RI 2019 Prelim P2 Ans

-1- Raffles Institution Preliminary Examination 2019 H2 Chemistry Paper 2 (Suggested Answers and Comments) 16 Sep 2019 Q1 (a) Assume 100 g of X. Si N H Mass / g 48.3 48.1 3.6 Amount / mol 48.3 28.1 = 1.72 48.1 14.0 = 3.44 3.6 1.0 = 3.60 Molar ratio 1 2 2 Empirical formula of X is SiN2H2. Comments • Generally well done. Q1 (b) The first ionisation energy of P is higher than that of Si. P has one more proton and hence a higher nuclear charge than Si. Though P also has one more electron than Si, the increase in shielding effect is minimal since this additional electron occupies the outermost shell. Consequently, the valence electrons in P experience a higher effective nuclear charge and are more strongly attracted by the nucleus. Comments • Most students were able to describe the trend across the period well. The key ideas of increased nuclear charge, relatively similar shielding effect and increased effective nuclear charge were all stated. • Students should be more concise. For ionization energy, the key consideration should be simply, i) nuclear charge, ii) shielding effect, hence, iii) effective nuclear charge, and attraction for valence electrons. Q1 (c) Al2O3(s) + 6HCl(aq) ⎯→ 2AlCl3(aq) +3H2O(l Or Al2O3(s) + 6H+(aq) ⎯→ 2Al3+(aq) +3H2O(l) Al2O3(s) + 2NaOH(aq) + 3H2O(l) ⎯→ 2Na[Al(OH)4](aq) Or Al2O3(s) + 2OH–(aq) + 3H2O(l) ⎯→ 2[Al(OH)4]–(aq) SiO2(s) + 2NaOH(aq) → Na2SiO3(aq) + H2O(l) Or SiO2(s) + 2OH–(aq) → SiO32–(aq) + H2O(l) Comments • Students are reminded to be familiar with all chemical reactions of the Period 3 oxides and chlorides. • This question was poorly answered. Students need to manage the knowledge of inorganic chemistry with understanding and technique, not just rote, as poor recall means inability to answer these standard questions. • Firstly, the acid base properties of oxides across period tends from basic to acidic (correlating to structures: ionic to covalent) . Thus, Al2O3, which has ionic with covalent character, is amphoteric. • Secondly, basic oxides neutralise acids. Acidic oxides neutralise bases. Amphoteric oxide neutralizes both acids and bases. • Thirdly, writing equations involve the following technique: i) The formula of the “salt” needs to be recalled. ii) Balance non-H and non-O atoms first (eg, Na, Al, P, SI)

-2- iii) Balance charges with H+ or OH– if ionic equation is written iv) Always balance the H and O atoms last using H2O Rather than memorise and recall equations (with so many coefficients to note), it is better to know the type of reaction, the expected products, and balance with proper technique. Following steps above i) Identify salts: Al2O3 -> Na[Al(OH)4] ii) Balance Na, Al: Al2O3 + 2NaOH -> 2Na[Al(OH)4] iii) No need. iv) Balance H and O with H2O Al2O3 + 2NaOH + 3H2O -> 2Na[Al(OH)4] Q1 (d) P4O10 has a s