Atom Economy
AQA A Level Chemistry revision notes on atom economy, desired products, waste products and how balanced equations are used to judge the efficiency of a chemical reaction.
GCSE Recap: Relative Formula Mass
Before you start, check that you can work out a relative formula mass when there are brackets and numbers in front.
Why Atom Economy Matters
Atom economy measures how efficiently atoms from reactants are converted into the desired product.
A reaction can have a high percentage yield but still have poor atom economy if a large proportion of the atoms form unwanted products. Atom economy links amount-of-substance calculations to sustainable chemistry and industrial process design.
Key idea: percentage yield compares actual yield with theoretical yield. Atom economy measures the proportion of atoms incorporated into the desired product.
The Percentage Atom Economy Formula
Percentage atom economy compares the relative formula mass of the desired product with the total relative formula mass of all products in the balanced equation.
Percentage atom economy = Mr of desired product ÷ sum of Mr of all products × 100
Use relative formula masses from the balanced equation. If a product has a coefficient in front of it, multiply its relative formula mass by that coefficient.
Exam focus: the numerator includes only the desired product or desired products. The denominator includes all products, with balancing numbers taken into account.
Desired Products and Waste Products
The desired product is the substance the reaction is intended to make. Other products may be described as waste products or by-products.
In atom economy questions, first identify the desired product. Then calculate what proportion of the total product mass is represented by that desired product.
| Term | Meaning | Exam implication |
|---|---|---|
| Desired product | The product the reaction is intended to make. | Include this in the numerator of the atom economy calculation. |
| Waste product | An unwanted product formed alongside the desired product. | Do not include this in the numerator unless the question says it is also a desired product. |
| By-product | A secondary product that may be unwanted, but could sometimes be sold or reused. | Selling a by-product can improve process economics, but it does not change the atom economy calculation. |
Quick Check: Set Up an Atom Economy Calculation
Drag the numbers into place for a reaction this page has not shown you.
Worked Example: Iron from Iron(III) Oxide
Question: Calculate the percentage atom economy for the reaction below if iron is the desired product.
Balanced equation: Fe2O3 + 3CO → 2Fe + 3CO2
| Stage | Working | Result |
|---|---|---|
| Mr of desired product | Desired product = Fe 2Fe = 2 × 55.8 |
111.6 |
| Sum of Mr of all products | 2Fe = 2 × 55.8 = 111.6 3CO2 = 3 × (12 + 2 × 16) = 132.0 |
243.6 |
| Atom economy | percentage atom economy = 111.6 ÷ 243.6 × 100 | 45.8% |
Answer: the atom economy is 45.8%. This means less than half of the product mass is the desired iron product; the rest is carbon dioxide.
Quick Check: Calculate Atom Economy
Work out each atom economy on paper before you flip the card.
Why One-Product Reactions Have 100% Atom Economy
If a reaction has only one product and all atoms from the reactants are used to make that product, the atom economy is 100%.
This is why many addition reactions are considered atom-efficient. For example, hydrogen adds across the C=C bond in ethene to make ethane.
CH2=CH2 + H2 → CH3CH3
There is only one product, so all atoms from ethene and hydrogen are found in ethane.
Remember: one-product reactions have maximum atom economy because there are no waste products in the balanced equation.
Quick Check: Compare Four Routes
Decide which route to the same product has the lowest atom economy.
Atom Economy and Sustainable Chemistry
Sustainable chemistry aims to design processes that reduce waste, reduce separation costs and make better use of starting materials.
| Feature | High atom economy | Low atom economy |
|---|---|---|
| Use of atoms | Most atoms become desired product. | Many atoms become waste products. |
| Waste | Less waste is produced. | More waste must be separated, treated or disposed of. |
| Industrial value | Usually more sustainable and efficient. | May be less sustainable unless by-products have useful applications. |
Quick Check: Atom Economy, Yield and Waste
In each round, pick the one statement that is accurate.
Common Exam Points
- Atom economy measures the proportion of atoms that become desired product.
- The formula is Mr of desired product ÷ sum of Mr of all products × 100.
- Use the balanced equation because coefficients affect the reacting masses.
- The denominator must include all products shown in the balanced equation.
- The numerator includes only the desired product, unless more than one product is desired.
- Reactions with only one product have 100% atom economy.
- High atom economy is desirable because it reduces waste and supports sustainable chemistry.
- Selling a by-product may improve the economics of a process, but the atom economy calculation still depends on which product is useful.
Why High Atom Economy Matters
A reaction with a high atom economy turns most of the mass of its reactants into the product wanted, so little is wasted. The advantages are usually asked for under three headings.
| Advantage | Explanation |
|---|---|
| Economic | Less raw material is bought per tonne of product, less waste has to be separated, treated and disposed of, and by-products may be sold rather than dumped. |
| Environmental | Less waste goes to landfill or is released, fewer resources are consumed and less energy is used in separation, so the process is more sustainable. |
| Ethical | Finite raw materials are conserved for future generations and the pollution burden on communities near the plant is reduced. |
Addition reactions, where all the reactant atoms end up in one product, have an atom economy of 100%. Substitution and elimination reactions always produce a by-product, so their atom economy is lower, and industry prefers routes that avoid them where a choice exists.
Exam sentence: A high atom economy process is more sustainable because it uses fewer raw materials and produces less waste, which reduces costs and environmental harm.
Quick Check: Explain the Benefits
Write a short explanation, then compare it with the mark points and the model answer.
QuickSnap
This text summary condenses the page into the essential exam ideas.
- Atom economy: the percentage of atoms that end up in the desired product.
- Formula: Mr of desired product ÷ sum of Mr of all products × 100.
- Balanced equation: needed because reacting masses depend on stoichiometric coefficients.
- 100% atom economy: usually occurs when there is only one product.
- Low atom economy: means a greater proportion of atoms become waste products or by-products.
- Sustainable chemistry: favours high atom economy because it minimises waste products.
Master Amount of Substance for AQA A Level Chemistry
Continue from these free revision notes into the full 3.1.2 Amount of Substance course, covering moles, Avogadro constant, empirical and molecular formulae, reacting masses, concentration, titrations, gas volumes, percentage yield and atom economy with guided video teaching, diagnostic MCQ practice, teacher-marked short-answer questions and a personalised progress report.
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Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across the full 3.1.2 Amount of Substance specification, including mole calculations, formulae, solution calculations, gas calculations, yield and atom economy.
See how the course works
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A student prepares a sodium hydroxide solution.
| Quantity | Value |
|---|---|
| Concentration of NaOH | 0.200 mol dm−3 |
| Volume used | 25.0 cm3 |
| Volume in dm3 | 0.0250 dm3 |
What amount of NaOH is present in the 25.0 cm3 sample?
Calculate the mass of CaCO3 that reacts with 25.0 cm3 of 0.200 mol dm−3 HCl.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
FAQs
These questions address common misconceptions students have when learning atom economy calculations.
What is atom economy?
Atom economy is the percentage of atoms that end up in the desired product. It shows how efficiently a reaction uses atoms from the reactants.
How do you calculate percentage atom economy?
Use the relative formula mass of the desired product divided by the sum of the relative formula masses of all products, then multiply by 100. Include balancing numbers from the equation.
Why do balancing numbers matter in atom economy?
Balancing numbers show how many formula units or molecules are produced. They must be included when calculating the relative formula masses used in the atom economy calculation.
When is atom economy 100%?
Atom economy is 100% when all atoms from the reactants are incorporated into the desired product. This often occurs in reactions with only one product, such as simple addition reactions.
What is the difference between percentage yield and atom economy?
Percentage yield compares the actual amount obtained with the theoretical amount. Atom economy measures how much of the product mass is the desired product.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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