A level Chemistry · Edexcel 9CH0 · Topic 5D
Yield and atom economy
Key points
- Theoretical yield is the maximum mass of product from the limiting reagent; the limiting reagent is the one that is used up first.
- Percentage yield = (actual yield ÷ theoretical yield) × 100.
- Yields are below 100% because of incomplete or reversible reactions, side reactions and loss of product during transfer and purification.
- Atom economy = (molar mass of desired product ÷ total molar mass of all reactants) × 100, using the balanced equation.
- Addition reactions have 100% atom economy; substitution and elimination reactions produce by-products so are lower.
- High atom economy means less waste, so processes are more sustainable and cheaper; selling the by-products can help too.
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A balanced equation tells you the most product you could possibly make, but real reactions never quite get there, and many also make by-products nobody wants. Percentage yield and atom economy measure these two different kinds of…
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