Percentage Yield and Atom Economy
Percentage Yield
In theory, a reaction should produce a calculated amount of product (the theoretical yield). In practice, you almost always get less. The percentage yield compares what you actually obtained with what you theoretically could have obtained.
percentage yield = (actual yield ÷ theoretical yield) × 100
The percentage yield is always between 0% and 100%. A yield of 100% means you obtained the maximum possible product, which rarely happens.
Why Is the Yield Less Than 100%?
Several factors reduce the yield:
1. The reaction is reversible — not all reactants are converted to products because the reaction reaches equilibrium
2. Product is lost during transfer — when pouring between containers, filtering, or evaporating, some product is always left behind
3. Side reactions — unwanted reactions produce different products, using up some of the reactants
4. Incomplete reaction — not all the reactant may have reacted, especially if it was impure
Worked Example
The theoretical yield of copper sulfate from a reaction is 16 g. A student actually obtains 12 g.
Percentage yield = (12 ÷ 16) × 100 = 75%
Why Yield Matters
In industry, a high percentage yield is important because:
- It reduces waste — fewer unreacted raw materials
- It is more cost-effective — more product for the same amount of reactant
- It reduces the environmental impact — less waste to dispose of
Atom Economy
Atom economy measures how much of the reactant atoms end up in the desired product (as opposed to waste by-products).
atom economy = (Mr of desired product ÷ sum of Mr of all products) × 100
A high atom economy means most atoms from the reactants end up in the useful product, with less waste.
Worked Example 1: High Atom Economy
2Mg + O₂ → 2MgO
The only product is MgO (the desired product).
Atom economy = (2 × 40) ÷ (2 × 40) × 100 = 100%
Reactions with only one product always have 100% atom economy.
Worked Example 2: Low Atom Economy
CaCO₃ → CaO + CO₂
Desired product = CaO (Mr = 56)
All products: CaO (56) + CO₂ (44) = 100
Atom economy = (56 ÷ 100) × 100 = 56%
44% of the mass of products is waste CO₂.
Worked Example 3
Ethanol can be made from ethene:
C₂H₄ + H₂O → C₂H₅OH
Only one product → atom economy = 100%
Ethanol can also be made by fermentation:
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Desired product: 2 × C₂H₅OH = 2 × 46 = 92
All products: 92 + (2 × 44) = 92 + 88 = 180
Atom economy = (92 ÷ 180) × 100 = 51.1%
The hydration route has a much higher atom economy than fermentation.
Comparing Yield and Atom Economy
| Percentage yield | Atom economy | |
|---|---|---|
| Measures | How much desired product you actually get | How much of the atoms end up in desired product |
| Depends on | Practical technique, losses | The reaction itself (the equation) |
| Can be improved by | Better technique, purer reagents | Choosing a different reaction pathway |
| Value | Can vary each time | Fixed for a given reaction |
Why Atom Economy Matters
High atom economy is desirable because:
- Less waste is produced, reducing disposal costs and environmental harm
- Raw materials are used more efficiently, reducing costs
- It supports sustainable chemistry — making the most of finite resources
- By-products may be useful (e.g. CO₂ from fermentation is used in carbonated drinks)
Exam Tips
- Percentage yield uses actual and theoretical mass (or moles) of the desired product
- Atom economy uses Mr values from the balanced equation — you do not need experimental data
- A reaction with only one product always has 100% atom economy
- If the question asks why yield is less than 100%, give at least two specific reasons (reversible, lost in transfer, side reactions)
- Both concepts are about efficiency, but they measure different things — understand the distinction