Yield and atom economy

No reaction gives you every bit of product you could get. Percentage yield compares the mass you actually collect with the maximum mass the equation says you could make. Atom economy looks at the reaction itself: what percentage of the mass of the reactants ends up in the product you want. Industry wants both to be high, to save money and cut waste.

Key facts

  • Percentage yield = mass of product made ÷ maximum (theoretical) mass × 100.
  • Yields are below 100% because the reaction may not finish, some product is lost when it is separated, and other reactions can happen.
  • 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

Heating some calcium carbonate could make 28 g of calcium oxide, but only 22.4 g is collected. Find the percentage yield.

  1. Percentage yield = mass made ÷ maximum mass × 100.
  2. Put the numbers in. 22.4 ÷ 28 × 100 = 80%

Answer: 80%

Example 2

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 3

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%