Dunman (2024) 4E 6092 Prelim P1+P2_MS

1 DUNMAN SECONDARY SCHOOL 6092 CHEMISTRY 4E Preliminary Examination 2024 Marking Scheme PAPER 1 1 2 3 4 5 6 7 8 9 10 A B A A C C C D C A 11 12 13 14 15 16 17 18 19 20 B C C C C D A C C D 21 22 23 24 25 26 27 28 29 30 D C D C B D B A B D 31 32 33 34 35 36 37 38 39 40 A A D D D B A B C B PAPER 2 Section A Qn Answer Mark (max) 1(a) A, B, E 1 1(b) C 1 1(c) D 1 1(d) E 1 1(e) D and B 1 2(a) Acidic oxide : SiO2 / P2O5 / SO2 Basic oxide : Na2O / MgO Amphoteric oxide : Al2O3 [1] any 1-2 correct; [2] all correct 1 2(b) Correct sharing of electrons [1] Correct outermost electrons for each atom [1] 2

2 Qn Answer Mark (max) 2(c) • Silicon dioxide has a giant covalent structure. • The extensive network of silicon and oxygen atoms • are held by many strong covalent bonds, • which require a lot of energy to overcome it, hence high melting point. • Sulfur dioxide has a simple covalent structure. • The molecules are held by weak intermolecular forces of attraction which require a little amount of energy to overcome it, hence low melting point. 6 points [3], 4-5 points [2], 2-3 points [1] 3 2(d) movement : from slide past each other and exchanging position within liquid to move randomly at high speeds in all direction. arrangement : from closely packed and disorderly arranged to far apart and disorderly arranged. 2 2(e) • Dichlorine molecule exists as neutral molecules . They do not have mobile ions or electrons to carry charge. • Hypochlorous acids have mobile ions which are free to move around to carry charged. 2 3 formula of salt Formulae of reagents used method used MgCl2 (aq) Mg / MgCO 3 / MgO HCl (aq) addition of excess solid to acid filtration evaporation and crystallisation (NH4)2SO4 NH3 (aq) H2SO4 (aq) Titration between alkali and acid Evaporation and crystallisation BaCO3 (s) Ba(NO3)2 (aq) Na2CO3 (aq) / K2CO3 (aq) / (NH4)2CO3 (aq) Mixing of the two salt solutions to form precipitate Filtration to collect residue 5

3 Qn Answer Mark (max) 4(a) The total energy absorbed to break 1 mol of O-O bond and 2 mol of O -H bond in H2O2 is less than the total energy released to form 4 mol of O-H in H2O and 1 mol of O=O in O2. Since there is net release of energy to the surrounding, it is an exothermic reaction. [1] total energy absorbed to break bond is less than total energy released to form bond [1] correct number and type of bonds 2 4(b)(i) catalyst 1 4(b)(ii) [1] for 2 correct [2] for 3-4 correct [3] for all correct - reactants and products - activation energy with catalyst - activation energy without catalyst - enthalpy change - shape of graph 3 5(a) Insert a filter paper soaked in acidified potassium manganate(VII) [1] If purple KMnO4 turns colourless, SO2 is still present. OR If purple KMnO4 remains purple, SO2 is not present [1] 2 5(b)(i) No. of moles of iodine = 12/ 1000 x 0