GCSE · Chemistry · AQA · Spec 8462

Atom economy

Heat calcium carbonate and, however carefully you work, only part of its mass can become calcium oxide. The balanced equation sets that share before anything reacts.

Chemistry · Atom economy

Heat calcium carbonate: how much of it becomes calcium oxide?

calcium oxide wanted: 5/10. Calcium oxide wanted 280 kg. Calcium carbonate to heat 500 kg. Carbon dioxide (unwanted) 220 kg. Atom economy 56%

Calcium oxide wanted280 kgCalcium carbonate to heat500 kgCarbon dioxide (unwanted)220 kgAtom economy56%

CaCO₃ → CaO + CO₂. Relative formula masses: CaCO₃ = 100, CaO = 56, CO₂ = 44. The masses shown assume all the calcium carbonate reacts.

Comparing reactions

Where does each reaction sit?

Don't calculate yet. The product in brackets is the one wanted. Drag each reaction to where you think its atom economy sits, judging by how much of the mass goes into the other products.

NH₃ — N₂ + 3H₂ → 2NH₃ (ammonia)MgCl₂ — MgO + 2HCl → MgCl₂ + H₂O (magnesium chloride)C₂H₅OH — C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (ethanol)H₂ — CH₄ + H₂O → CO + 3H₂ (hydrogen)

Worked calculation

Problem

Iron is made by this reaction: Fe₂O₃ + 3CO → 2Fe + 3CO₂. Calculate the percentage atom economy for iron. (Relative atomic masses: Fe = 56, C = 12, O = 16)

Check the working

Find the line that goes wrong

What is the atom economy for titanium in TiCl₄ + 4Na → Ti + 4NaCl? (Ti = 48, Cl = 35.5, Na = 23)

A student's answer — which line goes wrong?

Desired product: titanium, Ti = 48
TiCl₄ = 48 + (4 × 35.5) = 190
Na = 23
Sum of reactants = 190 + 23 = 213
Atom economy = (48 ÷ 213) × 100
Atom economy = 22.5%

What do you think?

A perfect reaction: what is its atom economy?

A student heats copper carbonate to make copper oxide: CuCO₃ → CuO + CO₂. Everything goes perfectly: every gram of copper carbonate reacts and none of the copper oxide is lost.

Which is closest to what you think about the atom economy for copper oxide?

It is 100%, because none of the copper oxide was lost.Atoms are never destroyed, so every balanced reaction has an atom economy of 100%.Atom economy belongs to the reaction, so it is the same whichever product you want.It is below 100%, fixed by the equation: only copper oxide's share of the reactant mass counts.

How sure are you?

I'm sureI think soI'm guessing

It is 100%, because none of the copper oxide was lost.

This is a common misconception

Losing none of the product tells you how much you collected compared with what you expected, which is a different measure. Atom economy comes from the balanced equation. Carbon dioxide is also made, and the mass that goes into it can never end up as copper oxide however carefully you work.

Atoms are never destroyed, so every balanced reaction has an atom economy of 100%.

This is a common misconception

Atoms are conserved, so the total mass of products equals the total mass of reactants. But atom economy counts only the desired product. The carbon and oxygen atoms that leave as carbon dioxide are not in the copper oxide, so they do not count.

Atom economy belongs to the reaction, so it is the same whichever product you want.

This is a common misconception

The top of the fraction is the desired product. Copper oxide is 79.5 of the 123.5 reactant mass, about 64%. If carbon dioxide were the product wanted, the top would be 44, giving about 36%. Always name the desired product first.

It is below 100%, fixed by the equation: only copper oxide's share of the reactant mass counts.

This matches the evidence

Yes. Copper oxide's share of the reactant mass is 79.5 ÷ 123.5 × 100 = 64.4%. That figure was set by the balanced equation before the reaction was run, and no amount of care can raise it.

Atom economy is calculated from the balanced equation for a named desired product. A reaction can go perfectly and still send much of its reactant mass into other products.

Higher

Choosing a reaction pathway

Two routes to the same product: which should be used?

The claim

Route A should be chosen to make product P. (Both routes and all their figures are hypothetical.)

Place each piece of evidence to load the balance. Mark the strong ones — they count double.

Atom economy (given): Route A 100%, Route B 48%.
Evidence 1: does it support or challenge the claim?SupportsChallenges
Strong evidence
Rate: Route A's reaction is fast under the conditions used; Route B's is slow.
Evidence 2: does it support or challenge the claim?SupportsChallenges
Strong evidence
Equilibrium position: Route A is reversible, and under the conditions used its equilibrium position lies to the left. Route B is not reversible.
Evidence 3: does it support or challenge the claim?SupportsChallenges
Strong evidence
Yield: Route A 30%, Route B 85%.
Evidence 4: does it support or challenge the claim?SupportsChallenges
Strong evidence
By-products: Route B's by-product is a useful substance that can be sold.
Evidence 5: does it support or challenge the claim?SupportsChallenges
Strong evidence

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Atom economy (also called atom utilisation) measures how much of the starting materials end up as useful products.
Percentage atom economy = (relative formula mass of desired product from equation ÷ sum of relative formula masses of all reactants from equation) × 100.
Take every mass from the balanced equation: multiply by each coefficient and include every reactant.
Atom economy depends on which product is wanted and does not change with the amount you make.
High atom economy matters for sustainable development and for economic reasons.
Higher only: a reaction pathway is chosen by weighing atom economy together with yield, rate, equilibrium position and usefulness of by-products.

The big picture

Atom economy is a measure of the amount of starting materials that end up as useful products. It is calculated from the balanced equation: the relative formula mass of the desired product, divided by the sum of the relative formula masses of all the reactants, multiplied by 100. Because it comes from the equation, it is fixed before the reaction is run, whatever the scale, and it depends on which product is wanted. Reactions with a high atom economy turn less of their starting materials into unwanted products, which is important for sustainable development and for economic reasons.

Key points

1Atom economy is the percentage of the reactants' mass, from the balanced equation, that ends up in the desired product.

2The calculation needs the balanced equation: coefficient × relative formula mass for the desired product, divided by the total for all the reactants, × 100.

3A reaction whose only product is the desired product has an atom economy of 100%.

4Atom economy is different from how much product you actually collect: a reaction can go perfectly and still have a low atom economy.

5A low atom economy means much of the starting materials ends up as unwanted products, which is less sustainable and more costly.

6Higher only: a lower atom economy can still be acceptable if the by-products are useful, or if the yield, rate or equilibrium position is better.

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Worked example

Problem

Sodium hydrogencarbonate decomposes when heated: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Calculate the percentage atom economy for sodium carbonate. (Relative atomic masses: Na = 23, H = 1, C = 12, O = 16)

⚠ Watch out

Dividing by the unwanted products instead of by all the reactants. For heating calcium carbonate, 56 ÷ 44 × 100 gives 127%, which is impossible: the bottom of the fraction must be the total mass of ALL the reactants, 100.

🧠

Memory hook

Want over all, times a hundred: the product you WANT on top, ALL the reactants underneath, coefficients included.

✓

Check yourself

2H₂O₂ → 2H₂O + O₂, with oxygen as the desired product. Using H₂O₂ = 34 and O₂ = 32, what is the percentage atom economy? (32 ÷ (2 × 34) × 100 = 47.1%)

Flashcards

(9)

What does atom economy measure?

The amount of starting materials that end up as useful (desired) products.

In the atom economy fraction, what goes on top and what goes underneath?

On top: the desired product's mass from the equation. Underneath: the total mass of every reactant from the equation. Then × 100.

Why must you use the coefficients in the balanced equation?

Each coefficient tells you how many of that formula react or form, so its relative formula mass must be multiplied by it before you add up or divide.

What is the atom economy of a reaction whose only product is the desired product?

100%: all of the reactant mass ends up in the desired product.

If you make ten times as much product, what happens to the atom economy?

Nothing. The masses of reactants, product and unwanted products all scale together, so the percentage stays fixed by the equation.

Can the same reaction have two different atom economies?

Yes, one for each product you might want. The top of the fraction is whichever product is named as the desired one.

A reaction goes perfectly and none of the product is lost. Is its atom economy 100%?

Not necessarily. Atom economy comes from the balanced equation; if other products are formed, some reactant mass always ends up in them.

Why are reactions with a high atom economy preferred?

Less of the starting materials ends up as unwanted products, which is important for sustainable development and for economic reasons.

Higher only: besides atom economy, what data help decide which reaction pathway to use?

Yield, rate, equilibrium position and the usefulness of by-products.

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Where does each reaction sit?

Find the line that goes wrong

A perfect reaction: what is its atom economy?

Two routes to the same product: which should be used?

More AQA GCSE Chemistry topics