📘 IB Understanding
Atom economy is a measure of efficiency in green chemistry. It tells us what fraction of the atoms in the reactants end up in the desired product rather than in waste by-products.
The Formula
\( \text{% Atom Economy} = \frac{\text{Molar mass of desired product}}{\text{Total molar mass of all reactants}} \times 100 \)
This formula is provided in the IB Chemistry data booklet.
Key Differences: Yield vs Atom Economy
| Percentage Yield (R2.1.4) | Atom Economy (R2.1.5) | |
|---|---|---|
| Measures | How well the experiment was carried out | How efficient the reaction pathway is |
| Requires experiment? | Yes (needs actual product mass) | No (calculated from the equation alone) |
| Type of waste | Physical losses (spills, transfers) | Stoichiometric by-products |
| Changed by | Better lab technique | Choosing a different reaction pathway |
Worked Example
C₂H₄ + H₂O → C₂H₅OH
Step 1: Molar mass of desired product:
M(C₂H₅OH) = 46.08 g mol⁻¹
Step 2: Total molar mass of all reactants:
M(C₂H₄) + M(H₂O) = 28.06 + 18.02 = 46.08 g mol⁻¹
Answer:
\( \text{AE} = \frac{46.08}{46.08} \times 100 = \) 100%
This is an addition reaction: all atoms are incorporated into a single product, giving 100% atom economy!
Reaction Type and Atom Economy
✅ Addition Reactions
Always 100% atom economy. All reactant atoms join into a single product molecule. No by-products.
Example: C₂H₄ + HBr → C₂H₅Br
❌ Substitution/Elimination
Always < 100% atom economy. These reactions inherently produce by-products alongside the desired product.
Example: C₃H₈ + Br₂ → C₃H₇Br + HBr
Green Chemistry Connection
♻️ Sustainability
There is a direct inverse relationship between atom economy and waste:
- High atom economy = most atoms end up in the product = less waste = more sustainable
- Low atom economy = many atoms become unwanted by-products = more waste = greater environmental harm and disposal costs
Industries prefer catalysed addition pathways wherever possible to maximise atom economy. Both high yield AND high atom economy are needed for truly efficient processes.
Think About It
A pharmaceutical company uses a 6-step synthesis with 100% yield at each step but only 30% overall atom economy. Is this process sustainable?
No! Despite perfect yields, 70% of the total reactant mass becomes waste by-products at the molecular level. A green chemist would redesign the pathway to use addition reactions where possible, reducing inherent waste.