2019 VJC H2 Chem Prelim P3 Ans

1 Victoria Junior College 2019 H2 Chemistry Prelim Exam 9729/3 Suggested Answers Section A Answer all the questions in this section. 1 (a) Account for the reactions that occur when MgC l2 and PCl5 are separately dissolved in water. Predict the pH of the resulting solutions formed and write equations for the reactions that occur. [4] Mg2+ has high charge density due to small ionic radius (and high charge). Mg2+ has high polarising power and hence MgC l2 undergoes hydration and partial hydrolysis, producing a slightly acidic solution. MgCl2 + 6H2O  [Mg(H2O)6]2+ + 2Cl– hydration [Mg(H2O)6]2+ + H2O ⇌ [Mg(H2O)5OH]+ + H3O+ hydrolysis pH of resulting solution = 6.5 PCl5 undergoes complete hydrolysis due to the presence of energetically accessible vacant 3d orbitals on phosphorus which can accommodate lone pair from water molecules. The solution produced is strongly acidic. PCl5 + 4H2O  H3PO4 + 5HCl pH of resulting solution = 2 (b) A sample consists of a solid mixture of MgO and Al2O3. Describe briefly an experimental procedure that will enable you to separate the mixture and recover each of the oxides in its pure form. [3] 1. Add excess NaOH(aq) to the sample. 2. Filter the resulting mixture. The residue collected is mainly MgO. 3. Wash the residue with distilled water and press between pieces of filter paper to obtain dry solid MgO. 4. To the filtrate, add HCl(aq) dropwise till the maximum mass of precipitate is formed. 5. Filter the resulting mixture. The residue collected is mainly Al(OH)3. 6. Wash the residue with distilled water and press between pieces of filter paper to obtain dry solid Al(OH)3. 7. Heat the residue till constant mass to obtain dry solid Al2O3. [Note: Al(OH)3 is expected to decompose on heating to form Al2O3, similar to Group 2 hydroxides.] (c) The highest fluoride of xenon, XeF6, can be obtained by heating the octafluoroxenates of the Group 1 metals, M2XeF8, where M represents the Group 1 metal. M2XeF8 → 2MF + XeF6 Suggest reasons why the sodium salt (M = Na) decomposes below 100 °C, whereas the caesium salt (M = Cs) requires a temperature of 400 °C. Hence explain why MgXeF 8 is not known to exist. [3]

2 Down group 1, as ionic radius increases, charge density of cations decreases, polarising power of cations decreases, which polarises the XeF82– anion to a lesser extent. The anion becomes more thermally stable and hence higher temperature is required to decompose caesium salt compared to sodium salt. Mg2+ has higher charge density than Na+, making MgXeF8 very unstable and hence does