Ethyl ethanoate synthesis: yield and separation decisions
A Level 2 student prepares ethyl ethanoate in a school laboratory. They mix 10.0 mL of ethanol (density 0.789 g mL−1) with 10.0 mL of ethanoic acid (density 1.05 g mL−1) and add 2.0 mL of concentrated sulfuric acid. The mixture is refluxed for 20 minutes, then the product is distilled. The distillate is washed with sodium carbonate solution, dried, and redistilled. The final mass of ethyl ethanoate collected is 8.20 g. The equation is CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O. M(ethanol) = 46.1 g mol−1; M(ethanoic acid) = 60.1 g mol−1; M(ethyl ethanoate) = 88.1 g mol−1. The student's data sheet gives boiling points: ethanol 78 °C, ethanoic acid 118 °C, ethyl ethanoate 77 °C, water 100 °C. The student is asked to judge the success of the synthesis and the separation steps.
1. Determine the limiting reagent and the theoretical yield of ethyl ethanoate.
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n(ethanol) = (10.0 mL × 0.789 g mL−1)/46.1 g mol−1 = 7.89/46.1 = 0.171 mol. n(ethanoic acid) = (10.0 mL × 1.05 g mL−1)/60.1 g mol−1 = 10.5/60.1 = 0.175 mol. The equation shows a 1:1 ratio. Ethanol is limiting because 0.171 mol < 0.175 mol. Theoretical n(ethyl ethanoate) = 0.171 mol. Theoretical mass = 0.171 mol × 88.1 g mol−1 = 15.1 g.
2. Calculate the percentage yield and explain why simple distillation cannot fully separate ethanol from ethyl ethanoate.
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Percentage yield = (8.20/15.1) × 100 = 54.3%. Simple distillation cannot fully separate ethanol from ethyl ethanoate because their boiling points differ by only about 1 °C (78 °C and 77 °C). They co-distil, so the distillate contains both. A fractionating column or repeated fractional distillation would be needed for a better separation. Washing with sodium carbonate removes acid impurities but does not remove ethanol.
3. Explain the purpose of reflux and of washing with sodium carbonate. Include an equation for the reaction of sodium carbonate with ethanoic acid.
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Reflux heats the mixture without losing volatile ethanol or ester; the condenser returns vapours, allowing time for the slow reversible esterification to approach equilibrium. Sodium carbonate solution neutralises unreacted ethanoic acid and the sulfuric acid catalyst, converting them to salts that stay in the aqueous layer. Carbon dioxide is produced. Equation: Na2CO3(aq) + 2CH3COOH(aq) → 2CH3COONa(aq) + CO2(g) + H2O(l). This also reduces the smell and corrosiveness and helps isolate the ester.