14.5 - Atom Economy
Atom economy of a reaction
- Industrial reactions often make more than one product. Any product other than the desired one is waste.
- Atom economy (or atom utilisation) measures the proportion of reactant mass that ends up as the desired product. It's given as a percentage and calculated from the balanced equation.
Formula for atom economy
Where:
- = Relative formula mass, using the numbers in the balanced equation
Using balancing numbers
- Both lines of the fraction use the balancing numbers: multiply each relative formula mass () by the balancing number in front of it in the equation.
- For example, 3H2 means multiplying the of H2 by 3, and 2Fe2O3 means multiplying the of Fe2O3 by 2.
Interpreting atom economy
- 100% atom economy - Every reactant atom ends up in the desired product, with no waste. A reaction with only one product always has an atom economy of 100%.
- Below 100% - Some of the reactant mass is lost as waste products.
- Atom economy is a theoretical value worked out from the balanced equation alone, not from practical measurements.
Worked example - Atom economy of a reaction
Copper is extracted from copper(II) oxide by heating it with carbon:
2CuO + C → 2Cu + CO2
Calculate the atom economy of this reaction for the production of copper. Give your answer to 2 significant figures.
Desired product = copper, Cu. Relative atomic masses (from the periodic table): C = 12, O = 16, Cu = 63.5
Step 1 - Total relative formula mass of the reactants
The equation uses 2CuO, so that balancing number is used here.
Mr(CuO) = 63.5 + 16 = 79.5
2 × Mr(CuO) = 2 × 79.5 = 159
Total Mr of reactants = 159 + 12 = 171
Step 2 - Relative formula mass of the desired product
The equation makes 2Cu, so the balancing number is used here. Carbon dioxide is the other product and plays no part in the top of the fraction.
2 × Ar(Cu) = 2 × 63.5 = 127
Step 3 - Substitution and calculation
Atom economy = (127 ÷ 171) × 100 = 74.269...%
Step 4 - Round the answer
74.269...% to 2 significant figures is 74%.
So about 74% of the mass of the copper(II) oxide and carbon ends up as copper. The other 26% leaves as carbon dioxide.
Why a high atom economy matters
Choosing a reaction with a high atom economy is important for both economic reasons and sustainable development.
Benefits of a high atom economy
- Conserves raw materials - A higher atom economy means less starting material is needed to make a given amount of product, reducing costs and preserving finite resources.
- Reduces waste - Less waste is produced, cutting disposal costs and reducing the risk of environmental pollution.
Improving industrial efficiency
- Use an alternative reaction with fewer unwanted products, such as an addition reaction where all reactant atoms become one product (100% atom economy).
- Find a use for by-products - If a by-product can be recycled or sold to another manufacturer, it becomes a useful product rather than waste.