MSHS 2024 Chem Prelim MS

1 Secondary Four Chemistry Preliminary Examination 2024 Mark Scheme Paper 1 1 2 3 4 5 6 7 8 9 10 B B D B C C D C C C 11 12 13 14 15 16 17 18 19 20 B B D A D B D D A D 21 22 23 24 25 26 27 28 29 30 C C B A C C D C B B 31 32 33 34 35 36 37 38 39 40 C A C C D C C C A D Paper 2 Section A Qn No. Answer Mark 1a A 1 b D (Accept E: GeCl4 bp: 86.5oC ) 1 c B 1 d C 1 e G 1 Total 5 2a Water vapour produced escaped from the tube OR Water was removed through evaporation. 1 b To ensure that reaction is complete/ no more water (vapour) was produced. 1 c Energy taken in = (2.00 ÷ 238)× 88.1 = 0.740336134 kJ = 0.740 kJ 1 1 d Endothermic/thermal decomposition 1 Total 5

2 Qn No. Answer Mark 3a half equation(s) in each beaker Difference stated In beaker A, the anode would decrease in size /becomes smaller as copper is a reactive electrode and dissolves to form copper(II) ions. Cu(s) → Cu2+ (aq) + 2e In beaker B, there is no change in size for anode. OH - ions are preferentially discharged to form oxygen gas. 4OH- (aq) → 2H2O(l) + O2(g) + 4e- OR There is no effervescence in beaker A as copper is a reactive electrode and dissolves to form copper(II) ions. Cu(s) → Cu2+ (aq) + 2e In beaker B, there is effervescence observed at the anode. OH- ions are preferentially discharged to form oxygen gas. 4OH- (aq) → 2H2O(l) + O2(g) + 4e 1 1 bi Number of moles of electrons = 289500 ÷ 96500 = 3 moles 1 ii Cu2+ (aq) + 2e → Cu (s) No of moles of electrons = 3 moles No of moles of Cu= 1.5 moles Mass of Cu produced in both beakers = 1.5 x 64 g/mol = 96g 1 1 d Universal Indicator will turn from green to violet. Hydrogen ions are preferentially discharged to form hydrogen gas, concentration of OH- more than that of H+, solution becomes alkaline/pH of solution increases. 1 1 Total 7 4a Carbon monoxide produced/chlorine used is toxic/poisonous. 1 b Argon is unreactive/inert. It prevents oxygen/water vapour in the air from reacting with sodium used /titanium produced. 1 1 c Titanium chloride is a m etal chloride which is usually an ionic compound. Ionic compounds have high melting points and are solids at room temperature. 1 1

3 Qn No. Answer Mark d TiCl4 + 4 Na Ti + 4 NaCl From eqn, 1 mol TiCl4 reacts with 4 mol Na 190 g TiCl4 reacts with 92 g Na 190 kg TiCl4 reacts with 92 kg Na 40 kg TiCl4 reacts with (40÷190) x 92 kg = 19.4 kg Na TiCl4 is the limiting reactant since 40 kg of TiCl4 reacts completely with 19.4 kg of Na but 20 kg of Na is available. 1 1 1 e Actual mass = (92.3 ÷ 100) × 13.5 kg = 12.5 kg 1 Total 9 5a Magnesium (most reactive), chromium, iron, nickel (least reactive) Magnesium oxide is not reduced by carbon at a high temperature (1750 °C) because it is a very stable oxide. This shows that magnesium is the most reactive metal, forming the most stable oxide. Nickel(II) oxide is reduced at the lowest temperature (above 300 °C ) because it is the least stable oxide . This shows that nickel is the least reactive metal, forming the least stable oxide. 1 1 1 b Any t wo characteristic properties of iron, highlighting its difference from potassium. Iron has high melting and boiling points whereas potassium has low melting and boiling points. Iron is hard whereas potassium is soft. Iron forms compounds which are coloured whereas compounds of potassium are usually not coloured/white. Iron has variable oxidation states in its compounds but not potassium. 2 ci Add aqueous sodium hydroxide/ammonia to a sample of iron(II) chloride solution a little at a time and then in excess. A green precipitate is formed, which is insoluble in excess , showing that iron(II) ions are present. 1 1 ii Fe2Cl6 1 Total 8

4 Qn No. Answer Mark 6a Energy absorbed = [(5 x 413) + 347 + 358 + 467] + [(3 x 495)] = 4722 kJ Energy released = [(4 x 799) + (6 x 467)] = 5998 kJ Overall energy change = 4722 + [–5998] kJ = –1276 kJ = –1280 kJ 1 1 1 b The reaction is exothermc because energy released when bonds are formed in 2 moles carbon dioxide and 3 moles of water is greater than energy absorbed when bonds are broken in 1 mole of ethanol and 3 moles of oxygen. 1 1 1 Total 6 7a Both immediate and long-term impact are acceptable answers. Immediate impact : Milk bottle made of glass has a greater impact on the environment because it uses more energy to process the raw materials for glass (6750 kJ) than for polymer (1710 kJ) Long-term impact: Milk bottle made of polymer is used only once while a milk bottle made of glass is used 25 times during its lifetime. Hence it has a greater impact on the environment because it would use more energy (1710 kJ x 25) to process the raw materials than a glass bottle (6750 kJ). 1 1 b Advantage – it can be reused up to 25 times Disadvantage – it is heavier so it takes more energy to transport 1 1 c Recycling of polymer conserves crude oil which reduces pollution/reduces the need to extract crude oil. 1 Total 5 8a C12H26 C2H4 + C10H22 1 b C H O Mass (g) 1.44 0.36 0.96 Molar mass (g/mol) 12 1 16 Number of moles 1.44 ÷12 = 0.12 0.36 ÷ 1 = 0.36 0.96 ÷ 16 = 0.06 Simplest mole ratio 0.12 ÷ 0.06 = 2 0.36 ÷ 0.06 = 6 0.06 ÷ 0.06 = 1 C2H6O 1 1 c Hydration Ethanol 1 1 di Purple acidified potassium manganate(VII) turns colourless. Ethane-1,2-diol is a reducing agent. 1 1

5 Qn No. Answer Mark ii 1 Total 8 9ai Any 2 of the 3 2 ii chromatography 1 b

6 Qn No. Answer Mark c In the manufacture of proteins, the different monomers join together with the elimination of water but in the formation of polymer N, the monomers join together without loss of any small molecules. 1 Total 5 10a energy level reactants and products enthalphy 1 1 1 b N2 + 3 H2 ⇌ 2 NH3 The lower the temperature, the higher the yield of ammonia 1 1 c The production of hydrogen in the steam-methane reforming reaction produces carbon dioxide, which is a greenhouse gas. OR The steam -methane reforming reaction requires a high temperature of 800°C – 900°C, likely from the burning of fossil fuels which produces carbon dioxide, a greenhouse gas. 1 1 d Electricity from solar/geothermal/wind/nuclear energy. 1 ei It requires a lower pressure to maintain ammonia which has a higher boiling point in the liquid state at room temperature compared to hydrogen. It requires less energy to lower the temperature to the boiling point of ammonia which is higher than that of hydrogen. 1 ii It is expensive to have high pressure tanks to store and transport hydrogen gas. 1 f Nitrogen and water. Combustion of ammonia does not produce carbon dioxide/carbon monoxide 1 1 Total 12 energy progress of reaction H = +165.2 kJ/mol CO2 + 4H2 CH4 + 2H2O

7 Section B Qn No. Answer Mark 11a As the colour intensity of the solution increases from chlorine to bromine to iodine/the darker the colour of the solution, the higher the absorbance reading on the colorimeter. 1 b Cl2(aq) + 2 KI (aq) → 2KCl (aq) + I2 (aq) Chlorine is more reactive than iodine , it displaces iodine from potassium iodide. From 0 to 5 cm 3, the absorbance reading increases from 0.00 to 0.34 and remains