Atom economy

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)

  1. Total Mᵣ of the reactants. 40 + 12 + 3 × 16 = 100
  2. Mᵣ of the useful product, CaO. 40 + 16 = 56
  3. Atom economy. 56 ÷ 100 × 100 = 56%
  4. The other 44% leaves as carbon dioxide, which is wasted unless it is captured and used.

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)

  1. Total Mᵣ of reactants: 28 + 3 × 2 = 34
  2. Mᵣ of useful product: 2 × 17 = 34
  3. Atom economy = 34 ÷ 34 × 100 = 100.0%

Answer: 100%

Example 3

Which kind of reaction always has an atom economy of 100%?

  1. If there is only one product, every atom ends up in it.
  2. Addition reactions, such as ethene + water → ethanol, have one product.
  3. So their atom economy is 100%.

Answer: a reaction with only one product, such as an addition reaction

Example 4

What does a high atom economy mean?

  1. Atom economy compares the mass of useful product with the mass of all reactants.
  2. A high value means little is wasted as unwanted products.
  3. This is better for sustainability and cost.

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)

  1. Total Mᵣ of reactants: 2 × 102 = 204
  2. Mᵣ of useful product: 4 × 27 = 108
  3. Atom economy = 108 ÷ 204 × 100 = 52.9%

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)

  1. Fermentation: the useful product is 2 × C2H5OH. 2 × 46 = 92
  2. The reactant is glucose, Mᵣ = 180. 92 ÷ 180 × 100 = 51.1%
  3. From ethene there is only one product, so every atom ends up in the ethanol: 100%.
  4. Fermentation wastes about half the mass as carbon dioxide, but it uses a renewable raw material; ethene comes from crude oil.

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?

  1. A high yield means most of the possible product is collected.
  2. A low atom economy means many atoms end up in unwanted products.
  3. Waste products cost money to dispose of, unless a use is found for them.

Answer: it makes a lot of waste products that must be dealt with