RI Prelim_P2_QP

1 © Raffles Institution 9647/02/S/15 [Turn Over RAFFLES INSTITUTION 2015 YEAR 6 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS INDEX NUMBER CHEMISTRY 9647/02 Paper 2 Structured Questions September 2015 2 hours Candidates answer on the Question Paper. Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your name, class and index number in the spaces provided at the top of this page. Write in dark blue or black pen in the spaces provided. 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. A Data Booklet is provided. Do not write anything on it. 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 1 / 12 2 / 7 3 / 23 4 / 17 5 / 13 Total / 72 This document consists of 21 printed pages.

2 © Raffles Institution 9647/02/S/15 1 Planning (P) Cobalt(III) complex salts played a critical role in early studies of the structures of octahedral transition metal complexes. One such complex salt is the nitrate salt of pentaammineaquacobalt( III), [Co(NH3)5(H2O)](NO3)3, which is prepared from cobalt(II) nitrate, Co(NO3)2. 2HNO3(aq) + 2Co(NO3)2(s) + H2O2(aq) + 10NH3(aq) 2[Co(NH3)5(H2O)](NO3)3(s) 1.45 g of solid Co(NO3)2 is added to 45 cm3 of concentrated ammonia. This is followed by the addition of 25 cm3 of hydrogen peroxide solution. A rapid increase in the concentration of hydrogen peroxide in the reaction mixture can result in hydrogen peroxide undergoing self-decomposition. Subsequently, 30 cm 3 of concentrated nitric acid is added. The initial re action may be highly exothermic. Once the reaction is completed, a 100 cm3 crude mixture is obtained. The product, [Co(NH3)5(H2O)](NO3)3, is precipitated from the crude mixture to separate it from any unreacted Co(NO3)2. This is done by adding an approximately equal volume of a suitable solvent to the crude mixture. The table below shows the respective solubilities of Co(NO3)2 and [Co(NH3)5(H2O)](NO3)3 in three possible solvents. Solute Solubility / g per 100 g of solvent Water Ethanol Hexane Co(NO3)2 10.2 9.5 0.9 [Co(NH3)5(H2O)](NO3)3 10.5 2.1 1.1 The precipitated [Co(NH3)5(H2O)](NO3)3 is subsequently purified by recrystallisation from a given solution of ammonium nitrate as a solvent. (a) Explain the role of H2O2 in the reaction. ……………………………………………………………………………..………… ……………………………………………………………………………..………… [1] For Examiner’s Use

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