Esterification Yield and Le Chatelier's Principle
In an organic synthesis practical, a student refluxes a mixture of 0.20 mol ethanol and 0.10 mol ethanoic acid with a few drops of concentrated sulfuric acid as catalyst. After refluxing for 45 minutes, the mixture is distilled and the distillate is washed with sodium carbonate solution and dried. The student obtains 6.50 g of ethyl ethanoate. The teacher says the yield is respectable but not optimal, and asks the student to explain the result in terms of equilibrium rather than simply saying the reaction is slow. The student had used an excess of ethanol because it is cheaper and less corrosive than ethanoic acid. The molar mass of ethyl ethanoate is 88.0 g mol-1.
1. Calculate the percentage yield of ethyl ethanoate.
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The balanced equation is CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O. Ethanoic acid is the limiting reagent: 0.10 mol. The theoretical amount of ester is therefore 0.10 mol. Theoretical mass = 0.10 × 88.0 = 8.80 g. Percentage yield = actual mass / theoretical mass × 100 = 6.50 / 8.80 × 100 = 73.9%. The calculation assumes the ester is pure after washing and drying; any residual water or ethanol would make the true yield slightly lower.
2. Explain why the yield is below 100% and why using excess ethanol improves it. Refer to the equilibrium in your answer.
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Esterification is reversible and reaches an equilibrium rather than going to completion. At equilibrium all four substances are present, so some ethanoic acid remains unreacted. Adding extra ethanol increases the concentration of a reactant on the left-hand side. By Le Chatelier's principle, the system shifts to the right to reduce that increase, producing more ester and water. The excess ethanol is not consumed completely, but it raises the equilibrium yield of ethyl ethanoate. Some product is also lost during distillation and washing, so the isolated yield is lower than the equilibrium yield.
3. Suggest one change that would improve the yield, and one reason why using excess ethanoic acid instead of excess ethanol is less desirable.
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Remove water as it forms, for example by distilling the ester or using a Dean–Stark apparatus. Removing a product shifts the equilibrium to the right and increases the yield. Using excess ethanoic acid is less desirable because ethanoic acid is corrosive and unpleasant, it is harder to separate from the ester, and it requires more sodium carbonate in the wash. Excess acid also increases the risk of acid-catalysed hydrolysis of the ester during work-up. Ethanol is cheaper, easier to remove by distillation, and gives the same equilibrium shift.