HCI Prelim P2

This document consists of 20 printed pages. HWA CHONG INSTITUTION C2 Preliminary Examinations Higher 2 CANDIDATE NAME CT GROUP 14S CENTRE NUMBER INDEX NUMBER CHEMISTRY Paper 2 Structured Questions Candidates answer on the Question Paper Additional Materials: Data Booklet 9647/02 2 September 2015 2 hours INSTRUCTIONS TO CANDIDATES 1) Write your name, CT group, centre number and index number clearly in the spaces at the top of this page. 2) Answer all questions in the spaces provided in this Question Paper. INFORMATION FOR CANDIDATES The number of marks is given in brackets [ ] at the end of each question or part question. A Data Booklet is provided. Do not use staples, paper clips, highlighters, glue or correction fluid. You may use a calculator. You are reminded of the need for good English and clear presentation in your answers. For Examiner’s Use 1 / 12 2 / 10 3 / 17 4 / 9 5 / 9  6 / 15  Deductions Total / 72       Calculator Model:

© Hwa Chong Institution 2015 9647 / 02 / C2 Prelim 2015 2 1 Planning (P) For Examiner's use When excess ammonia is added to an aqueous solution containing transition metal ion M2+, a complex ion [M(NH3)n]2+, where n has a maximum value of 6, is formed. When this solution is shaken with trichloromethane (density = 1.49 g cm–3), an equilibrium mixture consisting of two immiscible layers is obtained. The aqueous layer contains the complex ion and uncomplexed ‘free’ ammonia while the organic layer contains only ‘free’ ammonia. The ratio of the concentrations of ‘free’ ammonia in the two immiscible layers at equilibrium is a constant, known as the partition ratio, K. NH3(aq) ⇌      NH3(trichloromethane) K=[NH3(trichloromethane)][NH3(aq)]=0.4 To determine the value of n in [M(NH3)n]2+, the equilibrium mixture is prepared. A sample is withdrawn from the organic layer and the ‘free’ ammonia is titrated with standard hydrochloric acid. As the aqueous acid is unable to dissolve in the organic layer, an equal volume of water is added to the organic layer before titration so that the ‘free’ ammonia can be neutralised by the acid. The titration results can be used to determine the concentration of ‘free’ ammonia in the organic and aqueous layers. The amount of ammonia used to form the complex ion and hence the value of n can then be determined. (a) Explain why the value of the partition ratio, K, of ammonia between trichloromethane and water is less than 1. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………[1] (b) With the aid of an equation, explain why the concentration of ‘free’ ammonia in the aqueous layer cannot be determined by titrating the aqueous layer with standard hydrochloric acid. ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………[2]

© Hwa Chong Institution 2015 9647 / 02 / C2 Prelim 2015 3 (c) You are required to write a plan to determine the formula of the complex ion, [M(NH3)n]2+. You are provided with: • aqueous ammonia, of concentration 1.0 mol dm–3 • aqueous M2+, of concentration 0.10 mol dm–3 • hydrochloric acid, of concentration 0.050 mol dm–3 • trichloromethane • the apparatus and chemicals normally found in a school or college laboratory. Your plan should give details of: • the preparation of the equilibrium mixture containing the complex ion using 50.0 cm3 of 0.10 mol dm–3 M2+ and appropriate volumes of aqueous ammonia and trichloromethane; • withdrawal of sample(s) from the organic layer; • the titration of the ‘free’ ammonia in the organic layer. For Examiner's use ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ………………………………………………………………………………………………………………………………………………………………………………………………………………

© Hwa Chong Institution 2015 9647 / 02 / C2 Prelim 2015 4 ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………… ……………………………………………………………………………………………………[7] For Examiner's use 1 2 3 4 5 6 7 (d) In one experiment, an equilibrium mixture was prepared using 1 dm3 of 1.0 mol dm–3 aqueous ammonia, 1 dm3 of 0.10 mol dm–3 aqueous M2+ and 1 dm3 of trichloromethane. The concentration of ‘free’ ammonia in the organic layer was found to be x mol dm–3. Express the concentration of ‘free’ ammonia in the aqueous layer and the value of n in terms of x.   [2] [Total: 12]

© Hwa Chong Institution 2015 9647 / 02 / C2 Prelim 2015 5  2 This question is about the chemistry of Kuro Tamago (lit. “black eggs”), a local specialty of egg hard-boiled in the hot springs of Owankundai, Japan. When raw eggs are boiled in hot spring water of Owankundai (pH = 9.2), a chemical reaction between aqueous iron(II) and sulfide ions (from hydrogen sulfide, H2S), produces a black solid, FeS, that adheres to the porous egg shells. Iron sulfide is precipitated by the following reaction. reaction 1: Fe2+(aq) + S2–(aq) →        FeS(s) Hydrogen sulfide gas from volcanic systems is released into the hot spring water and behaves as a dibasic (diprotic) weak acid. H2S(g) + aq ⇌ 2H+(aq) + S2–(aq) In a saturated solution of hydrogen sulfide, [H+]2[S2–] = 1.0 × 10–23 mol3 dm–9 For Examiner's use (a) (i) Calculate the maximum concentration of sulfide ions present in hot spring water. [2] (ii) Hence, calculate the minimum concentration of Fe2+ present in hot spring water in order for precipitation to occur. (Ksp of FeS = 4.9 × 10–18 mol2 dm–6) [1] ΔHppt

© Hwa Chong Institution 2015 9647 / 02 / C2 Prelim 2015 6 (b) Use the following data, together with relevant data from the Data Booklet, to calculate the for reaction 1. standard enthalpy change of formation of FeS(s) –102 kJ mol–1 standard enthalpy change of atomisation of Fe(s) +415 kJ mol–1 standard enthalpy change of atomisation of S(s) +279 kJ mol–1 sum of first two electron affinities of sulfur +337 kJ mol–1 enthalpy change of hydration of Fe2+(g) –1981 kJ mol–1 enthalpy change of hydration of S2–(g) –1372 kJ mol–1 [2] For Examiner's use (c) When a precipitate is formed, , in J mol–1, is given by the following expression. (i) Use the data given in (a)(ii) to calculate , in kJ mol–1, for FeS. [1] ΔGppt= 2.303RT log KspΔGpptΔGppt ΔHppt

© Hwa Chong Institution 2015 964