Revision notes · Quantitative chemistry
Yield and atom economy of chemical reactions
Percentage yield4.3.3.1
Definition: Percentage yield is the actual amount of product obtained from a chemical reaction expressed as a percentage of the maximum theoretical yield.
Percentage yield measures how efficient a chemical process is at making the expected mass of product.
- •Calculated using the formula: percentage yield = (mass of product made ÷ maximum theoretical mass) × 100.
- •A yield of 100% is rarely achieved because the reaction may be reversible, product may be lost during separation, or reactants may react in unexpected side reactions.
- •The theoretical yield is the maximum possible mass of product calculated from the balanced equation.
- •High percentage yield reduces wasted reactants and lowers manufacturing costs in industrial processes.
⚠️ Common mistake: Confusing percentage yield (which uses measured masses from an experiment) with atom economy (which uses relative formula masses).
🧠 Remember: Yield = (Actual mass ÷ Theoretical mass) × 100.
Atom economy4.3.3.2
Definition: Atom economy is a measure of the proportion of starting materials that end up as useful desired products.
Atom economy evaluates how sustainable a chemical reaction pathway is by looking at waste production.
- •Calculated using the formula: atom economy = (relative formula mass of desired product ÷ total relative formula mass of all reactants) × 100.
- •Reactions that produce only one product always have an atom economy of 100%.
- •High atom economy conserves raw materials, creates less waste, and is more cost-effective and sustainable.
- •Industrial chemists choose reactions based on atom economy, percentage yield, rate of reaction, cost, and position of equilibrium.
⚠️ Common mistake: Forgetting to multiply the relative formula mass (Mᵣ) by the balancing numbers (coefficients) in front of the formulas in the balanced equation.
🧠 Remember: Atom economy uses Mᵣ values; percentage yield uses experimental masses.
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