Copy of CHS 2024 S4 OP Prelim Paper 1 and 2 Answers

1 2024 Sec 4 Preliminary Examination Mark Scheme Paper 1 Paper 2 Section A Qn Answers Mark Guidance 1a D 1 R: multiple answers 1b B 1 1c J 1 1d I 1 1e H 1 2 Polymer A Structure of repeat unit ; Type of reaction : addition ; Polymer B Structure of repeat unit ; Type of reaction : condensation ; 4 R: structure of polymer 1 2 3 4 5 6 7 8 9 10 B A D A B A D B C C 11 12 13 14 15 16 17 18 19 20 C A C C D D D D C A 21 22 23 24 25 26 27 28 29 30 D C B B B C C D A A 31 32 33 34 35 36 37 38 39 40 B A C D B B B D B A

2 Qn Answers Mark Guidance 3a At lower temperatures, less/slower reaction between nitrogen and oxygen ; 1 OWTTE 3b More oxygen + less incomplete combustion / more complete combustion ; 1 3ci 2CO + 2NO → 2CO2 + N2 1 3cii catalysts provide an alternative reaction pathway with lower activation energy ; greater proportion of molecules with energy greater than or equal to activation energy ; greater frequency of effective collisions ; 3 A: number & particles 4ai filter out excess solid / CuO / to obtain filtrate ; heat filtrate to saturation + leave to cool (for crystals to form) ; filter out crystals + wash crystals with little cold distilled water + dry between pieces of filter paper ; 3 4aii Copper is unreactive / does not react with dilute acids 1 4bi NaOH(aq) / Na2CO3(aq) ; H2SO4(aq) ; 2 4bii solid Na2O is soluble in water + not possible to separate excess reagent from Na2SO4 (by filtration) ; 1 OWTTE 5ai Hydrogen / H2 ; Place a lighted splint at the mouth of test -tube + gas extinguishes a lighted splint with a ‘pop’ sound ; 2 5aii Mass of solid increases ; Iron gains oxygen to form iron oxide ; OWTTE 5aiii (order of change in mass) Pb, Mg, Fe ; Pb does not react with steam + no change in mass ; 1 mole of Mg gains 16 g while 1 mole of Fe gains 21.3 g ; 3 OWTTE R: no mention of numbers

3 Qn Answers Mark Guidance 5b Iron is more reactive than copper ; Iron loses electrons / oxidises more readily + provides sacrificial protection to copper ; 2 ORA 6ai 1 A: crosses to represent electrons 6aii The electron removed for H2 is from a shell which is closer to the nucleus ; 1 R: Na attained noble gas electronic configuration 6b Reactivity increases + melting point decreases down Group 1 ; 1 6c Energy is absorbed to overcome repulsion between like charges / negatively-charged ions and electrons ; 1 6d correct diagram for first ionisation energy of sodium – 1 m correct diagram for first electron affinity of oxygen – 1 m labels – 1 m (I: Ea) first ionisation energy of sodium energy progress of reaction first electron affinity of oxygen energy progress of reaction Na(g) O(g) + e− Na+(g) + e− O−(g) H H Sodium nucleus contains 11 protons + 12 neutrons

4 Qn Answers Mark Guidance 7a Mobile electrons in graphite acts as charge carriers + mobile ions in dilute aqueous sodium chloride act as charge carriers ; Graphite electrodes remain chemically unchanged + water (in dilute aqueous sodium chloride) decomposes (to form hydrogen and oxygen gas) ; 2 A: H+ and OH− ions selectively discharged A: chemically changed 7bi (gas collected in test-tube Y) oxygen / O2 + (gas collected in test-tube Z) hydrogen / H2 ; 1 7bii (anode / positive electrode) 4OH−(aq) → 2H2O(l) + O2(g) + 4e− ; (cathode / negative electrode) 2H+(aq) + 2e− → H2(g) ; Number of moles of electrons lost at anode is the same as the number of moles of electrons gained at the cathode + 2 moles of H2 is produced with 1 mole of O2 ; same amount of gas occupies the same volume at the same temperature and pressure (and so, volume ratio is the same as mole ratio) ; 4 OWTTE A: balanced overall equation A: for every 4 moles of electrons transferred 7ci (gas collected in test-tube W) chlorine / Cl2 + (gas collected in test-tube X) hydrogen / H2 ; 1 7cii 1:1 1 7d (colour in beaker 1) violet / purple + (colour in beaker 2) green ; In beaker 1, H + and Cl− ions are selectively discharged (at the cathode and anode respectively) + leaving behind alkaline NaOH solution ; In beaker 2, H + and OH− ions are selectively discharged (at the cathode and anode respectively) + leaving behind a (more concentrated) solution of neutral NaCl ; 3 A: blue A: comparison of concentration of H+ and OH- ions 8ai • Correct arrangement of electrons for Al3+ & ‘3+’ charge • Correct arrangement of electrons for F- & ‘-’ charge • Charges balanced 2

5 Qn Answers Mark Guidance All 3 correct – 2 m 1 – 2 correct – 1 m 8aii (any 3) • Compounds formed between metals and non -metals are usually ionic (so, they have high melting points and conduct electricity when molten) ; • Aluminium chloride has a simple molecular structure • Less energy is needed to overcome weak intermolecular forces of attraction (resulting in low melting points) • Exists as (electrically neutral) molecules when molten / no mobile charge carriers when molten (resulting in poor electrical conductivity) OR • Compounds formed between metals and non -metals are usually ionic (so, they have high melting points and conduct electricity when molten) ; • Aluminium chloride should have a giant ionic structure ; • More energy is needed to overcome strong electrostatic forces of attraction between (oppositely -charged) ions (resulting in high melting points but melting point of aluminium chloride is low) • Ionic compounds conduct electricity when molten due to mobile ions (but aluminium chloride is a poor electrical conductor when molten) Minus 1 mark if student states that aluminium chloride is an ionic compound or has a giant ionic structure 3 8bi Beryllium oxide is amphoteric + magnesium oxide is basic 1 A: reacts with acids & alkalis + reacts with acids 8bii (any 2) • High melting/boiling point • Semi-conductor / non-conductor of electricity • Insoluble in water / organic solvents • Brittle / Hard 1 R: High density 9ai Sulfur is oxidised as oxidation state of sulfur increases from +2 in S2O32- to +2.5 in S4O62- ; Iodine is reduced as oxidation state of iodine decreases from 0 in I2 to -1 in I- ; 2

6 Qn Answers Mark Guidance 9aii No. of moles of S2O32- = 24.70 × 10-3 × 0.100 = 2.47 × 10-3 mol/dm3 ; No. of moles of HClO = no. of moles of I2 = 2.47 × 10-3 ÷ 2 = 1.24 × 10-3 mol ; 2 Allow ECF 9b (Yes) % Cl in NaClO = 35.5 74.5 ×100 = 47.7 % % Cl in Ca(ClO)2 = 71 143 ×100 = 49.7 % Cl2 contains 100% chlorine 2 correct calculations + statement – 2m Any one correct calculation – 1m 2 9ci (any one) • Different precisions/significant figures • Different volumes of water used 1 9cii Mass of chlorine = 2.86 × 177 × 10-3 = 0.50622 g ; Solubility = 0.50622 ÷ 71 ÷ 100 × 10-3 = 0.0713 mol/dm3 ; 2 Allow ECF 9ciii (any one) • higher solubility of chlorine + less harmful chlorine (gas) released (OWTTE) • lower water evaporation + less harmful chlorine (gas) released (OWTTE) • reduces energy consumption/less energy needed + less crude oil / fossil fuels used • less CO2 produced from burning fuels to heat the pool + which causes global warming 9di NH3(aq) + 2HOCl(aq) → NHCl2(aq) + 2H2O(l) ; 1 State symbols not required 9dii (any one) • pH > 5.5 / 6 or pH between 5.5 / 6 to 8 ; • low concentration of Cl2/HOCl/NH3 ;