# Explain why the theoretical yield of a product calculated from stoichiometry is almost never achieved in practice, referring to at least three distinct reasons.

> IB DP Chemistry Higher Level (2023 syllabus) — R2.1 How much? The amount of chemical change · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Reversible reactions / equilibrium means the reaction does not go to completion, so not all limiting reagent is converted to product.
2. Side reactions consume some of the limiting reagent or product, producing unwanted by-products and reducing the amount of desired product formed.
3. 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.
4. 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

- [theoretical yield](https://www.gradenine.co.uk/glossary/theoretical-yield)
- [stoichiometry](https://www.gradenine.co.uk/glossary/stoichiometry)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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