AQA Chemistry (8462) 4.3.3.2 · Quantitative
Atom economy
Every atom you start with ends up somewhere. How much of the starting mass becomes the product you actually want, and why does industry care?
Triple Science · Foundation tier
Start here
Two factories. One product.
Both factories make ethanol, C₂H₅OH. Factory A ferments sugar: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Factory B adds steam to ethene: C₂H₄ + H₂O → C₂H₅OH. Both run perfectly: every molecule of starting material reacts.
If you had to guess, which factory turns the bigger share of its starting materials’ mass into ethanol: A or B?
That share is the atom economy (also called atom utilisation): a measure of the amount of starting materials that end up as useful products. It comes from the balanced equation and relative formula masses, so a chemist can work it out before the reaction is ever run.
Equations
Learn it · no chemistry equation sheet% atom economy = Mr of desired product ÷ sum of Mr of all reactants × 100
Both from the balanced equation.
sum of Mr of reactants = sum of Mr of products
Mass is conserved, so dividing by the products’ total gives the same answer.
The mass strip · drawn to scale
Call the share. Then run the reaction.
C₂H₄ + H₂O → C₂H₅OH
Mr: C₂H₄ 28 · H₂O 18 · C₂H₅OH 46
Commit first. What share of the reactants’ mass ends up in the ethanol?
Think again
“Sodium is made by electrolysing molten salt: 2NaCl → 2Na + Cl₂. Mr of Na is 23, so 23 ÷ 117 × 100 = 19.7%.”
Spot the flaw
CalculateWhere does the working go wrong? (Ar: Na = 23, Cl = 35.5)
One equation, three products
Electrolysing brine makes three useful products.
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
Mr: NaCl 58.5 · H₂O 18 · NaOH 40 · Cl₂ 71 · H₂ 2
Commit first. Taken as the desired product, which one gives the lowest atom economy?
A high atom economy matters for sustainable development and for economic reasons: fewer raw materials are wasted, and there is less waste to dispose of. A reaction with only one product, such as an addition reaction, has an atom economy of 100%.
Atom economy is not percentage yield. Atom economy belongs to the balanced equation, so it is the same every time the reaction runs. Percentage yield depends on how the reaction is carried out: how much product is actually collected.
Command words in this lesson
- Calculate
- Show the working from the equation and give the answer as a percentage, usually to 3 significant figures.
- Give
- One short answer, no explanation needed.
- Explain
- Give the reason as linked steps, each one leading to the next.
Key note · AQA 4.3.3.2 (8462)
Atom economy
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
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Ask MrBadmus
Not sure what goes on the top and bottom of the fraction?
The mass strip draws each formula unit as a bar whose length is its relative formula mass, and shows the products forming in one move. It says nothing about how fast a reaction goes or how much product is collected.