Summary AS Chemistry - 3.1.2 Amount of Substance: Percentage Yield and Atom Economy

Percentage Yield & Atom Economy
Percentage Yield:
Percentage yield tells you how wasteful a process is.
The equation for percentage yield is as follows:


Percentage Yield = (actual yield/theoretical yield) x 100


Percentage yield is never 100%.
Reasons for this include:
- Some may be lost in isolation (e.g. filter paper)
- Some may be lost in transfer
- Some of the reactants may not have reacted
- The reaction could be reversible
- Side reactions may be occurring at the same time


Atom Economy:
Atom economy tells you how wasteful a reaction is.
i.e. the proportion of reactants that become part of the desired product.
The equation for atom economy is as follows:


Atom Economy = (molecular mass of desired product/sum of molecular mass of reactants) x 100


A greater atom economy is desired because:
- There is less waste (easier to dispose & better for environment)
- More sustainable (since it’s a more efficient use of reactants)
- Easier and cheaper to separate the desired & waste products


When calculating atom economy, the numbers in front of the molecules in the equation
should be accounted for:

E.g. CH4(g) + H2O(g) → 3H2(g) + CO(g)