CJC H2 Chem 2013 Prelim P2 Soln

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2 9647/02/CJC JC2 Preliminary Exam 2013 1 Planning (P) Natural water that contains calcium and magnesium ions is said to be 'hard'. Tap water in England is hard water, which is evident from the limescale found in kettles. The main source of calcium and magnesium ions in hard water is limestones. However, neither calcium carbonate nor magnesium carbonate is appreciably soluble in water. It is said that the solubility is partly the result of carbon dioxide present in rain water. You are to design an experiment to investigate whether the presence of carbon dioxide in water, indeed, increases the solubility of magnesium carbonate. In addition to the standard apparatus available in a school laboratory, you are provided with the following materials,  magnesium carbonate  distilled water  distilled water saturated with carbon dioxide  methyl orange indicator  phenolphthalein indicator  standard hydrochloric acid (a) Write an expression for the solubility product, Ksp, of magnesium carbonate, stating its units. ……………………………………………………………………………………………....[1] (b) When a solution is saturated, the undissolved solid is in equilibrium with its aqueous solution. X(s) + aq X(aq) Describe briefly how you would prepare a saturated solution of magnesium carbonate at room temperature. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………. [3]  K sp = [Mg2+] [CO3 2–] mol2 dm–6  Dissolve solid in water until some solid remains undissolved.  Allow solution to stand for few hours (to establish equilibrium).  Filter to remove undissolved solid. The saturated solution is collected as the filtrate. 416

3 9647/02/CJC JC2 Preliminary Exam 2013 (c) The concentration of a saturated solution of magnesium carbonate may be determined by titration with standard hydrochloric acid. (i) Given that, at room temperature, Ksp of magnesium carbonate has a numerical value of 6.82 × 10 –6, suggest an appropriate concentration of the standard hydrochloric acid to be prepared. Show your working. (ii) Name the indicator used in this titration. ……………………………………………………………………………………….. [4] (d) Describe what further experiments you woul d carry out to show that solubility of magnesium carbonate is increased by the presence of carbon dioxide in water. State clearly the volume of solution used and the expected results. [3] (e) Recently phenolphthalein is classified as a carcinogen and is banned from use in the school laboratory. Suggest a suitable replacement for phenolphthalein indicator from the list below. indicator colour (lower pH) pH range colour (upper pH) congo red blue 3.0 – 5.0 red methyl red red 4.8 – 6.0 yellow phenol red yellow 6.4 – 8.2 red-violet cresol red yellow 7.1 – 8.8 violet ……………………………………………………………………………………………… [1] [Total: 12]  for a saturated solution of MgCO3, [Mg2+] [CO3 2–] = Ksp   [CO3 2–] = spK (since [Mg2+] = [CO3 2–] = [MgCO3]) = 610 82 . 6 = 2.61 × 10–3 mol dm–3  MgCO 3 + 2 HCl  MgCl2 + CO2 + H2O  For volume of analyte to be  volume of titrant at end-point,  [HCl ] = 2 × [MgCO3] = 2 × 2.61 × 10–3 = 5.22 × 10–3 mol dm–3  a suitable [HCl ] = 5.22 × 10–3 mol dm–3 methyl orange indicator OR 2MgCO3 + 2HCl  MgCl2 + Mg(HCO3)2  [HCl ] = [MgCO3] = 2.61 × 10–3 mol dm–3 (ii) phenolphthalein indicator cresol red  Prepare a saturated solution of MgCO 3 in water saturated with CO2.  Pipette 25.0 cm 3 of the saturated solution of MgCO3 prepared above (into a 250 cm3 conical flask). Add (3 drops of) methyl orange indicator. Titrate with standard / 5.22 × 10–3 mol dm–3 HCl (placed in a 50 cm3 burette) until the solution in the conical flask changed from yellow to orange. Expected results: vol of HCl > that for saturated solution of MgCO3. 417

4 9647/02/CJC JC2 Preliminary Exam 2013 2 Ethylbenzene is a highly flammable and colourless liquid. It is important in the petrochemical industry as an intermediate in the production of styrene which is a precursor for polystyrene, a common plastic material. Ethylbenzene has a characteristic gasoline odour with an odour threshold, which is the lowest concentration of a compound detectable by smell, of 2.3 parts per million (ppm). It also has density of 0.866 g cm–3 and boiling point of 136 °C. (a) Ethylbenzene can be produced by combining ethene with benzene in an acid- catalysed reaction as shown below. (i) Name the mechanism for this reaction. ………………………………………………………………………………………….. (ii) The first step of the mechanism involves the reaction between A lCl 3 and HCl . Show clearly how ethene interacts with the species formed from the first step to generate the attacking species for subsequent reaction with benzene. You should include in your answer curly arrows to depict electron movement and all charges. (iii) Suggest why 1,4-diethylbenzene is also formed in the overall reaction. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. (iv) If a room has dimensions of 5 m x 4 m x 3 m, calculate the minimum mass of ethylbenzene that would be present before its odour is detected. [6] Electrophilic substitution Volume of room = 5 x 4 x 3 = 60 m3 For odour detection, minimum vol of ethylbenzene = 610 3 . 2x 60 = 1.38 x 10–4 m3  minimum mass of ethylbenzene = (0.866 x 106) x 1.38 x 10–4 = 119.5 g Presence of electron donating / activating ethyl group makes ethylbenzene even more susceptible to further electrophilic attack at position 4 on benzene, thereby forming the disubstituted compound. 418

5 9647/02/CJC JC2 Preliminary Exam 2013 (b) Ethylbenzene can reversibly undergo catalytic dehydrogenation at 600 oC and 0.5 bar in the presence of a solvent to form styrene. [1 bar = 1 x 105 Pa] H > 0 (i) State two main assumptions of the kinetic theory of gases. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. ………………………………………………………………………………………….. (ii) Under the conditions used for the reaction, 0.0200 g of the equilibrium mixture occupies 38.0 cm 3. Calculate the average relative molecul