FHSS Chem Prelim Ans

1 FUHUA SECONDARY SCHOOL Sec 4E Chemistry 6092 Preliminary Examinations 2025 – Mark Scheme PAPER 1 1 2 3 4 5 6 7 8 9 10 C A B A D A A B A A 11 12 13 14 15 16 17 18 19 20 D C C B B A D D C B 21 22 23 24 25 26 27 28 29 30 B A D B A A C C B D 31 32 33 34 35 36 37 38 39 40 A B D B C C B C D C

1 FUHUA SECONDARY SCHOOL Sec 4E Chemistry 6092 Preliminary Examinations 2025 – Mark Scheme PAPER 2 Section A [70 marks] Q Answer Mark Remarks 1a B and E 1 b A, D and F 1 c A 1 d Two B3+ ions Three D2- ions 1 1 Total 5 marks 2a No. I disagree with the student. A catalyst provides an alternative pathway with lower activation energy for the reaction to proceed but the enthalpy change remains unchanged as the energy levels of reactant and product remain the same. Accept both not accepted be catalyst. 1 1 b Stage 1: S is oxidised as the oxidation state of sulfur increases from 0 in S to +4 in SO2 . O2 is reduced as the oxidation state of oxygen decreases from 0 in O2 to -2 in SO2 . Stage 2: SO2 is oxidised as the oxidation state of sulfur increases from +4 in SO2 to +6 in SO3 and O2 is reduced as its oxidation state decreases from 0 in O2 to -2 in SO3. ; ; ; ; 4; [3] c Stage 2 is a reversible reaction AND some of the sulfur trioxide formed decompose to form sulfur dioxide and oxygen. 1 di Shape of graph Label axes with units and include correct reactant and product Enthalpy change and activation energy, arrow must be correct ; ; ; 3; [2] ii Moles of SO3 = 500000 (32+3×16) = 625 Moles of SO2 = SO3 = 625 Total amount of heat released = 625/2 × 196 = 61 300 kJ 1 1 Total 10 marks H = +196 kJ/mol Activation energy

2 3ai 2H+(aq) + 2e →H2 (g) 1 ii Solution turned from colourless to reddish-brown. Chlorine is produced at electrode Q as chloride ions are selectively oxidised. Chlorine is more reactive than bromine [;] and displaces bromine from its halide solution.[;] Aqueous bromine formed is reddish-brown. 1 1 1 2; [1] iii No gas collected in the gas syringe / reading on syringe is 0 cm3 [;] as Cu2+ ions are selectively reduced at electrode P to form Cu. [;] No visible change / solution remained colourless [;] Oxygen gas, instead of chlorine gas is produced at electrode Q.[;] as OH⎯ ions are selected oxidised at electrode Q to form oxygen gas. [;] 3 5; [3] 3-4;[2] 1-2; [1] bi Electrons flow from Mg to Pb. 1 ii Electrode 3 [;] as Ag+ ions are selectively reduced by gaining electrons at electrode 3 to form Ag deposited on toy car. [;] Ag+(aq) + e → Ag(s) [;] 3 ; [2] iii pH decreases to below 7. As Ag+ ions and OH- ions are selectively reduced and oxidised at electrode 3 and 4 respectively [;], H+ and NO3 – ions are left in the electrolyte / concentration of H+ ions in electrolyte increases / electrolyte becomes increasingly acidic.[;] 1 1 2; [1] T