Organic chemistry Question 2 2006 Section A

TYPE: base hydrolysis / saponification [Accept substitution] (3) Q 2. A sample of soap was prepared in the laboratory by refluxing a mixture of approximately 5 g of animal fat, 2 g of sodium hydroxide pellets (an excess) and 25 cm of ethanol in an apparatus like that drawn on the right. (a) Why was the reaction mixture refluxed? Name the type of reaction which occurs during the reflux stage of the preparation. (8) Answer :to speed up the reaction / reaction is slow / to drive reaction to completion /to maximise (increase) yield [Allow even if incorrect reaction specified] [Allow ‘to prevent loss of vapour (ethanol, solvent)] (5) TYPE: base hydrolysis / saponification [Accept substitution] (3) Section A Q2(a) 2006

Section A Q2(b) 2006

(c) What is the purpose of the ethanol (c) What is the purpose of the ethanol? Why is it desirable to remove the ethanol after reflux? Describe with the aid of a labelled diagram how you would remove the ethanol after the reflux stage of the experiment. (12) Answer Section A Q2(C) 2006

Section A Q2(C) 2006

(d) Describe how a pure sample of soap was obtained from the reaction mixture. (9) Answer: dissolve residue in minimum of boiling (hot) water / add in a little boiling (hot) water // pour onto brine (salt water, sodium chloride solution) // filter // wash with more brine / wash with a little ice-water POUR INTO BRINE (6) ONE OTHER POINT (3) Section A Q2(d) 2006

(e) At the end of the experiment, what is the location (i) of the second product of the reaction, (ii) of the excess sodium hydroxide? (6) LOCATION: (i) second product: in the filtrate/ in the brine / Buchner flask (3) (ii) excess sodium hydroxide: in the filtrate / in the brine / Buchner flask (3) Section A Q2(e) 2006

OBSERVE: (i) immediate lather (suds, bubbles)(3) (f) What would you observe, upon shaking, if a little of the soap prepared in this experiment is added to (i) a test tube containing deionised water, (ii) a test tube containing mineral water from a limestone region? (6) ____________________________ OBSERVE: (i) immediate lather (suds, bubbles)(3) (ii) scum / no lather (suds, bubbles) / less lather / does not easily form lather (3) Section A Q2(f) 2006