2025 RI H1Chem Prelims P1 P2 Answers

1 Q1 no. of protons no. of neutrons no. of electrons 32S2− 16 16 18 34S 16 18 16 Ans: A Q2 Electronic configuration: 1s² 2s² 2p⁶ 3s² 3p⁶  1 The third shell has no unpaired electrons. 2 The valence electronic configuration of an isolated atom is 3s2 3p6. 3 The 2p orbitals have the same energy (degenerate) and same shape (dumbbell). Ans: B (1 only) Q3 electronic configuration A Ti2+ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d²  B Zn2+ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ C Cu2+ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹ D Fe2+ 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ Ans: B Q4 Ans: D Q5 The reaction involves the cleavage of the C−X bond. The weaker the C−X bond, the easier it can be broken and the faster is the rate of reaction. 1 While permanent dipole -permanent dipole forces between chloroethane molecules are stronger than those between bromoethane molecules, it does not contribute to the ease of breaking the C−X bond. species bonding A Al2O3 ionic B SO3 covalent only C NH4Cl ionic, covalent and dative  D H3O+ covalent and dative Suggested Answers for 2025 Y6 H1 Chemistry Preliminary Examination Paper 1: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 A B B D D D C C C C D C C B C B C A A C 21 22 23 24 25 26 27 28 29 30 B B C B C D A B C C Worked solutions for Paper 1 Al 3+ 2− O 2 3 S O O O N H H H H + Cl − O H H H +

2 2 Though C−Cl bond is more polar than C −Br bond, it does not increase the reactivity as the rate of reaction is observed to be more rapid with bromoethane than chloroethane.  3 Since C−Cl bond is stronger than C−Br bond, more energy is required to break the C −Cl bond, this leads to a slower rate of reaction in chloroethane. Ans: D (3 only) Q6 Liquefaction of gases can be achieved by: • increasing pressure on a gas to bring the gas molecules closer together so that the strength of the intermolecular forces becomes more significant. • decreasing temperature so that sufficient energy is removed for the gas molecules to move at a slower speed and to make the gas change from the gaseous to the liquid state. The ease of liquefaction of gases generally depends on the strength of intermolecular forces . The stronger the intermolecular forces between the gas molecules, the easier it will be to liquefy a gas. Ans: D Q7 physical state room temperature conductivity solubility in water ionic solid non- conductors in solids, conductors in molten / aq usually soluble in polar solvents, insoluble in non– polar solvents metallic solid, except for Hg conductors in solids / liquids insoluble in polar and non– polar solvents giant molecular solid non- conductors, except graphite insoluble in all solvents. simple molecular mostly gases/ volatile liqu