Atom Economy
Quick Notes
- Atom economy measures the proportion of atoms in reactants that end up in the desired products.
- It's a key principle in green chemistry, reducing waste in chemical processes.
- Atom economy
= (molar mass of desired product ÷ total molar mass of all products) × 100 - High atom economy = less waste, more sustainable.
- Low atom economy = more waste, less efficient.
Full Notes
What Is Atom Economy?
Atom economy tells us how much of the reactants end up in the main product, rather than in by-products. It focuses on the stoichiometry of a reaction and is theoretical – it isn’t based on experimental data.
It is important in green chemistry because it promotes:
- Efficient use of materials
- Reduced waste
- Lower environmental impact
Formula for Atom Economy
Higher atom economy = more efficient process.
Low atom economy = more waste, less sustainable.
Worked Examples: Calculating Atom Economy
Example Production of Iron (Fe)
Find the percentage atom economy for the production of iron (Fe) from the reaction between iron(III) oxide and carbon monoxide.
If there is only one product in the reaction, then the atom economy must be 100%.
Example Production of Ethanol (C2H5OH)
Find the percentage atom economy for the production of ethanol from the reaction between ethene and water.
Why Atom Economy Matters
- High atom economy = more efficient reaction, less waste
- Low atom economy = more by-products, less desirable in industrial or environmental terms
Examples:
- Addition reactions often have 100% atom economy (no by-products).
- Substitution or elimination reactions tend to have lower atom economy (some atoms are lost as waste).
Atom Economy vs Percentage Yield
Atom economy and percentage yield are two different ideas:
- Atom economy is based on the reaction equation and measures theoretical efficiency.
- Percentage yield is based on actual results and measures practical efficiency.
A reaction can have a high atom economy but a low yield if it doesn’t proceed well in practice.
The atom economy and the percentage yield both give important information about the “efficiency” of a chemical process. What other factors should be considered in this assessment?
While atom economy and percentage yield measure efficiency, other important factors include:
- Energy requirements (e.g., high temperatures or pressures increase environmental and economic costs)
- Toxicity or hazard of reactants, products, or by-products
- Sustainability of raw materials (renewable vs non-renewable sources)
- Waste generation and ease of disposal or recycling
- Reaction conditions (e.g., use of catalysts, solvents, or special equipment)
- Scalability and cost for industrial applications
Combined together, these factors give a fuller picture of a process’s green chemistry and real-world efficiency.
Summary
- Atom economy measures theoretical efficiency from balanced equations.
- It shows how much of the reactants end up in the desired product.
- High atom economy means less waste and a greener process.
- Atom economy is different from percentage yield, which is experimental.
- Other factors like energy, toxicity, and sustainability must also be considered.