Temasek 2024_Prelim_O_Chem_P2_MS

2024 4E Prelim Pure Chemistry 6092 Section A Mark Scheme S/N Mark Scheme Common Mistakes A1 (a) FeSO4 (b) Al2O3 (c) Ag2CO3 (d) (NH4)2SO4 (e) SiO2 or Al2O3 A2(a) The metal electrodes have a giant metallic lattice structure. [1] It consists of ‘sea of delocalised electrons’ that are mobile to act as charged carriers. [1] A2(b) Q, S, P, R A2(c) X: 1.6 V Y: 0.8 V Accept values ± 0.2 V of answer. A2(d) Oxidation A2(e) Yes [stop marking if wrong] Metal P is more reactive than Metal R, it is able to reduce the oxide of metal R / displace R from the oxide of R. [1]

A3(a) D A3(b) C [1] C is not soluble in solvent 2. [1] A3(c) 1 m for each correct plot of A and D. A3(d) Spray locating agent on the chromatogram. A4(a) It is used as a feedstock for chemicals. B D A C

A4(b) 0.5 m for pentane and propane, 1 m for correct drawing of ethene A4(c) Product 2. [0.5] [stop marking if wrong.] It has the same molecular formula [0.5] of C6H14 [0.5] as hexane, but different structural formula. [0.5] A5(a) Graph X [1] Ethanoic acid ionised partially in water to give a low concentration of hydrogen ions, hence the starting pH should be higher than HCl. [1] Or This is because ethanoic acid is a weak acid hence the starting pH should be higher than that of hydrochloric acid. [1] A5(b) Both graphs show that both acids require the same volume (50 cm3) of NaOH to neutralise.

A5(c) Phenolphthalein is a more suitable indicator. [1] The end-point falls within the colour change interval of phenolphthalein. [1] A6(a) Amide linkage A6(b) and 1m each A6(c) Mr of 1 repeat unit = 14x 2 +2 + 12 x 2 + 16 x 2 + 10 x (12 +2) = 226 [1] No. of repeat units in polymer C = 20800 / 226 = 92 [1]

A7(a) A7(b) 2N2H4(g) → 2NH3(g) + N2(g) + H2(g) Total energy absorbed for bond breaking = 2 x 160 + 8 x 390 = 3440 kJ [1] Total energy released for bond forming = 2 x 3 x 390 + 944 + 436 = 3720 kJ [1] Enthalpy change = 3440 – 3720 [1] = -280 kJ [shown]

A7(c) 1 m for correct named products lower than reactants 1 m for correct direction of arrow of enthalpy change -0.5 m for adding in activation profile A7(d) Dinitrogen monoxide can cause global warming which leads to: Any of the following: • Melting of ice cap, hence flooding of low-lying lands • Droughts A8(a) (i) Hydrogen gas that is produced escaped to the environment.

(ii) No. of moles of magneisum = 1.2/ 24 = 0.05 [0.5] No. of moles of nitric acid = 0.25 x 0.5 = 0.125 [0.5] Moles ratio of Mg : HNO3 1 : 2 0.05 : 0.100 [1] 0.05 moles of Mg requires 0.100 moles of HNO3, but