Explain how the concepts of limiting reagent and percentage yield are used together to evaluate the efficiency of a chemical synthesis, using the reaction of ethanol with ethanoic acid to form ethyl ethanoate and water as your example.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ethanol and ethanoic acid react according to the equation: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). In a synthesis, 0.50 mol of ethanoic acid is mixed with 0.30 mol of ethanol in the presence of a small amount of concentrated sulfuric acid catalyst.
Model answer (4 marks)
Ethanol is the limiting reagent because only 0.30 mol is present, whereas 0.50 mol of acetic acid is required for a 1:1 stoichiometry.
The theoretical yield of ethyl ethanoate is therefore 0.30 mol, calculated directly from the amount of the limiting reagent.
Percentage yield is calculated as (actual yield ÷ theoretical yield) × 100 %. A value below 100 % shows that not all of the limiting reagent has been converted to product.
Because the esterification is reversible, the equilibrium lies to the left; even with perfect conditions the reaction will not go to completion, so the percentage yield will always be less than 100 %. This demonstrates a fundamental limitation on synthesis efficiency.
The theoretical yield of ethyl ethanoate is therefore 0.30 mol, calculated directly from the amount of the limiting reagent.
Percentage yield is calculated as (actual yield ÷ theoretical yield) × 100 %. A value below 100 % shows that not all of the limiting reagent has been converted to product.
Because the esterification is reversible, the equilibrium lies to the left; even with perfect conditions the reaction will not go to completion, so the percentage yield will always be less than 100 %. This demonstrates a fundamental limitation on synthesis efficiency.
Examiner tips
- State the limiting reagent and give the reasoning; calculate theoretical yield from it; write the formula for % yield; explain the equilibrium effect on % yield
Common mistakes
- Confusing acetic acid with ethanol as the limiting reagent; calculating theoretical yield from the wrong reagent; forgetting to include the equilibrium argument
Mark scheme (4 marks)
- Ethanol is identified as the limiting reagent because it is present in the smaller molar amount (0.30 mol) relative to the 1:1 stoichiometric ratio required.
- The theoretical yield of ethyl ethanoate is 0.30 mol, calculated from the amount of the limiting reagent (ethanol) and the 1:1 mole ratio in the equation.
- Percentage yield is calculated as (actual yield / theoretical yield) × 100%, and a value below 100% indicates that not all of the limiting reagent was converted to product.
- The reversible nature of the esterification equilibrium means the reaction does not go to completion, so the percentage yield will always be below 100% even without experimental losses; this illustrates a fundamental limitation on synthesis efficiency.
Key terms in this question
limiting reagent · percentage yield
Related
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