IB Chemistry HLTopic 5 — How Much? Quantifying Chemical ChangePaper 1 & 2Core idea~9 min read
Atom Economy
You can run a reaction perfectly, collect every last crystal, and still throw away half of what you started with. Not because you were careless — because the equation itself sends half the atoms somewhere you did not want them. Atom economy measures that.
📚 What you need to know
Atom economy = M of the desired product ÷ total M of all reactants × 100.
It is worked out from the balanced equation only — no experimental data needed.
Remember to include the balancing numbers in both the top and the bottom.
Addition reactions have 100% atom economy, because there is only one product.
It is completely different from percentage yield: yield judges the process, atom economy judges the reaction.
A low atom economy means more waste, more raw material and higher costs.
Atom economy is fixed for a given reaction — the only way to improve it is a different route.
Where do the atoms actually go?
Atom economy
atom economy = (M of desired product ÷ sum of M of all reactants) × 100
Every atom you buy has to end up somewhere. Some finish up in the product you wanted; the rest leave as by-products. Atom economy is simply the fraction of the reactant mass that lands in the useful column.
This is why cement production is such a large source of carbon dioxide. The waste is not a leak or an inefficiency — it is chemistry doing exactly what the equation says.
Yield and atom economy answer different questions
These two get muddled constantly, so be clear about what each one is for. Percentage yield asks “how well did the experiment go?” Atom economy asks “was this a sensible reaction to choose in the first place?”
Notice which side you can improve by being careful in the lab. Only the left one. The right-hand number is decided the moment you choose your reaction.
If an exam question asks you to “suggest how the process could be made greener”, a high yield is not the answer they want. Look for a route with fewer by-products — that is an atom economy answer.
Why addition reactions win
An addition reaction joins two molecules into one, so there is nothing else for the atoms to become. Every atom you put in ends up in the product, and the atom economy has to be 100%.
The 100% case
C2H4 + Br2 → C2H4Br2 one product, so nothing is wasted
Substitution and elimination reactions always throw something away — a molecule of water, a hydrogen halide, a salt. That by-product is pure waste as far as atom economy is concerned, even if you can sell it afterwards.
Reaction
Desired product
Atom economy
C2H4 + Br2 → C2H4Br2
C2H4Br2
100%
C2H5OH + HCl → C2H5Cl + H2O
C2H5Cl
78.2%
ZnO + C → Zn + CO
Zn
70.0%
CaCO3 → CaO + CO2
CaO
56.0%
CH4 + H2O → CO + 3H2
H2
17.8%
Look at that last row. Making hydrogen from methane and steam has an atom economy under 18%, because most of the mass you feed in is carbon and oxygen that leave as carbon monoxide. That single number is a large part of the argument for making hydrogen a different way.
Worked examples
WORKED EXAMPLE
Calculate the atom economy for making quicklime: CaCO3(s) → CaO(s) + CO2(g). M: CaCO3 100.09, CaO 56.08.
Step 1: identify the desired product
The useful product is CaO, M = 56.08Step 2: total the reactants
Only one reactant here: 100.09Step 3: divide and multiply by 10056.08 ÷ 100.09 × 100 = 56.03Atom economy = 56.0%No masses, no yields, no lab data. Just the equation and the periodic table.
WORKED EXAMPLE
Zinc is extracted by heating its oxide with carbon: ZnO(s) + C(s) → Zn(s) + CO(g). Calculate the atom economy, taking zinc as the desired product.
Step 1: molar massesZnO = 65.38 + 16.00 = 81.38; C = 12.01; Zn = 65.38Step 2: add up ALL the reactants81.38 + 12.01 = 93.39Step 3: divide and multiply by 10065.38 ÷ 93.39 × 100 = 70.01Atom economy = 70.0%The oxygen and carbon leave as carbon monoxide. That is 30% of the mass fed into the furnace, and it is unavoidable with this route.
WORKED EXAMPLE
Chloroethane can be made two ways. Route A: C2H4 + HCl → C2H5Cl. Route B: C2H5OH + HCl → C2H5Cl + H2O. Which has the better atom economy, and why?
Route A: one product only
Every atom ends up in C2H5Cl, so atom economy = 100%Route B: work it outM(C₂H₅Cl) = 64.52; reactants = 46.08 + 36.46 = 82.5464.52 ÷ 82.54 × 100 = 78.2Compare and explain
Route A is an addition reaction with a single product. Route B is a substitution, so water is formed as waste.
Route A is better: 100% against 78.2%The explanation is the mark here. “Route A has only one product, so no atoms are wasted” says everything the examiner is looking for.
💡 Exam tip
Include the coefficients in your molar masses. 3H2 is 6.06, not 2.02.
Add up every reactant on the bottom, even the ones that seem minor.
Show your working even for a 100% answer. Questions often ask for it explicitly.
Read which product is “desired”. The same reaction has a different atom economy depending on what you want.
Use the “sum of products” version if you prefer: desired product mass ÷ total mass of all products gives the same answer for a balanced equation.
Link low atom economy to real consequences: more raw material, more waste to handle, higher cost, bigger environmental impact.
⚠ Common mix-up
Confusing it with percentage yield. Atom economy never uses a mass you measured.
Dividing by the desired product instead of by the reactants. The useful product goes on top.
Forgetting a reactant from the bottom line, which makes your answer too high.
Ignoring balancing numbers in either the numerator or the denominator.
Assuming a high yield means low waste. A 99% yield of a reaction with 40% atom economy still wastes most of the mass.
Thinking better technique improves atom economy. Only a different reaction route can.
That completes “how much”. You can now go from a balanced equation to masses, gas volumes, concentrations, limiting reactants and both measures of efficiency. Next comes “how far” — because many of these reactions never actually finish. Up next: Features of Dynamic Equilibrium.
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