SAJC H2 Chem 2012 Prelim P2 QP

ST ANDREW’S JUNIOR COLLEGE JC2 Preliminary Examinations Chemistry 9647/02Higher 2 Paper 2 10 September 2012 2 hours Candidates answer in the spaces provided on the question paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. Answer all questions. You are reminded of the need for good English and clear presentation in your answers. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use: Question Marks 1 2 3 4 5 Total This document consists of 17 printed pages including this page.

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3 [Turn Over No. Experiment Observations Deductions with relevant balanced equations 1. Add ½ spatula of solid potassium carbonate to each separate samples (1 cm 3) of the 4 solutions in clean, dry test tubes. 2. 1 Planning (P) Four aqueous solutions are provided. Each solution contains one of the following: Aluminium bromide, Zinc sulfate, Silver nitrate, Barium hydroxide You are to plan a series of test-t ube experiments using only the four solutions and solid potassium carbonate, in order to deduce which compound is present. No other reagents should be used. You should commence your plan by adding potassium carbonate to each of the solutions. Your completed plan should include observations, deductions and balanced equations for positive tests. [12]

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5 [Turn Over 2 (a) Transition metals are o ften used to p roduce ornamental stained -glass windows. Iron–based colours and pigments have been used since ancient times for stained-glass windows in European churches. (i) Explain why iron compounds are usually coloured. (ii) Explain why the melting point of iron is significantly higher than that of calcium. Stained-glass windows are darkened from corrosion and age. Microprobe analysis showed that the tiny cracks and defects on the glass surface were enriched with insoluble MnO2. (iii) Using the Data Booklet, suggest a suitable redu cing agent to dissolve MnO2 without reducing iron (III) to iron (II). [7]

6 [Turn Over 2 (b) Iron (III) chloride is often used as a catalyst in the conversion of benzene to produce carbonyl compounds. CO / HCl FeCl3 CHO (i) Write an equation to show the generation of the electrophile to form benzaldehyde. (ii) Explain why iron (III) chloride mus t be anhydrous in order to generate the electrophile. [3]

7 [Turn Over 2 (c) By making use of the reaction in 2(b) and suitable starting materials, suggest a synthetic pathway (in no more than 4 steps) to convert benz ene into the following compound. State clearly the reagents and conditions for each stage, and draw the structural formulae of every intermediate formed. O [4]

8 [Turn Over 2 (d) Iron (III) chloride can also be used in the alkylation of benzene to yield alkyl benzene , for example methylbenzene (toluene) . Toluene is less toxic than benzene, and thus can replace benzene as an aromatic solvent in chemical preparation. Using the following data, ∆Hθ/ kJ mol-1 Enthalpy change of formation of CO2 (g) - 393.5 Enthalpy change of formation of H2O (l) - 285.8 Enthalpy change of combustion of C6H5CH3 (l) - 3910 Enthalpy change of vaporisation of C6H5CH3 (l) + 38 (i) Calculate the standard enthalpy change of formation of toluene, ΔHfθC6H5CH3 (l). (ii) Using your answer to 2 (d)(i), relevant data from the table above and the Data Booklet, draw an appropriate energy cycle to determine the enthalpy change of formation of benzyl radical C6H5CH2• (g).

9 [Turn Over 2 (d) (iii) Hence, deduce the thermodynamic stability of the product, benzyl radical C6H5CH2• as compared to its reactants. Explain. [7] [Total: 21 marks] 3 (a) Chelation is the formation of coordinate bonds between a ligand and a single central atom or metal ion. The ethanedioate ion (C 2O42–) is an example of a chelating ligand. Two complexes which are successfully synthesised are [Al(C2O4)3]3– and [Al(C2O4)2]–. (i) Draw a dot -and-cross diagram for C 2O42–. Hence, suggest the bond angle about either carbon atom. (ii) State the coordination number of aluminium in the complex [Al(C2O4)3]3–. [3]

10 [Turn Over 3 (b) Aluminium is able to form ethanedioate complex such as K 2n-3[Al(C2O4)n]. To determine the identity of the complex, the complex is first decomposed in an acidic solution to give ethanedioic acid and then titrated against potassium manganate (VII). Al(C2O4)n(3-2n) + n H2SO4 ® n H2C2O4 + Al3+ + n SO42– (n = 2 or 3) 2 H+ + 2 CO2 + 2 e ⇌ H2C2O4 E = - 0.49 V (i) Use the Data Booklet to explain how the results of titration would differ when HCl is used instead of H2SO4 in Step 3. (ii) By using collision theory of reaction kinetics, e xplain why the solution needs to be warmed in Step 4. [4] Volumetric Analysis for ethanedioate content Step 1 Weigh 1.77 g of the complex, K2n-3[Al(C2O4)n], in a 50 cm3 beaker. Step 2 Dissolve it in deionised water and transfer to a 250 cm 3 volumetric flask and top up to the mark with deionised water. Step 3 Pipette 25.0 cm3 of the solution into a conical flask and add about 40 cm 3 of aqueous H2SO4. Step 4 Warm the solution to about 50 0C. Step 5 Titrate the contents of the flask with potassium manganate (VII) from a burette until the first pink colour appears.