YJC_H2_CHEM_P3_Answer Prelim

1 9647 / YJC / 2012 / JC2 Preliminary Examinations / Paper 3 answers YJC 2014 Prelim Paper 3 answers 1 (a) Na2O : dissolves readily in H 2O to form a strongly alkaline solution with pH = 13 (accept 11 – 14) . Na2O + H2O → 2NaOH SO3 : dissolves in H 2O forming a strongly acidic solution with pH = 2 (accept 0 – 3) . SO3 + H2O → H2SO4 (b) (i) Buffer solutions resist pH changes when a small amount of acid or alkali is added. (ii) Small amount of H+: CO3 2− + H+ (or H3O+) → HCO3 − ( + H2O) The H+ added is removed by the large reservoir of CO3 2−. Small amount of OH−: HCO3 − + OH− → CO3 2− + H2O The OH− added is removed by the large reservoir of HCO3 − (iii) Buffer most effective when [HCO 3 −] = [CO 3 2−] (maximum buffering capacity), using Hasselbalch equation, pH = pKa = −lg (5.61 × 10−11) = 10.3 Range of effectiveness = pKa ±1 = 9.3 to 11.3 (iv) By Hasselbalch equation, pH = pKa + lg ][ ]2   3 3 HCO [CO When ][ ]2   3 3 HCO [CO increases from 0.50 to 0.85, pH change = lg (0.85) – lg (0.50) = −0.0706 – (−0.301) = +0.230 OR ratio 0.5, pH = 9.95 ratio 0.85, pH = 10.18 Difference = 10.18 – 9.95 = 0.230 (c) (i) Mr AlCl3 = [27 + 3(35.5)] = 133.5 Mr (AlCl3)n = 267, n = 2 Molecular formula is Al2Cl6. Cl Cl Cl Cl Cl Cl Al Al x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x xx xxxx xx xx xx

2 9647 / YJC / 2012 / JC2 Preliminary Examinations / Paper 3 answers (ii) Al2O3 (iii) When large amount of water added, aluminium chloride forms Al(H2O)6 3+. Al(H2O)6 3+ (+ H2O) ⇌ Al(H2O)5(OH)2+ + H+ (or H3O+) pH = 3.0 (d) (i) Electrophilic substitution (ii) Step 1: Cl2 + AlCl3 → Cl+ + AlCl4 − Step 2: Step 3: (iii) 2 (a) Shape: Tetrahedral Angle: 109.5° (b) (i) (ii) (iii) (iv) CH3 Cl + CH3 Cl H + CH3 Cl H + AlCl4 - CH3 Cl + HCl + AlCl3 ClCl Cl S O O OO xx xx H H x x x x x x x x xx xx xx xx x x x x

3 9647 / YJC / 2012 / JC2 Preliminary Examinations / Paper 3 answers For (iii) and (iv), accept if Br/OH is in 3rd position of cyclic ring instead of 2nd. (c) (i) Eᶱcell = EPbO2/Pb2+ − EPb2+/Pb = +1.47 – (−0.13) = +1.60 V (ii) 2 PbSO4(s) + 2 H2O(l) → Pb(s) + PbO2(s) + H2SO4(aq) + HSO4 −(aq) + H+(aq) OR 2 PbSO4(s) + 2 H2O(l) → Pb(s) + PbO2(s) + 2 H2SO4(aq) OR 2 PbSO4(s) + 2 H2O(l) → Pb(s) + PbO2(s) + 2 HSO4 −(aq) + 2 H+(aq) (iii) As time passes, more of the reactants are being used up, causing the concentration of the reactants (H2SO4) to decrease. As a result, Eᶱcell decreases. (iv) Hydrogen fuel cell . The reactants are constantly replenished as it is supplied from the air, thus e.m.f. remains constant. . (d) (i) Ksp = [Pb2+][CrO4 2−] = s2 = 1.69 × 10−14 mol2 dm−6 (where s is the solubility of PbCrO4.) s = 1.30 × 10−7 mol dm−3 (ii) Precipitation occurs when ionic product