Percentage yield and atom economy
Atom economy is a way to measure the atoms wasted when making a chemical. The higher the atom economy, the 'greener' the process. 100 per cent atom economy means that all the atoms in the reactants have been converted to the desired product.
Atom economy = Mr of desired products∕sum of Mr of all products × 100
For example, calculate the atom economy for making hydrogen by reacting zinc with hydrochloric acid: Zn + 2HCl → ZnCl2 + H2
Mr of H2 = 1 + 1 = 2
Mr of ZnCl2 = 65 + 35.5 + 35.5 = 136
Atom economy = 2∕136 + 2 × 100 = 2∕138 × 100 = 1.45%
Industrial processes need as high an atom economy as possible, because this:
For example, hydrogen can also be made from methane and steam: CH4 + 2H2O → CO2 + 4H2
Atom economy = 4 × 2∕44 + (4 × 2) × 100 = 8∕52 × 100 = 15.4%
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