catholic high school chemistry prelim 2023 answers

1 2023 CHS S4 Prelim Mark Scheme Paper 1 Paper 2 Section A A1 (a) Put a tick (✓) if a redox reaction would occur. (i) a copper strip added to lead(II) nitrate solution (ii) aqueous chlorine added to potassium iodide solution ✓ (iii) hydrogen gas passed over heated copper(II) oxide ✓ (iv) potassium nitrate solution warmed with sodium hydroxide solution 4 correct – 3m , 3 correct – 2m, 2 correct – 1m (b) Difference: Reaction rate is slower when used with aqueous bromine OWTTE; Explain: Chlorine is more reactive than bromine/ gains electrons more readily (and displaces iodine from iodide solution faster); OWTTE A2 (a) As the number of Cl atoms in the compound incre ases, the boiling point increases. A: has more Cl atom substituted/ less H atoms R: increasing Mr/ molecular size (b) (i) C2H6 (ii) C2H4Cl2 (iiii) 5 (c) and 1m for each correct structure 1 2 3 4 5 6 7 8 9 10 B A C A A B D B C C 11 12 13 14 15 16 17 18 19 20 B B A B B A B D D D 21 22 23 24 25 26 27 28 29 30 D A C D B B B D C C 31 32 33 34 35 36 37 38 39 40 A D C D D D C C D C

2 A3 (a) Carbon dioxide = C; phosphorus pentoxide = B; both has simple molecular/ covalent structure; where a small amount of energy is needed to overcome weak intermolecular forces; phosphorus pentoxide is a larger molecule with stronger intermolecular forces (b) (i) 1 similarity (1m) + 1 difference (1m) Similarity: both has giant molecular/ covalent structure; both consists of only the same type of atoms difference: Silicon Graphite Difference 1 Only has strong covalent bonds between atoms Has both strong covalent bonds and weak intermolecular forces Difference 2 Not layered Layered Difference 3 Tetrahedral structure or 1 atom bonded to 4 other atoms Hexagonal structure or 1 atom bonded to 3 other atoms (ii) No. + A semiconductor needs to have presence of mobile charged carriers such as ions or electrons; All 4 valence electrons are used in bonding (and absence of ions/ contains only atoms) A4 (a) (i) removes carbon dioxide from atmosphere + hence regulating amount of carbon dioxide OWTTE (ii) 2NO + 2CO → 2CO2 + N2 ; 2CO + O2 → 2CO2 A: 2C8H18 + 25O2 → 16CO2 + 18H2O A: 2NO2 + 4CO → 4CO2 + N2 ; R: formation of sulfur dioxide (fuel does not contain S) Still releases large amount of carbon dioxide into the atmosphere + explain how carbon dioxide causes environmental problems (see e.g. below) Global warming/ Melting of ice caps/ rise in sea levels/ flooding in low -lying areas etc

3 (b) (i) The set-up using light will produce gas X but the set -up without light will not produce any gas X (or oxygen gas); Or In the absence of light, carbon dioxide is produced while in the presence of light, gas X / oxygen is produced (+ CO2) I: if students discussed observations such as presence of effervescenc