2019 VJC H2 Chem Prelim P3 QP

2  VJC 2019 9729/03/PRELIM/19 [Turn over 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] (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] (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 ex plain why MgXeF8 is not known to exist. [3] (d) Suggest identities for the following substances A to D, writing equations where appropriate. When magnesium is heated with nitrogen under inert conditions, an ionic compound, A is produced. When water is added to A, a colourless gas B which turns damp red litmus paper blue is produced. B reacts with chlorate(I) ion, ClO– in a 2 : 1 mole ratio to form a colourless liquid C with empirical formula NH2. The reaction of C with sulfuric acid in a 1:1 mole ratio produces a salt D, N 2H6SO4, which contains one cation and one anion per formula unit. [4] (e) Real gases do not obey the ideal gas equation exactly. Many chemists have tried to come up with gas equations that describe the behaviour of real gases. In 1873 J D van der Waals introduced an approximate gas equation that is applicable for all real gases. The van der Waals equation is P = nRT V−nb − a n2 V2 where a and b are constants which are characteristic of each gas. The other symbols carry their usual meaning and units as in the ideal gas equation. (i) Using what you have learnt about the differences between ideal and real gases, suggest what the constants a and b represent. [2] (ii) The values of the constants a and b for CO 2 are a = 0.3658 Pa m 6 mol–2 and b = 4.29 x 10–5 m3 mol–1. Use your answer in (e)(i) to suggest how the value of the constant a for xenon (Xe) will compare with CO2. Explain yo