Yield & Atom Economy (AQA GCSE Chemistry): Revision Notes
Atom economy
What is atom economy?
Atom economy shows us how much of the starting materials (reactants) end up as useful products in a chemical reaction. It's measured as a percentage.
The higher the atom economy, the less waste is produced. This is important because it helps us choose the best reaction pathways for industrial processes and laboratory work.
Atom economy is a key concept in green chemistry, helping chemists design more environmentally friendly reactions by minimising waste production.
How to calculate atom economy
You can work out atom economy using this formula:
Percentage atom economy = (mass of desired product / mass of all reactants) × 100
You need to use the masses from a balanced chemical equation to do this calculation.
The formula helps us quantify exactly how efficient a reaction is at converting starting materials into the product we actually want.
Worked example
Worked Example: Calculating Atom Economy for Bromoethane Production
Let's look at making bromoethane from ethene and bromine:
C₂H₄ + Br₂ → C₂H₄Br + HBr
Step 1: Identify the desired product
- Desired product: C₂H₄Br (bromoethane)
Step 2: Calculate the mass of desired product
- Mass of desired product (C₂H₄Br) = 109
Step 3: Calculate the mass of all reactants
- Mass of all reactants (C₂H₄ + Br₂) = 190
Step 4: Apply the atom economy formula
- Atom economy = (109/190) × 100 = 57%
Interpretation: This means 57% of the reactant mass becomes useful product. The other 43% becomes waste (HBr).
Why atom economy matters
High atom economy reactions are better for both industry and the environment because they are:
- More sustainable - less waste means fewer raw materials are used up, preserving natural resources
- More economical - less waste means lower costs for buying materials and disposing of waste products
Industries often choose reaction pathways with higher atom economy not just for environmental reasons, but because it directly impacts their bottom line through reduced material costs and waste disposal fees.
Reactions with 100% atom economy
Some reactions have perfect atom economy of 100%. This happens when only one product is formed, so no atoms are wasted.
Polymerisation reactions are excellent examples of this:
- Many small molecules (monomers) join together
- They form one large molecule (polymer)
- No waste products are made
- All reactant atoms end up in the useful product
Key Point: When a reaction produces only one product, it automatically has 100% atom economy because no atoms are lost as unwanted by-products.
For example, when ethene molecules polymerise to make poly(ethene), no atoms are wasted - every atom from the reactants becomes part of the final polymer product.
Remember!
Key Points to Remember:
- Atom economy tells us what percentage of reactants becomes useful product
- Higher atom economy = less waste
- Formula: (mass of desired product / mass of all reactants) × 100
- Reactions with only one product have 100% atom economy
- High atom economy is better for the environment and cheaper for industry