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 your answer briefly. [1] (iii) Use the  ideal gas equation and  van der Waals equation to calculate the pressure exerted by 1 mol of CO 2 at a temperature of 30 °C and volume of 1 dm3. [3] [Total: 20]

3  VJC 2019 9729/03/PRELIM/19 [Turn over 2 (a) Malonic acid, CH 2(CO2H)2 is an organic weak dibasic acid . It is a building block chemical to produce numerous valuable compounds, including the flavo ur and fragrance compound, cinnamic acid, and the pharmaceutical compound , valproate. The two pKa values of CH2(CO2H)2 are 2.83 and 5.69. (i) Define the term weak acid. [1] (ii) Calculate the pH of 25.0 cm3 solution of 0.100 mol dm–3 CH2(CO2H)2. [1] (iii) Calculate pH of the resulting solution when 50 cm 3 of 0.100 mol dm –3 NaOH was added to the solution in (a)(ii). [2] (iv) Using your answer s in (a)(ii) and (a)(iii), as well as the pKa values provided, sketch a graph to show how the pH of the solution changes as 50 cm 3 of 0.100 mol dm –3 NaOH is gradually added to 25.0 cm 3 of 0.100 mol dm –3 CH2(CO2H)2. Clearly indicate the corresponding volumes of NaOH in your graph. [2] (b) Malonic acid can be converted to its corresponding β–diester. β–diesters are commonly used as starting compounds in the Michael addition reaction, where they react with α,β-unsaturated ketones. It is one of the most useful methods for the formation of C–C bonds. (i) Suggest reagents and conditions to convert malonic acid to dimethyl malonate, CH2(COOCH3)2. State the type of reaction. [2] (ii) The fi rst step in the mechanism of Michael addition involves an acid base reaction where the strong base catalyst extracts an α-hydrogen from the β–diester. Reagents similar to the malonate ester can undergo the same type of reaction. The pKa values of malonate ester and another similar reagent are as follows: pKa β-ketoester 11 β-diester 13 Explain the difference in pKa values between the two compounds. [2]

4  VJC 2019 9729/03/PRELIM/19 [Turn over (iii) Compound A, C8H9ClO, contains a non-aromatic six-membered ring. A reacts with 2,4-dinitrophenylhydrazine to form an orange precipitate but does not react with Tollen’s reagent. 1 mole of A reacts with 3 moles of H2 gas in the presence of solid platinum. When A is warmed with aqueous sodium hydroxide, compound B, C8H10O2 is formed. B gives a pale yellow precipitate when warmed with alkaline aqueous iodine. When B is warm ed with acidified potassium permanganate, compounds C, C3H2O5 and D, C5H6O5 are formed. D also gives a pale yellow precipitate when warmed with alkaline aqueous iodine. A is able to undergo Michael addition with dimethyl malonate, CH2(COOCH3)2, to form E, a compound with 18 carbons. Deduce the structural formulae of compounds A, B, C, D and E, explaining clearly your reasoning for all reactions described. [10] [Total: 20]

5  VJC 2019 9729/03/PRELIM/19 [Turn over 3 (a) A Latimer diagram provides a concise way of representing large amount of information about the different oxidation states of an element. In a Latimer diagram, the most highly oxidised form of an element is written on the left, with successively lower oxidation states to the right. The different species are connected by arrows, and the standard electrode potential in volts is written above each arrow. The Latimer diagrams for chlorine in acidic and alkaline medium are shown below. In acidic medium: In alkaline medium: (i) Define the term standard electrode potential. [1] (ii) The standard electrode potentials in a Latimer diagram are not additive . For example, the standard electrode potential for converting C lO4– to C lO– in acidic medium is not the sum of +1.19 V and +1.21 V and +1.66 V. However, their respective standard Gibbs’ free energy changes are additive. Using relevant data given below, show that the standard electrode potential for converting ClO4– to ClO– in acidic medium is 1.34 V. electrode reaction Eo / V ∆Go / kJ mol–1 ClO4– + 2H+ + 2e– ⇌ ClO3– + H2O +1.19 –220.7 ClO3– + 2H+ + 2e– ⇌ ClO2– + H2O +1.21 –233.5 ClO2– + 2H+ + 2e– ⇌ ClO– + H2O +1.66 –320.4 ClO– + 2H+ + e– ⇌ ½Cl2 + H2O +1.64 –158.3 ½Cl2 + e– ⇌ Cl– +1.36 –131.2 [2] (iii) With the exception of the conversion of Cl2 to Cl–, the standard electrode potentials in alkaline m