Yield and atom economy
Percentage yield
The is the amount of product you actually make. The theoretical yield is the maximum you could make, calculated from the equation.
Example: 12 g of magnesium could make 20 g of magnesium oxide, but only 16 g is collected.
percentage yield = (16 ÷ 20) × 100 = 80%
Yields are below 100% because:
- the reaction may be , so it doesn't go to completion
- some product is lost when it is separated, filtered or poured
- some reactants may react in unexpected ways (side reactions)
Key terms
Atom economy
measures how much of the reactants' mass ends up in the , rather than in waste by-products.
atom economy = (Mᵣ of desired product ÷ sum of Mᵣ of all reactants) × 100
Use the balanced equation, including the balancing numbers.
Example: making calcium oxide: CaCO₃ → CaO + CO₂ (CaCO₃ = 100, CaO = 56)
atom economy = (56 ÷ 100) × 100 = 56%
Example: making ethanol by fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (glucose = 180, ethanol = 46)
atom economy = (2 × 46 ÷ 180) × 100 = 51.1%
A reaction with only one product, such as C₂H₄ + H₂O → C₂H₅OH, has an atom economy of 100%.
Key terms
Why high atom economy matters
Reactions with a are better for : they use resources efficiently and produce less waste. They are also usually cheaper, because less of what you pay for is thrown away.
A low atom economy is less of a problem if the by-products can be sold or used. For example, carbon dioxide from fermentation can be used in fizzy drinks.
Choosing a reaction pathway (Higher tier only)
There is often more than one way to make a product. Chemists choose a by weighing up:
- rate of reaction
- , if the reaction is reversible
- whether the are useful
Example: ethanol can be made by fermenting sugar (atom economy 51.1%, slow, but uses renewable plants) or by hydrating ethene (atom economy 100%, fast, but ethene comes from crude oil, which is non-renewable).