TJC H2 Chem 2013 Prelim P2 Soln

1 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use TJC Paper 2 Answers 1 Planning (P) (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.  Diethyl ether γ-butyrolactone is polar with the presence of electronegative O atom and is able to form permanent dipole-permanent dipole interactions (pd-pd) with polar diethyl ether (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.  Upper layer. Diethyl ether is less dense (  = 0.713 g cm –3) than water (  = 1.000 g cm –3). Therefore, diethyl ether will be the upper layer and water will be the bottom layer. (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 1352

(b) The enth You Hc, diag Your 1 2 3 Give 1 2 the exper Chemica  Chem It is to r reaction pressure funnel. Chemica  Chem It is to d water. 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 rimental pro l reagent A mical reage react with . Carbon d e to prev e l reagent B mical reage dry the org a 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 ocedures 4 a : ent A – aqu excess a q dioxide wi ent buildin g : ent B – anh anic layer 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 2 2/TJC Prelim 2 and 5. eous sodiu queous hy ll be rele a g u p o f th ydrous Mg containing 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 um carbona drochloric ased and i t he internal gSO4/ NaSO g γ-butyrola 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 ate acid pres t is essen pressure O4/ CaCl2/ C actone by r 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 qu e ntial to rel e in the s e CaSO4. removing t ent to dete r ange of co m the apparat chool labora s nge of comb water by 1 urn Over [5] ench the ease the eparating traces of rmine the mbustion, tus in the atory. bustion of oC. [5] For Examiner’s Use 1353

3 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use Experimental Procedures: Experimental procedures: 1. Weigh the empty copper calorimeter using an electronic balance. 2. Using a burette, add 50.00 cm 3 of distilled water into the calorimeter. Cover it with a lid and place a thermometer in it. Clamp the flask at a suitable height so that the spirit burner can be placed below easily. 3. Record the initial temperat ure of the distilled water. 4. Fill the spirit burner with γ-butyrolactone 5. Weigh the spirit burner (and lid) containing γ-butyrolactone and record the mass. 6. Place the spirit burner about 1 cm under the copper calorimeter, and light the wick. After observing that the temperature has risen for 10 C [fixed temperature rise], record the final temperature and extinguish the flame. 7. Cool the experimental set-up and reweigh the spirit burner and cap. 8. Calculate the mass of γ-butyrolactone used. Recording of raw data: Processing of raw data: Mass of calorimeter / g m1 Mass of spirit burner before combustion / g m 2 Mass of spirit burner after combustion / g m 3 Mass of γ-butyrolactone used for combustion / g (m 2 - m3) Initial temperature of water / C T1 Final temperature of water / C T2 Analysis of raw data: M r of γ-butyrolactone = 4(12) + 6 + 2(16) = 86 Quantity of heat produced in combustion = heat absorbed by the water + heat absorbed by the copper calorimeter Q = [50 x 4.2 x (T2 - T1)] + [m1 x 0.385 x (T2 – T1)] J ൌ െ ૚ሻሿ ૜ ૡ૟ J mol–1 ● logical, coherent steps/measurements that would lead to fruitful results ● appropriate choice of apparatus ● tables with clear headings and units ● calculation of quantity of heat produced during combustion 1354

4 9647/02/TJC Prelim 2013 [Turn Over For Examiner’s Use ● calculation of Hc (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]  Combustion of the crude γ-butyrolactone could be incomplete. Reason: The draught shield used for minimising heat loss to the surrounding could have blocked the constant air supply, resulting in incomplete combustion and lesser amount of heat evolved from the incomplete combustion. OR Some of the γ-butyrolactone may have vaporised. Reason: This would result in a smaller final total mass of spirit burner and γ- butyrolactone measured. Thus the mass of γ-butyrolactone combusted determined (m2 – m 3 calculated) is larger than actual values, and a less exothermic Hc would be calculated. [Total: 12] 1355

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