bonds. 1 4eii -2673 kJ/mol 1 TOTAL 9

3 Qn Suggested answers Mark 5a iron, chromium, manganese 1 5b [Stating both properties] (i) exhibiting variable oxidation states in their compounds and (ii) forming coloured compounds [Explanation for variable oxidation states] iron has oxidation states of +2 (pale green solution) and +3 (Fe2O3), OR manganese had oxidation states of +2 (MnSO4) and +3 (Mn2O3) [Explanation for coloured compounds] reference to green Cr2O3(s), red manganese (II,III) oxide, dark brown Fe2O3(s) 1 1 1 5c Mn(s) + 2H+(aq) → Mn2+(aq) + H2(g) [1] – balanced ionic equation with correct species [1] – state symbols 2 5d [Experiment 1] grey solid / coating / deposit formed, and pale green solution turns pale pink [Experiment 2] no visible reaction 1 1 5e Mercury is lower than iron in the reactivity series [must state, but no mark] When heated, iron( II) carbonate (only) decomposes to its oxide [iron(II) oxide], while mercury(II) carbonate decomposes to its metal element [mercury]. Thus, iron(II) carbonate is more thermally stable than mercury(II) carbonate, and iron is more reactive than mercury. 1 1 TOTAL 10 6a [ethene gas] bubble (excess) ethene gas into aqueous bromine [ethanoic acid] changes from green to orange (OR yellow / red) 1 1 6bi Each carbon atom is bonded to two hydrogen atoms Mass of one mole of poly(ethene) molecule = 6000 x Mr(CH2) = 6000 x 14 = 84,000 g Mass of one single poly(ethene) molecule = 84,000 / (6.02 x 1023) = 1.40 x 10-19 g (to 3 s.f.) 1 1 6bii [Economic] Recycling plastics is energy-intensive / manpower-intensive as the plastic waste must first be sorted and cleaned. The process of recycling plastics may thus be more costly than disposing of plastics in landfills (and/or the market value of the recycled plastics) [Environmental] If wastewater generated from the recycling of plastics is not treated properly before being discharged into water bodies, this can lead to water pollution. 1 1

4 Qn Suggested answers Mark 6c [1] – correct bond pairs [1] – correct lone pairs on oxygen 2 6di 1 6dii methanol 1 6e C H O mass (in 100 g sample) / g 62.1 10.3 100 – 62.1 – 10.3 = 27.6 no. of moles / mol 62.1 / 12 = 5.175 10.3 / 1 = 10.3 27.6 / 16 = 1.725 mole ratio 3 6 1 [1] % by mass of oxygen = 27.6% [1] no. of mol of each element [1] empirical formula: C3H6O; and molecular formula: C6H12O2 [carboxylic acids contain two oxygen atoms] 3 TOTAL 13 7a Concentration of sodium ions = 100 ppm = 100 x 10-3 g per 1000 cm3 = 0.100 g/dm3 Mass of sodium ions in 250 cm3 sample = (250/1000) x 0.100 = 0.0250 g Amount of sodium ions (in mol) in 250 cm3 sample = (0.0250 / 23) = 0.00109 mol 1 1 7bi From Fig. 7.1, I agree that retention time is affected by ionic charge. All ions with a higher 2+ charge have a longer retention time than ions of a lower 1+ charge. However, I disagree that retention time is (fully) affected by ions’ relative masses. While ions of lower relative mass generally have shorter retention times, this is not true for Mg2+ (Ar = 24), which has a lower relative mass but a longer retention time than K+ (Ar = 39). 1 1 7bii Retention time within range of 7 to 9 min. Nitrate ion has a charge of 1- and a relative (molecular) mass of 62. Thus, the retention time is likely between that of Cl- ion (Ar = 35.5) and Br- ion (Ar = 80). 1 1 7c CH3COOH 1

5 Qn Suggested answers Mark 7di At t = 10 min, the reaction mixture contains ethanoic anhydride (R2), aspirin (P), ethanoic acid / known by-product (BP), and one unknown substance. 1 7dii The student concluded that the reaction was complete as salicylic acid, the limiting reactant, was used up. (Thus there was no spot corresponding to R1 / salicylic acid at t = 10 min.) 1 7e Ethanoic anhydride (R2) and aspirin (P) have similar solubilities in ethanol, as evidenced by their similar Rf values in Fig. 7.4. Thus, some ethanoic anhydride might have left the column (and got collected) at the same time as aspirin. 1 7f Differences between ion and column chromatography [Any two] • Io