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No atoms are created or destroyed in a chemical reaction. However, the atoms in the reactants may not become the desired product. They instead end up forming by-products.

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 of a reaction is a measure of how many reactant atoms form a desired product.

Calculating percentage atom economy

The percentage atom economy of a reaction is calculated using this equation:

The atom economy of a reaction is 100% if there are no unwanted by-products in the reaction. Usually the atom economy is less than 100%. The more atoms that end up in the by-products, the lower the atom economy.

Worked 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. (A_r of H = 1, A_r of C = 12, A_r of O = 16)

M_r of CH₄ = 12 + (4 × 1) = 16

M_r of H₂O = (2 × 1) + 16 = 18

Total M_r of reactants = 16 + 18 = 34

A_r of H₂ = (2 × 1) = 2

Total M_r of desired product = 3 × 2 = 6 (there are three H₂ in the balanced equation)

Atom economy = 17.6% (to 3 significant figures)