TJC H2 Chem 2013 Prelim P2 QP

s C A NA CIV GR C E NU CH 96 Pa Ca Ad RE Wr Wr Yo Do An A D At Th ANDIDATE AME VICS ROUP ENTER UMBER HEMISTRY 47/02 aper 2 Struc andidates an dditional Ma EAD THESE rite your Civ rite in dark b ou may use o not use sta nswer all qu Data Bookle the end of t he number o S ctured Ques nswer on th terials: Data E INSTRUC vics Group a blue or blac a soft penc aples, pape estions. et is provide the examina of marks is g PRELIM HIGHER / stions e Question a Booklet CTIONS FIR and name o ck pen on bo il for any dia er clips, high ed. ation, fasten given in brac This do MINARY EX R 2 9647/02 Paper. RST on all the wo oth sides of agrams, gra hlighters, glu n all your wo ckets [ ] at ocument co XAMINATIO 1 2/TJC Prelim 2 INDEX NUMBER ork you hand the paper. aphs or roug ue or correc ork securely the end of e onsists of 21 ONS 2013 d in. gh working. ction fluid. y together. each questi printed pag on or part q ges. [Tu 17th Septe question. For Exami 1 2 3 4 5 Total urn Over mber 2013 2 hours ner’s Use /12 /20 /10 /15 /15 For Examiner’s Use s 1331

2 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use 1 Planning (P) Scientists in the 1970s have discovered that the reduction of cyclic anhydrides using sodium borohydride (NaBH 4) is more convenient and versatile method than lithium aluminium hydride (LiAlH4). γ-butyrolactone, a pro-drug to GHB, can be synthesized from its starting cyclic anhydride using NaBH4, as shown in the diagram below. (a) A research student wishes to synthesize γ-butyrolactone to determine the enthalpy change of combustion of γ -butyrolactone. Professor Hamilton pointed out to him that the product synthesized has to be purified before further use. This is because in the preparation of crude γ-butyrolactone, sodium borohydride was added in excess to ensure maximum yield in synthesizing γ-butyrolactone. After the reaction is complete, the excess sodium borohydride is dec omposed by acidifying the reaction mixture to pH 6 or below (slowly and while stirring) using excess 6 mol dm -3 aqueous hydrochloric acid. Hydrogen gas is evolved during this process as the excess sodium borohydride decomposes. The purification of crude γ -butyrolactone involves perform ing a liquid-liquid extraction initially using a separating funnel seen in Figure 1 below, before final purification of the product by recrystallization or distillation. Figure 1: Liquid-liquid extraction using a separating funnel 1332

3 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use (i) Which of the following two organic solvents would be a good choice to dissolve crude γ-butyrolactone to perform a liquid-liquid extraction? Explain your answer. (ii) One common mistake when performing a liqui d-liquid extraction is to mix-up the layers and hence the organic product would be lost in the discarded layer. Using the following data in the table, Solvent Density (g cm -3) Benzene 0.877 Diethyl ether 0.713 Water 1.000 Identify if the organic solvent identified in (i) would be the upper or lower layer? Explain your answer. (iii) The experimental procedures of the purification of crude γ-butyrolactone via liquid- liquid extraction is described below: 1. Dissolve the crude γ-butyrolactone in 30 cm3 of the organic solvent. 2. Cool the flask in an ice bath and then add 20 cm 3 of 6 mol dm -3 hydrochloric acid dropwise, while stirring, to quench the reaction. 3. Place the reaction mixture in a separating funnel and separate the aqueous layer from the organic layer. Reject the aqueous layer. 4. Add about half its volume of chemical reagent A. Shake the reaction mixture in the separating funnel cautiously, releasing the pressure at intervals. 5. Transfer the organic layer into a conical flask and add some chemical reagent B. Filter off chemical reagent B and evaporate the solvent at room temperature. State the identities of chemical reagents A and B , explaining the role each plays in the experimental procedures 4 and 5. [5] 1333

(b) The enth You Hc, diag Your 1 2 3 Give 1 2 Chemica Chemica following e alpy change are to pla n under lab o ram above r method sh 1. appropr i 2. how the 3. how thi s γ -butyro en: 1) Specific 2) 4.2 J of l reagent A l reagent B experimenta e of combus n a step-b y oratory cond and any ap hould cover iate quantiti raw data is s data is to b olactone heat capac heat is need Cop Calor γ-butyrol 9647/02 : : al set-up ca stion of γ-bu y-step meth o ditions, of γ paratus req the followin es to be use s to be recor be processe city of coppe ded to raise pper rimeter lactone 4 2/TJC Prelim 2 n be used utyrolactone od to dete r -butyrolact quired that is ng points: ed in the ex rded ed to determ er calorimete e the temper 2013 to design a e. rmine the e tone. You s s normally fo xperimental mine the en er = 0.385 J rature of 1 c an experime nthalpy ch a should use t ound in a sc procedures thalpy chan J g−1 K−1 cm3 (1 g) of [Tu ent to dete r ange of co m the apparat chool labora s nge of comb water by 1 urn Over rmine the mbustion, tus in the atory. bustion of oC. [5] For Examiner’s Use 1334

5 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use Experimental Procedures: Recording of raw data: Analysis of raw data: 1335

6 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use (c) Upon analysis of his raw data, the research student obtained a value less than the literature value of -2010 kJ mol-1. (i) Other than heat loss to the surrounding, suggest another reason to explain his observation, explaining your answer clearly. [2] [Total: 12] 1336

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8 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use (b) Proteins are necessary in the human diet, as they are an important sou