Atom Economy – IGCSE Chemistry Definition

Atom economy is a measure of how efficiently a chemical reaction converts the atoms in the reactants into the desired product. It is calculated from the balanced equation, not from experimental data. A high atom economy means most of the reactant atoms end up in the useful product, with little waste. A low atom economy means a large proportion of the atoms end up in unwanted by-products.

Formula

Atom economy = (Mr of desired product / total Mr of all products) x 100

Alternatively: Atom economy = (sum of Mr of desired products / sum of Mr of all reactants) x 100

Both forms give the same answer for a balanced equation. The first form is more commonly used at IGCSE.

Examples

Addition reactions have 100% atom economy because all the atoms end up in a single product. For example, the hydration of ethene: C₂H₄ + H₂O → C₂H₅OH. Only one product is formed, so atom economy is 100%.

Reactions that produce by-products have lower atom economy. For example, extracting iron: Fe₂O₃ + 3CO → 2Fe + 3CO₂. The desired product is iron (2 x 56 = 112). Total Mr of all products = 112 + (3 x 44) = 112 + 132 = 244. Atom economy = (112/244) x 100 = 45.9%.

Why it matters

In industry, high atom economy is desirable because it means less waste, lower costs for raw materials, and less pollution. Chemical engineers design processes to maximise atom economy where possible. This is a sustainability issue examined in the context of green chemistry.

Exam context

Paper 4 may ask candidates to calculate atom economy from a balanced equation or to compare the atom economy of two different routes to the same product. Understanding that atom economy is different from percentage yield is an important distinction that examiners test.

Worked exam question

Hydrogen can be made by reacting methane with steam: CH₄ + H₂O → CO + 3H₂. Calculate the atom economy for hydrogen production. (Mr: H₂ = 2, CO = 28) (2 marks)

Total Mr of all products = 28 + (3 x 2) = 34 [1]. Atom economy = (6/34) x 100 = 17.6% [1].

Common mistakes

Candidates confuse atom economy with percentage yield. Atom economy is calculated from the equation alone and has nothing to do with how much product is actually obtained. It is a theoretical measure of waste. Another error is forgetting to include the coefficient when calculating the Mr of products (e.g. 3H₂ has a total Mr of 6, not 2).