2025 Prelim P3 MS (for exchange)_H2 Chem_HCI

2025 HCI C2 H2 Chemistry Prelim Exam Mark Scheme 1 HWA CHONG INSTITUTION 2025 C2 H2 CHEMISTRY PRELIMINARY EXAMINATION MARK SCHEME Paper 3 1 (a) (i) Let the solubility of MgCO3 and Mg(OH)2 be x and y respectively, in mol dm–3. For MgCO3 Ksp = [Mg2+][CO32–] [0.5] 1.0 × 10–5 = (x)(x) x = 3.16 × 10–3 [0.5] mol dm–3 For Mg(OH)2 Ksp = [Mg2+][OH–]2 [0.5] 1.1 × 10–11 = (y)(2y)2 y = 1.40 × 10–4 [0.5] mol dm–3 (ii) [Mg2+][OH–]2 = 1.1 × 10–11 [0.5] (3.0 × 10–5)[OH–]2 = 1.1 × 10–11 [OH–] = 6.06 × 10–4 mol dm–3 [0.5] (iii) [0.5] CO32– ions are added first to precipitate Ca2+ as CaCO3 [0.5] such that the filtrate contains mainly Mg2+ / very little Ca2+. [1] This must be controlled to prevent precipitation of Mg2+ as MgCO3 (b) (i) Lattice energy is the heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions. [1] (ii) [3] total for correct Born-Haber cycle Hat (Mg) + 1st + 2nd IE (Mg) + Hf (OH) + EA (OH) x 2 + LE (Mg(OH)2) = Hf (Mg(OH)2) 148 + 736 + 1450 + (6 x 2) – (173 x 2) + LE (Mg(OH)2) = –925 LE (Mg(OH)2) = –2925 (or 2900 or 2930) kJ mol–1 [1] energy / kJ mol–1 Mg(OH)2(s) Mg(s) + O2(g) + H2(g) Mg(g) + O2(g) + H2(g) Mg2+(g) + 2e– + O2(g) + H2(g) Mg2+(g) + 2e– + 2OH(g) Hf(Mg(OH)2) = – 925 Hat(Mg) = + 148 1st + 2nd IE (Mg) = +736 + 1450 2 × ∆Hf (OH) = 2(+6) LE (Mg(OH)2) = ? Mg2+(g) + 2OH–(g) 2 × EA (OH) = 2(–173) 0

2025 HCI C2 H2 Chemistry Prelim Exam Mark Scheme 2 (iii) [1] correct trend: decreasing magnitude of LE: MgO > Mg(OH)2 > Ca(OH)2 or LE for MgO is larger than for Mg(OH)2 + LE for Mg(OH)2 is larger than for Ca(OH)2 [1] explain why LE of MgO > LE of Mg(OH)2: The cation Mg2+ is the same for both compounds. • O2– anion has larger charge [0.5] • but smaller ionic radius than OH– [0.5] [0.5] explain why LE of Mg(OH)2 > LE of Ca(OH)2: The anion OH– is the same for both compounds and Mg2+ cation has the same charge as Ca2+ but Mg2+ has smaller ionic radius than Ca2+ [0.5] link between ionic radius and charge with magnitude of LE (e.g. quote LE eqn or link to strength of ionic bond / attraction bet. oppositely-charged ion at least once) (c) (i) Al : 1s2 2s2 2p6 3s2 3p1 [0.5] Mg: 1s2 2s2 2p6 3s2 [0.5] [0.5] The 3p subshell of A l is further away from the nucleus than the 3s subshell (or at higher energy level). [0.5] There is weaker attraction between the nucleus and the outermost electron of Al (or less energy is needed to remove the 3p electron) resulting in lower ionisation energy. (ii) MgCl2 dissolves in water to form aqueous ions. MgCl2(s) + 6H2O(l) → [Mg(H2O)6]2+(aq) + 2Cl–(aq) or MgCl2(s) + aq → Mg2+(aq) + 2Cl–(aq) [1] Mg2+ has slightly high charge density . Slight hydrolysis occurs, forming a slightly acidic solution of pH 6.5. [Mg(H2O)6]2+(aq) + H2O(l) ⇌ [Mg(H2O)5(OH)]+(aq) + H3O+(aq) [1]