Atom Economy
A focused revision guide to percentage atom economy, useful products, waste products and why sustainable chemistry aims to design reactions that make better use of reactant atoms.
Why Atom Economy Matters
Atom economy measures how efficiently a reaction turns the atoms in the reactants into the useful product.
A reaction can have a high percentage yield but still be wasteful if many atoms from the reactants end up in unwanted side products. Atom economy therefore links mole calculations to sustainable chemistry and industrial process design.
Key idea: percentage yield asks how much product was collected. Atom economy asks how many reactant atoms were built into the useful product.
The Percentage Atom Economy Formula
The percentage atom economy compares the molar mass of useful products with the total molar mass of all reactants in the balanced equation.
Percentage atom economy = mass of useful products ÷ mass of all reactants × 100
Use relative formula masses from the balanced equation. If there is a coefficient in front of a substance, multiply its relative formula mass by that coefficient.
Exam focus: the numerator includes only the useful product or useful products. The denominator includes all reactants, with balancing numbers taken into account.
Useful Products and Waste Products
The useful product is the substance the reaction is designed to make. Other products may be described as waste products or by-products.
In atom economy questions, first identify which product is useful. Then calculate how much of the total reactant mass becomes that useful product.
| Term | Meaning | Exam implication |
|---|---|---|
| Useful product | The desired product made by the reaction. | Include this in the numerator of the atom economy calculation. |
| Waste product | An unwanted product formed alongside the useful product. | Do not include it in the numerator, even though its atoms came from reactants. |
| By-product | A secondary product that may be unwanted, but could sometimes be sold or reused. | Selling a by-product can improve process economics, but it does not change the atom economy calculation. |
Worked Example: Iron from Iron(III) Oxide
Question: Calculate the percentage atom economy for the reaction below if iron is the useful product.
Balanced equation: Fe2O3 + 3CO → 2Fe + 3CO2
| Stage | Working | Result |
|---|---|---|
| Mass of useful product | Useful product = Fe 2Fe = 2 × 55.8 |
111.6 |
| Mass of all reactants | Fe2O3 = 2 × 55.8 + 3 × 16 = 159.6 3CO = 3 × (12 + 16) = 84.0 |
243.6 |
| Atom economy | percentage atom economy = 111.6 ÷ 243.6 × 100 | 45.8% |
Answer: the atom economy is 45.8%. This means less than half of the reactant mass becomes the useful iron product.
Why One-Product Reactions Have 100% Atom Economy
If a reaction has only one product and all atoms from the reactants are used to make that product, the atom economy is 100%.
This is why many addition reactions are considered atom-efficient. For example, hydrogen adds across the C=C bond in ethene to make ethane.
CH2=CH2 + H2 → CH3CH3
There is only one product, so all atoms from ethene and hydrogen are found in ethane.
Remember: one-product reactions are ideal for atom economy because they do not generate waste products.
Atom Economy and Sustainable Chemistry
Sustainable chemistry aims to design processes that reduce waste, reduce separation costs and make better use of starting materials.
| Feature | High atom economy | Low atom economy |
|---|---|---|
| Use of atoms | Most reactant atoms become useful product. | Many reactant atoms become waste products. |
| Waste | Less waste is produced. | More waste must be separated, treated or disposed of. |
| Industrial value | Usually more sustainable and efficient. | May be less sustainable unless by-products have useful applications. |
Common Exam Points
- Atom economy measures the proportion of reactant atoms that become useful product.
- The formula is mass of useful products ÷ mass of all reactants × 100.
- Use the balanced equation because coefficients affect the reacting masses.
- The denominator must include all reactants, not just the limiting reagent.
- The numerator includes only the useful product, unless more than one product is useful.
- Reactions with only one product have 100% atom economy.
- High atom economy is desirable because it reduces waste and supports sustainable chemistry.
- Selling a by-product may improve the economics of a process, but the atom economy calculation still depends on which product is useful.
Check Your Understanding
Use these short tasks to consolidate the atom economy formula, useful products, waste products and the link to sustainable chemistry.
QuickSnap
This text summary condenses the page into the essential exam ideas.
- Atom economy: the percentage of reactant atoms that end up in the useful product.
- Formula: mass of useful products ÷ mass of all reactants × 100.
- Balanced equation: needed because reacting masses depend on stoichiometric coefficients.
- 100% atom economy: usually occurs when there is only one product.
- Low atom economy: means a greater proportion of reactant mass becomes waste or by-products.
- Sustainable chemistry: favours high atom economy because it minimises waste products.
Master Formulae, Equations and Amounts of Substance for Edexcel A Level Chemistry
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A compound contains carbon, hydrogen and oxygen only. Its percentage composition by mass is shown.
| Element | Percentage by mass |
|---|---|
| C | 40.0% |
| H | 6.7% |
| O | 53.3% |
What is the empirical formula of the compound?
Explain how the volume of hydrogen can be used to calculate the amount of magnesium that reacted.
FAQs
These questions address common misconceptions students have when learning atom economy calculations.
What is atom economy?
Atom economy is the percentage of reactant atoms that end up in the useful product. It shows how efficiently a reaction uses its starting materials.
How do you calculate percentage atom economy?
Use mass of useful products divided by mass of all reactants, then multiply by 100. The masses are calculated from relative formula masses and the balanced equation.
Why do balancing numbers matter in atom economy?
Balancing numbers show how many formula units or molecules react. They must be included when calculating the total mass of useful products and the total mass of reactants.
When is atom economy 100%?
Atom economy is 100% when all atoms in the reactants are converted into the useful product. This often occurs in reactions with only one product, such as simple addition reactions.
What is the difference between percentage yield and atom economy?
Percentage yield compares the actual amount obtained with the theoretical amount. Atom economy measures how much of the reactant mass is incorporated into the useful product.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.