Explain how an atom economy calculation differs from a percentage yield calculation, and outline why a reaction with a high percentage yield may still be considered wasteful from a green chemistry perspective.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Percentage yield compares the actual (experimental) amount of product obtained to the theoretical (maximum possible) amount.
Atom economy is calculated from the balanced equation alone and compares the molar mass of the desired product(s) to the total molar mass of all reactants.
A reaction can have a high percentage yield yet a low atom economy if significant molar mass is incorporated into by‑products rather than the desired product.
Green chemistry favours high atom economy to minimise waste and by‑products, reducing environmental impact and resource consumption.
Atom economy is calculated from the balanced equation alone and compares the molar mass of the desired product(s) to the total molar mass of all reactants.
A reaction can have a high percentage yield yet a low atom economy if significant molar mass is incorporated into by‑products rather than the desired product.
Green chemistry favours high atom economy to minimise waste and by‑products, reducing environmental impact and resource consumption.
Examiner tips
- Use the exact definitions of percentage yield and atom economy; show the difference in data required (experimental vs theoretical).
- Explain that a high yield does not guarantee low waste – link to atom economy and green chemistry principles.
Common mistakes
- Confusing yield with atom economy, or vice versa; writing that atom economy uses experimental data. Failing to mention that atom economy is based on the balanced equation only, not on experimental results.
Mark scheme (4 marks)
- Percentage yield compares the actual (experimental) amount of product obtained to the theoretical (maximum possible) amount.
- Atom economy is calculated from the balanced equation alone and compares the molar mass of the desired product(s) to the total molar mass of all reactants.
- A reaction can have a high percentage yield yet a low atom economy if significant molar mass is incorporated into by-products rather than the desired product.
- Green chemistry favours high atom economy to minimise waste and by-products, reducing environmental impact and resource consumption.
Key terms in this question
atom economy · percentage yield · green chemistry
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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