IB Chemistry SLTopic 5 — Quantifying Chemical ChangePaper 1 & 2Core idea~11 min read
Atom Economy
A reaction can go perfectly to completion and still be wasteful, because most of what you bought was never destined to become the product at all. Atom economy asks a question percentage yield cannot: how much of this mass am I actually going to keep?
📚 What you need to know
Atom economy is the proportion of the total mass of reactants that ends up as the desired product.
Atom economy = (Mr of desired product ÷ sum of Mr of all reactants) × 100, using the coefficients.
It is calculated from the balanced equation, never from experimental data.
Addition reactions have 100% atom economy, because there is only one product.
A high atom economy means less waste, lower raw material and disposal costs, and a more sustainable process.
Atom economy and percentage yield measure different things and industry wants both to be high.
What it measures
Atom economy
AE = (Mr of desired product ÷ Σ Mr of all reactants) × 100
Notice what is not in that expression: any mention of how much you made, how long it took, or how skilled you are. Atom economy is a property of the reaction you chose, fixed the moment you wrote the equation. Two chemists running the same reaction with wildly different yields have exactly the same atom economy.
They answer different questions, so a reaction can score brilliantly on one and dreadfully on the other.
Multiply each Mr by its coefficient, and include every reactant — even ones that feel incidental. The denominator is the total mass you have to buy.
Doing the calculation
🧩 The method
Write the balanced equation.
Decide which product is the desired one. The question always says.
Numerator: Mr of that product × its coefficient.
Denominator: add up Mr × coefficient for every reactant.
Divide, multiply by 100, and show the working even if the answer is obviously 100%.
Titanium is expensive partly for this reason. Four fifths of the mass bought becomes something that has to be recycled or disposed of.
WORKED EXAMPLE
Titanium is extracted by the Kroll process: TiCl4 + 2Mg → Ti + 2MgCl2. Calculate the atom economy, taking titanium as the desired product. (Ar: Ti 47.87, Cl 35.45, Mg 24.31)
Step 1 — numeratorM(Ti) = 47.87Step 2 — denominator, all reactants with coefficientsM(TiCl₄) = 47.87 + 4(35.45) = 189.672 × M(Mg) = 2 × 24.31 = 48.62total = 189.67 + 48.62 = 238.29Step 3 — divide(47.87 ÷ 238.29) × 100 = 20.09atom economy = 20.1%Nothing is wrong with the reaction — it works beautifully. The waste is built into the equation itself, and the only fix is a different equation.
Why addition reactions win
If a reaction has only one product, every atom you put in comes out in the thing you wanted, so the atom economy is 100% by definition. That is what makes addition reactions so attractive industrially.
One product, no waste
C2H4 + H2O → C2H5OH
Reactions that produce a by-product — substitutions, eliminations, most precipitations — can never reach 100%, because some of the mass is committed to the by-product before the reaction even starts.
WORKED EXAMPLE
Ethanol can be made by hydration of ethene, C2H4 + H2O → C2H5OH, or by fermentation, C6H12O6 → 2C2H5OH + 2CO2. Compare the atom economies. (Mr: C2H4 28.06, H2O 18.02, C2H5OH 46.08, C6H12O6 180.18)
Hydration of ethenereactants = 28.06 + 18.02 = 46.08(46.08 ÷ 46.08) × 100 = 100100% — one product, nothing wastedFermentationdesired = 2 × 46.08 = 92.16(92.16 ÷ 180.18) × 100 = 51.1551.2% — almost half leaves as CO₂Yet fermentation uses a renewable feedstock and hydration uses crude oil. Atom economy is one measure of a good process, not the only one.
Why industry cares
A low atom economy is not just untidy, it is expensive, and the costs land in three places at once: you buy raw material that was never going to become product, you pay to separate the by-product from the product, and you pay to dispose of or sell what is left. On top of that sit the environmental costs of extracting and processing material that ends up as waste.
This is why atom economy is a central idea in green chemistry. But it is not the only measure of an efficient process — a full assessment also weighs the rate of reaction, the energy required, the solvents and catalysts needed, whether the by-product has a market, and the overall economics.
WORKED EXAMPLE
A manufacturer reports a reaction with a 95% yield and a 40% atom economy. Explain what each figure tells you, and why both matter.
The 95% yieldOf the product the equation says was possible, 95% was actually isolated. The reaction runs almost to completion and very little is lost in handling.The 40% atom economyEven so, only 40% of the MASS of the reactants is the desired product. The other 60% was always going to be by-product, however well the reaction is run.Why both mattergood technique, wasteful chemistryImproving the yield further is nearly pointless — there is only 5% left to gain. Real improvement means finding a different reaction route with fewer by-products.
💡 Exam tip
Show the working even when the answer is 100%. Marks are for the substitution, not the conclusion.
Include every reactant in the denominator, each multiplied by its coefficient.
If the equation is balanced, the reactant total should equal the product total — a quick check on your arithmetic.
Asked which route is better? Compare the atom economies, then mention yield, energy, feedstock and by-product use for the evaluation marks.
Remember atom economy comes from the equation; yield comes from the experiment.
⚠️ Common mix-up
Confusing atom economy with percentage yield. Different questions, different data, different fixes.
Forgetting the coefficients when adding up reactant masses.
Leaving out a reactant because it seems minor. Every atom bought counts.
Using experimental masses. Atom economy is theoretical, from Mr values.
Assuming a high atom economy means a green process. Energy, solvents and feedstock all still matter.
That completes How Much? Quantifying Chemical Change. You can balance an equation, convert freely between mass, gas volume, concentration and particles, find which reactant runs out first, and judge a reaction on both what it yields and what it wastes. Up next in Topic 5: How Fast? — the same reactions, timed.
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