Jurongville 2024 Prelim 4E Chemistry Paper 2 ans

Page 1 Qn Marking Scheme Remarks Mks 1ai Calcium oxide CaO 1 1aii Nitrogen monoxide NO 1 1aiii Aluminium oxide Al2O3 1 1aiv Sulfur dioxide SO2 1 1av Iron(II) oxide FeO 1 1b true false They all float on water. ✓ They lose electrons more easily down the group. ✓ They rapidly tarnish in air. ✓ They react with water to form two alkaline products. ✓ 1 mark for every 2 ticks 2 2a Average mass of one atom of that element relative to 1/12 the mass of an atom of carbon-12. Specify 1/12 the mass of a carbon-12 atom 1 2b ∑ isotope mass × isotope abundance ÷ 100 Let x be the relative abundance of Mg-24. Let y = 0.9 − x be the relative abundance of Mg-26. Ar(Mg) = 24.3 Ar(Mg) = (24 × x) + (25 × 0.1) + [26 × (0.9 − x)] 24.3 = 24x + 2.5 + 23.4 − 26x 2x = 1.6 x = 0.8 y = 0.9 − 0.8 = 0.1 relative abundance of Mg-24 = 80% relative abundance of Mg-26 = 10% 1 mark for each relative abundance in % 2 2c Mg-24 will diffuse the fastest, followed by Mg-25, and Mg-26 will diffuse the slowest. OR Mg-24 will diffuse the fastest. Mg-25 will diffuse slower than Mg-24, but faster than Mg-26. Mg-26 will diffuse the slowest. OR Mg-24 will diffuse faster than Mg-25 and Mg-26. Mg-25 will diffuse faster than Mg-26. Need to compare and order all 3 isotopes 1 2d element Mg B O mass / g 18.9 5.8 25.3 Ar 24 11 16 mol 0.7875 0.52727 1.58125 ÷ 1.4935 1 2.9989 mole ratio 3 2 6 empirical formula, Mg3B2O6 1 mark for finding moles 1 mark for Mg3B2O6 2 2e x = 7 and y = 4 cao 1

Page 2 Qn Marking Scheme Remarks Mks 3a In SF4, sulfur atom has 10 valence electrons. In XeF 4, xenon atom forms single bonds with fluorine atoms, despite xenon being a noble gas. 1 mark for S 1 mark for Xe 2 3b 1 mark for bonding electrons 1 mark for lone pairs 2 3c These fluoride compounds are covalent substances with simple molecular structure. Little energy is needed to overcome weak intermolecular forces of attraction between molecules. 1 mark for structure 1 mark for energy and bonding 2 4a 2Ca(NO3)2 → 2CaO + 4NO2 + O2 Calcium nitrate is reduced as the oxidation state of nitrogen decreases from +5 in calcium nitrate to +4 in nitrogen dioxide. Calcium nitrate is oxidised as the oxidation state of oxygen increases from −2 in calcium nitrate to 0 in oxygen. 1 mark for changes to N 1 mark for changes to O 4b Place a glowing splint near the gas. If the glowing splint relights, the gas is oxygen. Describe test and result 1 4ci Lead(II) nitrate will take the shortest time for nitrogen dioxide to appear, as it will be the first or fastest or easiest to decompose. In the reactivity series, calcium is the most reactive metal, followed by zinc, and lead is the least reactive metal. Lead forms the least thermally stable metal nitrate. 1 mark for Pb(NO3)2 1 mark for explanation 2 4cii Zinc oxide ZnO 1 5a Titration cao 1 5b