Atom Economy

Atom economy measures how much of the mass of the reactants ends up in the desired product, rather than in by-products. It is worked out from the balanced equation:atom economy = (M_r of desired product ÷ sum of M_r of all reactants) × 100Use the M_r values multiplied by the numbers in front of each formula in the equation.Worked example: calcium oxide is made by heating calcium carbonate.CaCO₃(s) → CaO(s) + CO₂(g)atom economy = (56.1 ÷ 100.1) × 100 = 56.0%Almost half of the mass of the reactant is lost as carbon dioxide.Addition reactions, where everything combines into one product, always have a 100% atom economy. For example:C₂H₄ + H₂O → C₂H₅OHSubstitution and elimination reactions always have lower atom economies, because a second product is formed.Atom economy and percentage yield are different. Percentage yield depends on how well the practical process is carried out; atom economy is fixed by the equation. A reaction can have a 100% yield and still a poor atom economy.Processes with a high atom economy are more sustainable: they produce less waste, use fewer raw materials and are often cheaper. The overall efficiency can also be improved by finding a use for the by-products.