Section 1 of 5What Atom Economy Measures
Atom economy compares the mass of the desired product with the total mass of reactants in the balanced equation, showing how much of the starting material ends up as useful product.
Desired product
- Desired product The molecule a chemist wants to make, which is the only product counted as useful when calculating atom economy.
Reactants
- Reactants All starting substances, counted using their relative formula masses multiplied by the coefficients in the balanced equation.
Waste product
- Waste product Any by-product formed alongside the desired product, which lowers the atom economy and must be disposed of.
High value
- High value A reaction with a high atom economy wastes few atoms, which lowers costs and reduces environmental harm.
Section 2 of 5Calculation Method
The same steps work for any balanced equation.
1
Write the balanced symbol equation, making sure the number of atoms of each element is equal on both sides.
2
Calculate the relative formula mass of each reactant and product, using relative atomic masses from the periodic table.
3
Multiply each relative formula mass by its balanced equation coefficient to get the total mass of reactants and the mass of the desired product.
4
Use the formula: atom economy = (mass of desired product ÷ total mass of reactants) × 100%.
5
Give the answer as a percentage and state whether the reaction is efficient or wasteful.
| Reaction | Desired product | Atom economy |
|---|---|---|
| CaCO₃ → CaO + CO₂ | Calcium oxide | 56% |
| C₂H₄ + H₂O → C₂H₅OH | Ethanol | 100% |
Section 3 of 5Why Atom Economy Matters
Why Atom Economy Matters
Sustainability
- Sustainability Processes with a high atom economy use finite raw materials more efficiently and produce less waste.
Cost
- Cost Less waste means less money spent on raw materials and on treating or disposing of by-products.
Limits
- Limits Atom economy ignores percentage yield, so a reaction with a high atom economy may still give little product in practice.
Addition reactions
- Addition reactions Reactions in which all reactant atoms end up in one product have an atom economy of 100%.
Section 4 of 5Worked Example
Question
- Calculate the atom economy for making calcium oxide from calcium carbonate: CaCO₃(s) → CaO(s) + CO₂(g).
- Use Ca = 40, C = 12 and O = 16.
- Mr of CaCO₃ = 40 + 12 + (3 × 16) = 100
- Mr of CaO = 40 + 16 = 56
- Both coefficients are 1, so the total mass of reactants is 100
- Atom economy = 56 ÷ 100 × 100 = 56%
Answer
- The atom economy is 56%.
- The remaining 44% is carbon dioxide (Mr 44), so the reaction wastes nearly half of its atoms.
Section 5 of 5Exam Checklist & Tips
3.3.3 – Key Points for the Exam
- Atom economy The mass of desired product divided by the total mass of reactants, multiplied by 100 percent.
- Relative formula mass (Mr) The sum of the relative atomic masses of all the atoms shown in a formula.
- Balanced equation A symbol equation with equal numbers of each type of atom on both sides.
- Coefficient The number in front of a formula in a balanced equation, showing the molar ratio of substances.
- By-product An unwanted substance formed in the same reaction as the desired product.
Common Mistakes
- Leaving out the balanced equation coefficients, so the total reactant mass is wrong.
- Confusing atom economy with percentage yield;
- atom economy is a theoretical value calculated from the equation alone.
- Calculating atom economy with the product mass instead of the total mass of reactants.
- Stating that a catalyst improves atom economy;
- a catalyst speeds up a reaction but does not change the equation.