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5.1.7Additional lessons

Atom Economy — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 95 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Definition and Formula

Definition
  • Definition Atom economy is the proportion of the total mass of reactants that ends up in the desired product.
Formula
  • Formula Atom economy = (molar mass of desired product ÷ sum of molar masses of all products) × 100.
Same value
  • Same value Because mass is conserved, the sum of masses of all products equals the sum of masses of all reactants.
Interpretation
  • Interpretation A high value means little waste is formed;
    • a low value means much of the reactant mass is wasted as by-products.

Part 2 of 5Calculation Method

1
Write a balanced equation and identify the desired product.
2
Calculate the molar mass of that product and multiply by its coefficient.
3
Add up the molar masses of all products, each multiplied by its coefficient, to get the total.
4
Divide the desired mass by the total mass and multiply by 100.
5
Quote the result to a suitable number of significant figures as a percentage.

Part 3 of 5Atom Economy and Yield

Atom Economy and Yield
Different ideas
  • Different ideas Percentage yield measures how well a reaction worked in practice, but atom economy is fixed by the equation.
High yield, low economy
  • High yield, low economy A reaction can give a large yield yet waste many atoms in by-products.
Addition reactions
  • Addition reactions These have 100 percent atom economy because all reactant atoms end up in one product.
Green chemistry
  • Green chemistry High atom economy reduces waste, saves raw materials and lowers disposal costs.

Part 4 of 5Worked Example

Question

  • Calculate the atom economy for iron in Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g).
  • (M: Fe = 55.85, Fe₂O₃ = 159.69, CO = 28.01, CO₂ = 44.01 g mol⁻¹)

  1. Mass of desired product2 × 55.85 = 111.70 g.
  2. Total mass of products111.70 + 3 × 44.01 = 111.70 + 132.03 ≈ 243.7 g (equal to the reactant total, 159.69 + 3 × 28.01 = 243.72 g).
  3. Atom economy = 111.70 ÷ 243.72 × 100 = 45.8 percent.

Answer Atom economy = 45.8 percent

Part 5 of 5Exam Checklist & Tips

5.1.7 – Key Points for the Exam

  • Atom economy The percentage of the mass of reactants that becomes the desired product.
  • By-product An unwanted substance formed alongside the desired product.
  • Addition reaction A reaction in which two reactants combine to form a single product.
  • Green chemistry Chemistry designed to reduce waste and hazards.

Common Mistakes

  • Treating atom economy and percentage yield as the same quantity.
  • Forgetting to multiply molar masses by coefficients.
  • Using only one by-product's mass when finding the total mass of products.
  • Claiming a high yield guarantees a high atom economy.

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