Atom economy
No atoms are gained or lost in a chemical reaction. However, some atoms in the reactantsSubstances present at the start of a chemical reaction. may not end up in the desired productsSubstances formed in a chemical reaction.. They instead form other products, and so are regarded as by-productA product from a side reaction which is formed during the creation of another product..
For example, hydrogen can be manufactured by reacting methane with steam:
methane + steam → hydrogen + carbon monoxide
CH4(g) + H2O(g) → 3H2(g) + CO(g)
In this reaction, carbon and oxygen atoms in the reactants do not form the useful product. Carbon monoxide is a waste gas.
The atom economy Atom economy = mass of desired product / total mass of products x 100. of a reaction is a measure of the amount of starting materials that end up as useful products. It is important for sustainable development and for economic reasons to use reactions with high atom economy.
Calculating percentage atom economy
The percentage atom economy of a reaction is calculated using this equation:
\(atom~economy = \frac {mass~of~desired~product}{total~mass~of~products} \times 100 \)
The highest possible value of atom economy is 100%, when all the reactant atoms end up in the desired product. If the atom economy is 50%, for example, then half the reactant atoms end up in the desired product or products.
Example
Hydrogen can be manufactured by reacting methane with steam:
CH4(g) + H2O(g) → 3H2(g) + CO(g)
Calculate the atom economy for the reaction. (Ar of H = 1, Ar of C = 12, Ar of O = 16)
Mr of H2 = 2 × 1 = 2
Mr of CO = 12 + 16 = 28
total Mr of all products = 3 × 2 + 28 = 34
Ar of H2 = (2 × 1) = 2
total Mr of desired product = 3 × 2 = 6 (there are three H2 in the balanced equation)
\(atom~economy = \frac {mass~of~desired~product}{total~mass~of~products} \times 100\)
\(atom~economy = \frac {6}{34} \times 100 \)
atom economy = 17.6%