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