Atom economy · Grade 12
In Grade 12 you get fluent with: atom economy; reactions with an atom economy of 100%; choosing a process: yield, atom economy and waste.
Atom economy looks at the reaction itself: what percentage of the mass of all the reactants ends up in the product you want. It comes straight from the balanced equation. A reaction with only one product wastes nothing, while one with many by-products has a low atom economy unless the by-products can be used.
Key facts
- Atom economy = Mᵣ of the useful product ÷ total Mᵣ of all the reactants × 100, using the numbers from the balanced equation.
- A reaction with only one product, such as an addition reaction, has an atom economy of 100%.
- A low atom economy means a lot of waste, unless the other products can be used or sold.
- To choose a way of making a product, compare the yield, the atom economy, the rate, the equilibrium position and the usefulness of any by-products.
Worked examples
Example 1
Find the atom economy for making calcium oxide by heating calcium carbonate. CaCO3 → CaO + CO2 (Aᵣ: Ca = 40, C = 12, O = 16)
Answer: 56%
Example 2
N2 + 3H2 → 2NH3. What is the atom economy for making chem NH3, to 1 decimal place? (Aᵣ: N = 14, H = 1)
Answer: 100%
Example 3
Which kind of reaction always has an atom economy of 100%?
Answer: a reaction with only one product, such as an addition reaction
Example 4
What does a high atom economy mean?
Answer: most of the atoms in the reactants end up in the useful product
Example 5
2Al2O3 → 4Al + 3O2. What is the atom economy for making chem Al, to 1 decimal place? (Aᵣ: Al = 27, O = 16)
Answer: 52.9%
Example 6
Ethanol can be made by fermentation, C6H12O6 → 2C2H5OH + 2CO2, or by adding steam to ethene, C2H4 + H2O → C2H5OH. Compare their atom economies. (Aᵣ: C = 12, H = 1, O = 16)
Answer: fermentation 51.1%; from ethene 100%
Example 7
A process has a high yield but a low atom economy. Why might a company still be concerned?
Answer: it makes a lot of waste products that must be dealt with