MSHS Chem Prelim Ans

2025 MSHS S4 Chemistry Prelim Paper 1 Answer 1 C 2 A 3 A 4 B 5 A 6 D 7 D 8 C 9 B 10 C 11 A 12 A 13 D 14 C 15 C 16 B 17 A 18 D 19 D 20 B 21 D 22 D 23 B 24 A 25 C 26 D 27 C 28 C 29 B 30 A 31 A 32 D 33 A 34 C 35 B 36 B 37 A 38 C 39 C 40 D

1 Secondary Four Chemistry Preliminary Examination 2025 Mark Scheme Paper 2 Section A Qn No. Answer Mark 1ai C 1 Ii D 1 iii B 1 iv E 1 v A 1 b The amount of B/carbon dioxide is increasing due to the complete combustion of fossil fuels in vehicles and industries. It takes years for trees to grow, so the absorption of carbon dioxide through photosynthesis is much slower than the release of carbon dioxide by complete combustion of fossil fuels. 1 1 Total 7 2a The ions in lithium are held together by electrostatic forces of attraction between positive lithium ions and the sea of delocalised electrons. The ions in lithium chloride are held together by the electrostatic forces of attraction between positively charged lithium ions and negative charged chloride ions. 1 1 b The melting and boiling points of lithium chloride are higher than that of lithium. The e lectrostatic forces of attraction between oppositely charged ions in lithium chloride is stronger than the electrostatic forces of attraction between lithium ions and the delocalised electrons in lithium metal. 1 1 (c) Both Student A and Student B are incorrect OR Student B is incorrect and Student A is partly correct. Lithium can conduct electricity in all states because of the mobile electrons. Lithium chloride can conduct electricity when molten / dissolved in water because the ions are mobile, but not in the solid state because the ions are held in fixed positions and are not mobile.. 1 1 1

2 Qn No. Answer Mark d One calcium atom (2, 8, 2) loses two electrons to t wo fluorine atoms , to form positively charged calcium ion (2, 8). Each fluorine atom (2, 7) gains one electron to form negatively charged fluoride ions (2,8). 1 1 Total 9 3a 2 oxygen atoms and 1 chlorine with 4 shared pair of electrons chlorine atom with 1 lone pair and 1 unshared electrons 1 1 b Chlorine has undergone disproportionation. Chlorine is oxidised because its oxidation state has increased from +5 in HClO3 to +7 in HClO4. Chlorine is also reduced because its oxidation state has decreased from +5 HClO3 to +4 in ClO2. 1 1 ci 2NaClO2 + Cl2 → 2ClO2 + 2NaCl 1 ii Chlorine is the oxidising agent. It oxidises sodium chlorate to chlorine dioxide / it causes the oxidation state of chlorine to increase from +3 in sodium chlorate to +4 in chlorine dioxide. 1 1 Total 7 4ai C3H4O4 1 ii C2H4O2 / CH3COOH Ethanoic acid 1 1 iii Ethyl ethanoate 1 iv 1 bi Malonic has a higher pH because it is a weak acid. 1 Total 6

3 Qn No. Answer Mark 5a More energy is released/given out in bond forming of 2 moles of water and 1 mole of oxygen than the energy absorbed/taken in bond breaking of 2 moles of hydrogen peroxide. 1 1 bi Number of moles of H2O2 in P = 50÷1000 x 2 = 0.1 Number of moles of H2O2 in Q = 0.1 Concentration of Q = 0.1÷ 0.08 = 1.25 mol/dm3 Or Concentration of Q = (50÷80) x 2 = 1.25 mol/dm3 1 1 ii From graph, Loss in mass for conical flask R is less than that for conical flask P and Q. Hence number of moles of H2O2 in R should be less than 0.1 mole. Speed of reaction for conical flask R is greater than that for conical flask P and Q. Hence concentration of H2O2 in R shoud be greater than 2.0 mol/dm3. Any volume that gives number of moles less than 0.1 for the suggested concentration. e.g. 30 cm3 Any concentration greater than 2.0 mol/dm3 e.g. 2.5 mol/dm3 No. of moles = 30 x 2.5 = 0.75 mol 1000 1 1 iii Increase the temperature of H2O2 solution. When the temperature raised, particles gain more energy and move faster. More particles have the minimum activation energy to react when they collide/more particles possess energy greater than or equal to activation energy. Hence, frequency of effective collisions increases and hence rate of reaction increases. 1 1 1 Total 9 6ai 2H+(aq) + 2e- → H2 (g) 2 Cl-(aq) → Cl2 (g) + 2e- For 2 moles of electrons that passed through the circuit, 1 mole of hydrogen and 1 mole of chlorine are formed. Equal number of moles of gases occupy equal volumes at r.t.p., 1 volume of hydrogen and 1 volume of chlorine are formed. 1 1

4 Qn No. Answer Mark ii Chlorine dissolves / soluble in acid / solution 1 b Mr of NaCl = 23 + 35.5 = 58.5 From eqn 2 moles NaCl produce 1 mole Cl2 2(58.5 g) NaCl produce 71 g Cl2 117 g NaCl produce 71 g Cl2 117 tonnes NaCl produce 71 tonnes Cl2 23.4 tonnes NaCl produce (23.4 ÷ 117) x 71 tonnes Cl2 = 14.2 tonnes No. of mole of Cl2 = 142 000 000 mol ÷ 71 g/mol = 200 000 Volume of Cl2 = 200 000 x 24 dm3 = 4.80 x 106 dm3 1 1 1 c Seawater is a concentrated sodium chloride solution. Electrolysis of concentrated sodium chloride solution produces chlorine which is toxic. Add water to dilute the seawater. Electrolysis dilute sodium chloride solution produces hydrogen and oxygen. 1 1 Total 9

5 Qn No. Answer Mark 7 Advantages of fermentation / disadvantages of hydration: • Sugar cane can be re-grown/is renewable but c rude oil is non- renewable/finite/cannot be replaced. • Fermentation of glucose uses a lower temperature (30 oC) than hydration of ethene (300 oC), so energy cost is lower than that for hydration of ethene. • Fermentation of glucose uses a lower pressure (1 atm) than hydration of ethene (60 –70 atm) so energy cost /cost of equipment to maintain the pressure is lower than that for hydration of ethene. • Yeast is a natural substance and not harmful whereas phosphoric acid is corrosive. Advantages of hydration / disadvantages of fermentation: • Hydration of ethene is a faster process whereas fermentation is a slower process. • Hydration of ethene produces pure ethanol whereas fermentation produces impure ethanol. Any four of the above points 4 Total 4 8ai 1 ii Add aqueous sodium hydroxide to a solution containing ammonium ions. Warm the mixture gently. If the gas turns damp red litmus paper blue (ammonia), then ammonium ions are present. 1 1 bi Condensation polymerisation 1 ii 1

6 Qn No. Answer Mark c Addition polymerisation of acrylamide in (a) produces only one product, the polymer whereas the condensation polymerisation in (b) produces the polymer and water as a by-product. The mass of the polymer in (a) is the same as the mass of all the monomers added together whereas the mass of the polymer in (b) is less than that of all the monomers. For a ddition polymerisation of in (a) , the monomer has a carbon-carbon double bond. For the condensation polymerisation in (b), the monomers has a functional group at each end of the molecule. Any two of the above differences. 2 Total 7 9a chromatography technique stationary phase mobile phase paper chromatography chromatography paper ethanol 2 bi Component D 1 ii Components B and C 1 iii Component D 1 ci 72 1 ii C5H12 It is a saturated hydrocarbon since it does not decolourise aqueous bromine. 1 1 iii C5H12 + 8O2 → 5CO2 + 6H2O reactants and products balanced equation 1 1

7 Qn No. Answer Mark iv 2 Total 12

8 Section B Qn No. Answer Mark 10ai (least) L