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 m