CJC_H2_CHEM_P3_SOL Prelim

CATHOLIC JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATIONS Higher 2 CHEMISTRY 9647/03 Paper 3 Free Response Wednesday 27 August 2014 2 hours Additional Materials: Data Booklet Answer Paper 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. Write your answers on the answer paper provided. You are reminded of the need for good English and clear presentation in your an swers. The number of marks is given in brackets [ ] at the end of each question or part question. At the end of the examination, fasten all your work securely together. [Turn over Answers

2 9647/03/CJC JC2 Preliminary Exam 2014 Answer any four questions. 1 (a) When an aqueous solution of Co 2+ is reacted with hydrochloric acid and ethane -1,2- diamine (en) under oxidising conditions, a green mixed -ligand cobalt( III) complex, [Co(en)2Cl2]+Cl-, with the following structure is formed: , where L = en (i) State the coordination number of the above complex. 6 (ii) Explain why the above complex is green in colour. Co3+: [Ar] 3d6 In the presence of ligands, the incompletely filled (or partially filled) 3d - orbitals become non -degenerate or split into 2 different energy levels. Hence d-d* electronic transition takes place as radiation in the red region of the visible spectrum is absorbed. The light energy not absorbed is thus seen as the colour of the complex, which is green. (iii) When 0.03 mol of aqueous silver nitrate is added to 0.01 mol of the above aqueous complex, a white precipitate weighing 1.44 g is formed. Identify an d calculate the amount of precipitate formed. With reference to the structure of the above complex, explain the above observations fully. The white precipitate is AgCl. No. of mol of AgCl = = 0.0100 mol AgCl ≡ [Co(en)2Cl2]+Cl- Although [Co(en)2Cl2]+Cl- has three chloride ions, only one of them is free to be precipitated out by Ag+, as the other two are covalently bonded to Co3+ . (iv) The formula [Co(en)2Cl2]+Cl- may refer to a structure that is different from what is shown above. Draw this other structure of [Co(en)2Cl2]+Cl- and state its relationship with the above structure.

3 9647/03/CJC JC2 Preliminary Exam 2014 , where L = en [9] (b) Cobalt is also found in nitrile hydratase, a bacterial enzyme which catalyses the conversion of nitriles to their corresponding amides: Nitrile hydratase has a quaternary structure with four subunits. Each pair of subunits binds a cobalt atom, which serves