Explain why the theoretical yield of a product calculated from stoichiometry is almost never achieved in practice, referring to at least three distinct reasons.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a multi-step organic synthesis, a chemist reacts excess ethanoic anhydride with a limited amount of 2-aminophenol to produce an amide product. Despite careful technique, the isolated yield is consistently below the theoretical yield calculated from the stoichiometry of the reaction.
Model answer (4 marks)
1. The reaction is reversible; equilibrium limits the extent of conversion so not all of the 2‑aminophenol is transformed into the amide.
2. Side reactions consume some of the 2‑aminophenol or the amide, forming by‑products that reduce the amount of desired product.
3. During isolation (filtration, recrystallisation, distillation) the product is lost, so the isolated yield is lower than the amount actually formed.
4. The theoretical yield assumes 100 % conversion and pure reagents; in practice kinetic barriers and impurities mean the limiting reagent is not fully consumed, further lowering the yield.
2. Side reactions consume some of the 2‑aminophenol or the amide, forming by‑products that reduce the amount of desired product.
3. During isolation (filtration, recrystallisation, distillation) the product is lost, so the isolated yield is lower than the amount actually formed.
4. The theoretical yield assumes 100 % conversion and pure reagents; in practice kinetic barriers and impurities mean the limiting reagent is not fully consumed, further lowering the yield.
Examiner tips
- Use the exact terms ‘equilibrium’, ‘side reactions’, ‘product loss’ and ‘kinetic/impurities’ as the mark scheme rewards.
- Show the four points clearly, each as a separate sentence or bullet, to demonstrate you have addressed all required reasons.
Common mistakes
- Confusing ‘yield’ with ‘efficiency’ or ‘purity’. Failing to mention that the yield is calculated from stoichiometry, not from the actual reaction progress. Using vague phrases like ‘reaction doesn’t go to completion’ without specifying equilibrium or side reactions.
Mark scheme (4 marks)
- Reversible reactions / equilibrium means the reaction does not go to completion, so not all limiting reagent is converted to product.
- Side reactions consume some of the limiting reagent or product, producing unwanted by-products and reducing the amount of desired product formed.
- Product is lost during separation, purification, or transfer steps (e.g., filtration, recrystallisation, distillation), so the isolated yield is lower than the amount actually formed.
- The theoretical yield assumes 100% conversion of the limiting reagent with perfect stoichiometry; in practice, reactions may not proceed completely due to kinetic limitations (e.g., insufficient activation energy being overcome for all molecules) OR the presence of impurities in the starting material reduces the effective moles of limiting reagent available.
Key terms in this question
theoretical yield · stoichiometry
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
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