2024 Prelims HCI H2 Chem P3 (Ans)

2024 HCI C2 H2 Chemistry Prelims / Paper 3 HWA CHONG INSTITUTION 2024 C2 H2 CHEMISTRY PRELIMINARY EXAMINATION SUGGESTED SOLUTIONS Paper 3 1 (a) (i) (ii) [0.5] for each point • Down the group, cationic radius increases , and charge remains the same. • Charge density of the cation decreases • Electron cloud of the anion is less polarized and the (N−O) covalent bonds are weakened to a smaller extent going down the group • More energy required to break these bonds hence temperature increases (iii) Number of moles of gaseous products increases as reaction takes place, entropy change is positive. [1] G = H − TS Since H is positive, for G to be negative and reaction to be spontaneous, the magnitude of TS needs to be greater than H. Hence, high temperature is needed. [1] (iv) [3] –602 = + 148+ ½(496) + 736 + 1450 – 142 + 844 + L.E

L.E = –3886 kJ mol–1 = –3890 kJ mol–1 (3 s.f.) [1] (ecf based on energy cycle drawn) (b) MgO dissolves sparingly with water / has limited solubility in water. pH = 9 [0.5] MgO + H2O → Mg(OH)2 Mg(OH)2 ⇌ Mg2+ + 2OH– [0.5] Al2O3 does not react with water [0.5]. pH = 7 [0.5] P4O10 reacts violently with water to give an acidic solution. pH = 2 [0.5] P4O10 + 6H2O → 4H3PO4 [0.5] (c) (i) AlCl3 is electron deficient and can accept a pair of electrons. [1] (ii) [0.5] for each product and (iii) [1] (iv) Chlorine atom is more electronegative than iodine atom, the I-Cl bond is polar and I atom in ICl is more electron deficient as compared to the iodine atom in I2. OR There is a partial positive charge on the I atom. (v) [1] for each step I–Cl + AlCl3 → I+ + AlCl4–

2024 HCI C2 H2 Chemistry Prelims / Paper 3 2 (a) Alkenes do not react/are unreactive with nucleophiles [0.5] as C-H and C=C bonds are non-polar so there is no electron deficient site for nucleophilic attack OR due to their electron rich  cloud/C=C bond which tends to repel nucleophiles [0.5] Carbonyl compounds react with nucleophiles [0.5] due to the presence of polar C=O bond/electronegative O which causes the C=O carbon to be electron deficient. [0.5] (b) (i) A racemic mixture contains equimolar amounts of two enantiomers. [1] Each enantiomer in a racemic mixture rotates plane -polarised light by the same magnitude in opposite directions , [1] hence there is no net rotation of the plane-polarised light. (ii) [1]: Choice of an alkene reaction that produces a carbocation intermediate, e.g. alkene + HCl / HBr / H2O / Cl2 / Br2 OR Choice of a carbonyl compound with a suitable nucleophile, e.g. HCN (in trace KCN) [1]: Explain why racemic mixture is obtained: • 0.5m (trigonal) planar geometry about positively charged carbon of the carbocation intermediate (for alkene reaction) or C=O carbon (for carbonyl reaction); • 0.5m equal probability of attack from top and bottom of the