RI 2024 Periodic Table 1 Tutorial Self-Check Answers

-1- Raffles Institution Year 5 H2 Chemistry 2024 Tutorial 12 – The Periodic Table 1 Answers to Self–Check Questions 1 A 2 D 3 D 4 B 5 (a) MgCl2 has an ionic structure. Its melting point is high as large amount of energy is required to break the strong electrostatic forces of attraction between oppositely charged Mg 2+ and Cl- ions. SiCl4 has a simple molecular structure . It is a volatile liquid (i.e. has low boiling point) as a relatively small amount of energy is required to overcome the weak instantaneous dipole –induced dipole interactions between its molecules. (b) Ba2+ and Mg2+ have the same charge but the ionic radius of Ba2+ is larger than that of Mg2+. This results in the lattice energy of BaO being less negative than that of MgO. A larger amount of energy is needed to break the lattice of MgO than BaO. Hence MgO is insoluble in water while BaO is soluble in water. Barium oxide dissolves in water to form Ba2+ and O2–. The strongly basic O2– ions cause the solution to be alkaline. O2–(aq) + H2O(l) → 2OH–(aq) Sulfur trioxide is soluble in water. It reacts with water to form H2SO4, which is a strong acid. SO3(g) + H2O(l) → H2SO4(aq) 6 (a) Sodium has 1 valence electron while oxygen has 6 valence electrons. Na loses 1 electron to become Na+ while O gains 2 electrons to become O 2−. To maintain electrical neutrality, the formula of sodium oxide is Na2O. [1] Silicon has 4 valence electrons. 1 Si atom shares one electron each with 4 O atoms while 1 O atom shares 1 electron each with 2 Si atom. Hence, silicon (IV) oxide has the formula of SiO2. [1] (Note: Do not explain in terms of oxidation numbers. The assignment of oxidation numbers for these elements is a result of the formulae of their oxides. Hence, the oxidation numbers cannot be used to justify these formulae.) (b) Sodium oxide dissolves in water to form sodium hydroxide solution: Na2O(s) + H2O(l) → 2NaOH(aq) [1] Since NaOH is a strong alkali, the resultant solution has a pH of about 13 or 14. Phosphorous oxide reacts readily with water to form phosphoric(V) acid: P4O10(s) + 6H2O(l) → 4H3PO4(aq) [1] Since H3PO4 is a weak acid, the resultant solution has a pH of about 2. [1] for both pH qqL.E. r +r −+ −+

-2- 7 (a) & (b) Sodium Chloride Magnesium Chloride Aluminium Chloride Silicon Chloride NaCl MgCl2 AlCl3 or Al2Cl6 SiCl4 [2] Ionic bonding Covalent bonding [2] Note: For MgCl2, it is ionic with some covalent character. (c) NaCl(s) dissolves when added to water. Hydration occurs. [1] Hydration: NaCl(s) + aq → Na+(aq) + Cl–(aq) Both MgCl2(s) and AlCl3(s) dissolves when separately added to water. Both hydration and hydrolysis occur. Hydration: MgCl2(s) + 6H2O(l) → [Mg(H2O)6]2+(aq) + 2Cl–(aq) Slight Hydrolysis: [Mg(H2O)6]2+(aq) ⇌ [Mg(H2O)5(OH)]+(aq) + H+(aq) [1] OR [Mg(H2O)6]2+(aq) + H2O(l) ⇌ [Mg(H2O)5(OH)]+(aq) + H3O+(aq) Hydration: AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl–(aq) Appreciable Hydrolysis: [Al(H2O)6]3+(aq) ⇌ [Al(H2O)5(OH)]2+(aq) + H+(aq) [1] OR [Al(H2O)6]3+(aq) + H2O(l) ⇌ [Al(H2O)5(OH)]2+(aq) + H3O+(aq) [With limited amount of water (e.g. a few drops of water): AlCl3(s) + 3H2O(l) ⇌ Al(OH)3(s) + 3HCl(g)] Note: Cations (e.g. Mg2+ and Al3+ ions) with high charge density and hence high polarising power are able to distort the electron cloud of the water molecules, weakening the O –H bond. In the presence of water, the O–H bond undergoes heterolytic fission readily to release H +. Since Al3+ has a higher charge density than Mg2+, it undergoes hydrolysis to a greater extent than Mg2+. Hence the resultant solution when MgC l2 is added to water has a pH of about 6.5 while that of A lCl3 has a pH of about 3. Hydrolysis occur when SiCl4(l) is added to water. Hydrolysis: SiCl4(l) + 2H2O(l) → SiO2(s) + 4HCl(aq) [1] (d) (i) Element S Cl Empirical Formula is SCl2. [1] % mass 31 69 Ar 32.1 35.5 Amount 0.9657 1.944 Mol ratio 1 2 (ii) The shape of the molecule is bent due to the two lone pairs and two bond pairs of electrons around the S atom. [1] (iii) It has a simple molecular structure / It is a covalent chloride and is expected to undergo hydrolysis readily in water to give an acidic solution of pH 1–2 due to the formation of HCl. [1] S Cl Cl