1.7.4Physical Chemistry
Yield and Atom Economy — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 32 of 42
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 6Percentage Yield
Theoretical yield
- Theoretical yield The maximum mass of product predicted from the balanced equation and the limiting reagent.
Actual yield
- Actual yield The mass of product actually collected in the experiment.
Formula
- Formula Percentage yield = actual yield ÷ theoretical yield × 100.
Why yield is below 100%
- Why yield is below 100% Reactions may be reversible, side reactions occur, and product is lost during transfer, filtering or purifying.
Part 2 of 6Atom Economy
Atom Economy
Definition
- Definition Atom economy measures how much of the mass of the reactants ends up in the desired product.
Formula
- Formula Atom economy = (molar mass of desired product ÷ sum of molar masses of all products) × 100.
High atom economy
- High atom economy Means little waste is made, such as in addition reactions that form only one product.
Low atom economy
- Low atom economy Occurs in reactions that produce large amounts of by-products, like substitution or elimination.
| Reaction type | Typical atom economy |
|---|---|
| Addition | 100% |
| Substitution | Less than 100% |
| Elimination | Less than 100% |
Part 3 of 6Yield versus Atom Economy
1
Yield tells you how well the experiment went in practice for a particular reaction.
2
Atom economy tells you how efficient the reaction design is, whatever the experimental skill.
3
A reaction can have a high yield but poor atom economy if it makes useful product plus much waste.
4
High atom economy reduces raw material use, waste disposal costs and environmental harm.
5
Industry also considers energy use, hazards and whether by-products can be sold or recycled.
Part 4 of 6Improving Sustainability
Choose addition routes
- Choose addition routes Reactions with only one product have 100% atom economy.
Use catalysts
- Use catalysts Catalysts let reactions run at lower temperature and pressure, saving energy.
Recycle by-products
- Recycle by-products Selling or reusing the waste stream increases the effective efficiency of a process.
Recycle reagents
- Recycle reagents Unreacted starting materials can be separated and fed back into the reactor.
Part 5 of 6Worked Example
Question
- 10.0 g of calcium carbonate is heated: CaCO₃(s) → CaO(s) + CO₂(g).
- 5.00 g of calcium oxide is collected.
- Calculate the percentage yield and atom economy for CaO.
- (Mr CaCO₃ = 100.1, CaO = 56.1, CO₂ = 44.0)
- Moles of CaCO₃ = 10.0 ÷ 100.1 = 0.0999 mol.
- Theoretical moles of CaO = 0.0999 mol, so theoretical mass = 0.0999 × 56.1 = 5.60 g.
- Percentage yield = 5.00 ÷ 5.60 × 100 = 89.2%.
- Atom economy = 56.1 ÷ (56.1 + 44.0) × 100 = 56.0%.
Answer Yield = 89.2%; atom economy = 56.0%
Part 6 of 6Exam Checklist & Tips
1.7.4 – Key Points for the Exam
- Theoretical yield Maximum possible mass of product from the equation.
- Percentage yield Actual yield as a percentage of theoretical yield.
- Atom economy Percentage of reactant mass that becomes useful product.
- By-product An unwanted substance made along with the desired product.
Common Mistakes
- Using the mass of the excess reagent to calculate theoretical yield.
- Calculating atom economy with only one product in the denominator.
- Quoting a percentage yield above 100% without commenting on impurities.
- Confusing yield with atom economy; one depends on the experiment, the other on the equation.
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