Percentage yield
- The yield is the amount of product made. Even though no atoms are gained or lost, it is not always possible to get the mass you calculate.
- Percentage yield = (mass of product actually made ÷ maximum theoretical mass of product) × 100.
- Reasons the yield is less than 100%: the reaction may be reversible and not go to completion; some product may be lost when it is separated from the reaction mixture; some reactants may react in unexpected ways (side reactions).
Theoretical yield (Higher)
- The maximum theoretical mass of product is calculated from the balanced equation, using moles.
- Example: 10 g of CaCO₃ (0.1 mol) can make at most 0.1 mol of CaO, which is 0.1 × 56 = 5.6 g.
Atom economy
- Atom economy is a measure of the amount of starting materials that end up as useful products.
- Atom economy = (Mr of desired product from the equation ÷ sum of Mr of all reactants) × 100.
- Reactions with only one product have an atom economy of 100%.
- A high atom economy is important for sustainable development and for economic reasons: less waste, fewer resources used, and less cost.
Choosing a reaction pathway
- Chemists consider the atom economy, the yield, the rate of reaction, the position of equilibrium (for reversible reactions), and whether by-products are useful.
- Example: CaCO₃ → CaO + CO₂ has an atom economy of (56 ÷ 100) × 100 = 56%, unless a use is found for the carbon dioxide.
Key terms
- Yield
- The amount of product obtained from a reaction.
- Percentage yield
- The actual mass of product as a percentage of the maximum theoretical mass.
- Theoretical yield
- The maximum mass of product that could be made, calculated from the equation.
- Atom economy
- The percentage of the mass of reactants that ends up in the desired product.
- By-product
- An unwanted product of a reaction.
- Reversible reaction
- A reaction in which the products can react to reform the reactants.
- Sustainable development
- Meeting the needs of people today without harming the ability of future generations to meet theirs.