RI 2022 Y6 CT Suggested Solutions

1 © Raffles Institution 2022 9729/J/22 2022 Y6 H2 Chemistry Term 3 Common Test – Suggested Solutions Section A Question 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Answer D B C B A A A D B C A B C D C MCQ worked solutions 1 IO3– + 5I− + 6H+ ⎯→ 3I2 + 3H2O I2 + 2S2O32− ⎯→ 2I− + S4O62− Mole ratio of IO3– : I2 : S2O32− = 1 : 3 : 6 Amount of S2O32− required = 6  amount of IO3– used = 6  25.0 1000  0.100 = 0.0150 mol [S2O32−] = 0.0150 20.00  1000 = 0.750 mol dm−3 2 Group 2 metals are oxidised when they react with oxygen. The more easily the Group 2 metals are oxidised, the greater their reactivity with oxygen. A Atomic radius of the metal atom (not ionic radius of metal cation) can be used to explain the trend. Down the group, the valence electrons are further away from the nucleus and are more easily lost, resulting in greater ease of oxidation of the metal. B As the reducing power of the Group 2 metals increases down the group, the more likely the Group 2 metals are oxidised and hence the greater their reactivity with oxygen. C Although the electronegativity of the elements decreases down the group, this does not explain why the Group 2 metals oxidise more easily down the group. D Although the magnitude of LE of the metal oxides decreases down the group, this does not explain why the Group 2 metals oxidise more easily down the group.

2 © Raffles Institution 2022 9729/J/22 3 Given their low m.p., elements C to F should be simple molecules with weak intermolecular forces of attraction. Hence, elements C to F should be in Group 15 to 18, and elements A to F should be in the same period. As the variation in m.p. of elements C, D and E corresponds to that of P4, S8 and Cl2, elements A to F must be in Period 3 and elements G to H in Period 4. Based on the melting point trend, the identities of the elements are as follows: Element A B C D E F G H Identity Al Si P S Cl Ar K Ca Type of structure Giant metallic Giant molecular Simple molecular Giant metallic Statement 1 Incorrect. SiO 2 is insoluble in water as a large amount of energy is needed to break the strong Si–O covalent bonds in the giant molecular structure. Statement 2 Correct. Aluminium and chlorine react to form AlCl3, which hydrolyses in water to give an acidic solution of approximately pH 3. AlCl3(s) + 6H2O(l) → [Al(H2O)6]3+(aq) + 3Cl–(aq) [Al(H2O)6]3+(aq) ⇌ [Al(H2O)5(OH)]2+(aq) + H+(aq) Statement 3 Correct. Phosphorus and sulfur are solid s at room temperature. Phosphorus can form oxides like P4O6 and P4O10 while sulfur can form oxides like SO2 and SO3. 4 Statement 1 Incorrect. From step 2 (slow step), rate  [B][C]. However, C is an intermediate which does not appear in the rate equation for the overall reaction. From step 1, [C] [A]2. Hence, rate  [A]2[B]. The correct rate equation should be rate = k[A]2[B]. Statement 2 Correct. step 1 2A ⇌ C step 2 B + C