CJC H2 CHEM P3 ANSWER Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CANDIDATE NAME CLASS 2T CHEMISTRY 9647/03 Paper 3 Free Response THURSDAY 25 AUGUST 2011 2 Hours Candidates answer on separate paper. Additional Materials: Answer Paper Data Booklet READ THESE INSTRUCTIONS FIRST Write your name and class on all the work you hand in. Write in dark blue or black pen on both sides of the paper. [PILOT FRIXION ERASABLE PENS ARE NOT ALLOWED] 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 any four questions. A Data Booklet is provided 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 of the question. At the end of the examination, fasten all work securely together. ANSWERS This document consists of 11 printed pages and 1 blank page. [Turn over]

2 9647/03/CJC JC2 Prelim Exam 2011 Answer any four questions. 1 Nanochemistry is an important field of science that explores the use of extremely small particles that have diameters about thousand time s thinner than the width of a human hair. Transition metals and its compounds are often the subject of interest in nanochemistry, as many useful properties are improved at this ultra small scale. (a) Nano-sized iron( III) hydroxide, Fe(OH) 3 exists as a precipitate and can be used as a catalyst to remove aqueous arsenate( V) ions, AsO 4 3- from contaminated water. The Fe(OH)3 with the adsorbed arsenate( V) ions are filtered off, and undergoes an ion- exchange with the arsenate( V) ions removed and Fe(OH) 3 precipitate recovered unchanged. (i) State the electronic configuration of iron in Fe(OH)3. 1s22s22p63s23p63d5 or [Ar]3d5 (ii) State the type of catalysis and briefly explain why transition metal compounds are suitable for such catalysis. Fe(OH)3 act as a heterogenous catalyst. As transition metal compounds has empty d orbitals which can accept the lone pair of electrons from reactant species, and allow adsorption onto the transition metal compounds. (iii) Suggest why the use of nano-sized particles will improve this catalytic property. Nano-scaled particles have a larger surface area resulting in more\faster adsorption of the arsenate ions. [4] (b) Nano-sized iron(II) hydroxide exists as a complex solid with the formula Fe(H 2O)4(OH)2 and has no net dipole moment. (i) State the colour and coordination number of the complex iron( II) hydroxide. Green\Dirty Green\Grey Green. Coordination number is Six. (ii) Suggest the structure of the complex solid, given that it is a non-polar species. Fe OH2 OH OH2 OH2 HO OH2 II H2O H2O : : : : : : [4]

3 9647/03/CJC JC2 Prelim Exam 2011 (c) Nano-sized iron(II) hydroxide, Fe(OH)2 is often unstable, due to oxidation by water even though oxygen is not present. The pH of the solution remains largely unchanged, a reddish-brown precipitate and effervescence are observed. (i) Identify the reddish-brown precipitate and suggest an equation for this reaction. Fe(OH)3 2Fe(OH)2 + 2H2O  2Fe(OH)3 + H2 (ii) The reaction is not expected to occur under standard conditions and can be considered reversible. Suggest a possible reason why it does in fact occur for an opened bottle of Fe(OH) 2 suspended in water. As H2 is formed, it is constantly removed from the system, hence the position of equilibrium will shift to the right, resulting in the reaction occurring. [3] (d) Nano-sized iron( II) hydroxide, Fe(OH) 2 is used for the purpose of removal of toxic selenite(IV) ions, SeO3 2- from water, by reducing selenite(IV) ions to selenium, Se. (i) Construct a half-equation for the reduction of SeO 3 2- to Se in an alkaline solution. 4e - + SeO3 2- + 3H2O  Se + 6OH- (ii) In water that is heavily contaminated with CN - ions, iron( II) hydroxide appears to dissolve. State the type of reaction that is occurring and construct a balanced chemical equation for it. Ligand displacement has occurred, forming soluble [Fe(CN)6]4- ions. Fe(OH)2 + 6 CN-  [Fe(CN)6]4- + 2OH- or Fe(H 2O)4(OH)2 + 6 CN-  [Fe(CN)6]4- + 2OH- + 4H2O (iii) By selecting E o values from the Data Booklet, and given that the redox potential of the reaction in (i) is – 0.37 V, explain and justify if SeO 3 2- can be removed from water contaminated with CN- ions. [Fe(CN)6]3- + e- ⇌ [Fe(CN)6]4- Eo = +0.36V = – 0.37 – (+0.36V) = –0.73 V < 0,hence the reaction is not spontaneous. Thus removal of SeO3 2- ions cannot occur. [5] (e) Electrochemistry is also widely used in the synthesis of nano-sized materials. For instance, anodising of aluminium can be conducted to produce extremely small aluminium oxide.

4 9647/03/CJC JC2 Prelim Exam 2011 Ka Describe with the aid of a labelled diagram, the set-up used during anodising of aluminium, and the reactions occurring at each electrode. [4] [Total: 20] 2 (a) Vitamin C, C 6H8O6, is also known as ascorbic acid. It is a monobasic acid and has a pKa of 4.10. When a 500 mg tablet of Vitamin C is swallowed, it dissolves in the stomach before being absorbed into the bloodstream. The stomach may be assumed to contain 1.0 dm3 of 0.10 mol dm-3 hydrochloric acid (i) Estimate the percentage of Vitamin C that is ionised in the stomach under these conditions. State the assumptions made during the calculation. [C6H8O6] = 176 10500 3 mol dm-3 = 2.84 10-3 mol dm-3 pKa = -log Ka = 4.10  Ka = 10-4.10 = 7.94  10-5 Vitamin C is a weak, monobasic acid, HA; HA ⇌ H+ + A- (2.84 10-3 – x) x x  2.84 10-3 mol dm-3 HCl → H+ + Cl- 0.1 mol dm-3 Assumptions: Vitamin C is a weak acid , only partially ionised in solution, so that; (2.84  10-3 – x)  2.84  10-3 mol dm-3, (0.1 + x)  0.1 mol dm-3, Power supply Pt - cathode (negative electrode) H2SO4 (aq) Al - anode (positive electrode) At the Cathode: 2e -+ 2H+  H2 At the Anode: H2O  O2 + 4e- + 4H+ 3O2 + 4Al  2Al2O3

5 9647/03/CJC JC2 Prelim Exam 2011 since x is very small. Ka = ][ ]][[ HA AH  = x xx   31084.2 ))(1.0( 7.94  10-5 = 31084.2 )1.0(  x  x = [H+] = 2.25  10-6 mol dm-3 % of Vitamin C that is ionised = 3 6 1084.2 1025.2      100 = 0.0794 % (ii) The structure of Vitamin C is State which hydrogen atom in the molecule is likely to be acidic, and justify your choice. The H atom of the OH group attached to Carbon 2 is likely to be acidic. The C=O group attached to Carbon 2 withdraws electrons due to the electronegative O atom, thus making the O-H bond more polar so that dissociation of the O-H bond can take place more readily. OR The H atom of the OH group attached to Carbon 2/3 is likely to be acidic. Ion formed is stabilized by delocalization of electrons / resonance stabilized. HO C C C O O 2 HO C C C O O 2

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