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 KOH(aq) + HNO3(aq) → KNO3(aq) + H2O(l) OR K2CO3(aq) + 2HNO3(aq) → 2KNO3(aq) + H2O(l) + CO2(g) 1 mark for formulae and coefficients 1 mark for state symbols 2 5c Heat the aqueous solution until it is saturated. Allow the saturated solution to cool so that the salt can crystallise. Filter to collect the crystals. Wash the crystals with a little cold distilled water. Dry the crystals between a few sheets of filter paper. 1 mark for heat, cool 1 mark for wash, dry 2 5di Endothermic cao 1 5dii ΔH = c × m × ΔT = 4.2 × 0.050 × (20.4 − 18.7) = 4.2 × 0.050 × 1.7 = + 0.357 kJ Show working 1 S F F O ●● ● ● ●×●× × × ×× ×× ×× ×× ×× ×× ×× ××

Page 3 Qn Marking Scheme Remarks Mks 5diii Product KNO3(aq) at higher level and arrow head for positive ΔH, no need show value or unit 1 6a Experiment 1, dilute CuSO4(aq), graphite electrodes At negative electrode: Cu2+(aq) + 2e− → Cu(s) At positive electrode: 4OH−(aq) → O2(g) + 2H2O(l) + 4e− Experiment 2, dilute CuSO4(aq), copper electrodes At negative electrode: Cu2+(aq) + 2e− → Cu(s) At positive electrode: Cu(s) → Cu2+(aq) + 2e− 1 mark for 1 ionic equation, must show state symbols 3 6b In graphite, each carbon atom is bonded to three other carbon atoms. This leaves 1 unbonded electron per carbon atom, which is free and mobile to conduct electricity. 1 mark for bonding 1 mark for mobile electron(s) 2 6c In experiment 1, colour of electrolyte changes or slowly fades from blue to colourless, and its pH becomes more acidic. In experiment 2, there is no change in the colour and pH of the electrolyte / the colour and pH of electrolyte remain the same. Mention changes to colour and pH 1 6d Increase in mass of Cu = 53 − 50 = 3 g Number of moles of Cu = 3 ÷ 64 = 0.046875 mol Cu2+(aq) + 2e− → Cu(s) Number of moles of electrons = 0.046875 × 2 = 0.09375 mol = 0.0938 mol (3 s.f.) 1 mark for moles of Cu 1 mark for moles of e− in 3 s.f. 2 7a Property of transition metals: - They have high melting points. - They have high densities. - They form ions with variable or different oxidation states. - They form coloured compounds. Any 1 property 1 KNO3(aq) ΔH

Page 4 Qn Marking Scheme Remarks Mks 7bi A reversible reaction can go both forward and backward at the same time. Mention both directions 1 7bii Pressure of 250 atm and temperature of 450 °C Values and units 1 7c Using powdered form increases surface area of catalyst. With more surface area in contact with reactant particles, the rate of reaction increases. More surface area + faster reaction rate 1 7d An increase in temperature results in an increase in rate of reaction. At a higher temperature, reacting particles have more kinetic energy. They gain energy and move faster. More reacting particles possess energy that is greater than or equal to activation energy. Frequency of effective collisions between reacting particles increases. 1 mark for energy > Ea 1 mark for increase frequency of effective collisions and rate 2 7e Reasons to not increase temperature: - If reaction is slow due to low temperature, catalyst added can help to increase reaction rate / speed up reaction - Decrease or lower yield of product / product yield / yield - Incur higher operational / energy / production costs - Increase energy consumption and greater environmental impact (more industrial emissions) - Need to purchase specialised materials that are heat -resistant / able to withstand high temperatures as well as schedule regular maintenance to keep equipment in optimal condition - Pose safety concerns when too high temperatures cause too fast uncontrolled reactions, or increase the risk of explosions, fires and other hazardous conditions - Promote unwanted side reactions / formation of by -products, affecting purity and quality of desired product 1 mark for 1 reason 2 8a Petroleum (crude oil) is first heated in a furnace and the vapour is passed into the fractionating (distillation) column. The fractions with lower boiling points rise up the column, cool and condense, and are collected at the top of the column. The fractions with higher boiling points condense at higher temperatures and are collected at the lower levels or sections or bottom of the column. 1 mark for processes and states (heat to vapour, cool to liquid) 1 mark for different boiling points and collection positions 2 8bi Use of kerosene: - Fuel for aircraft / jet engines - Fuel for cooking using oil stoves Use of diesel: - Fuel for diesel engines (in buses, lorries, trains) - Fuel for power generation, e.g. power heavy machinery Both uses, 1 use each 1

Page 5 Qn Marking Scheme Remarks Mks 8bii Other fractions: - Bitumen - Lubricating oil - Petroleum gas Any 1 fraction name 1 8ci product propene, C3H6 ethen