5.1.7Additional lessons
Atom Economy — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 65 of 98
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.
Section 1 of 6What Atom Economy Measures
Atom economy judges a reaction by how much of what goes in is useful.
Definition
- Definition Atom economy is the percentage of the total mass of the reactants that ends up in the desired product.
Formula
- Formula Atom economy = (molar mass of desired product ÷ sum of molar masses of all products) × 100.
Different from yield
- Different from yield Atom economy depends only on the equation, whereas percentage yield depends on the experiment.
Waste
- Waste A low atom economy means that a large share of the reactant atoms become by-products and are wasted.
Section 2 of 6Calculation
1
Write the balanced equation and find the molar mass of the desired product, multiplied by its coefficient.
2
Find the sum of the molar masses of all products, each multiplied by its coefficient.
3
Divide the first value by the second and multiply by 100.
4
Compare the result with other possible routes to the same product.
Section 3 of 6Why It Matters
Why It Matters
Addition reactions
- Addition reactions They have an atom economy of 100% because all atoms of the reactants appear in the single product.
Substitution and elimination
- Substitution and elimination These produce by-products, so their atom economy is below 100%.
Green chemistry
- Green chemistry Higher atom economy conserves raw materials, reduces waste and lowers disposal costs.
Economic value
- Economic value By-products that can be sold or recycled improve the overall economics of a process.
| Reaction type | Typical atom economy |
|---|---|
| Addition | 100% |
| Substitution | Less than 100% |
| Decomposition to useful product plus waste | Depends on masses |
Section 4 of 6Atom Economy and Yield Together
Atom Economy and Yield Together
High yield, low economy
- High yield, low economy A reaction can give a high percentage yield and still waste most of the atoms as by-products.
High economy, low yield
- High economy, low yield A reaction with a perfect equation may still give little product if conditions are poor.
Complete judgement
- Complete judgement Chemists consider yield, atom economy, energy use and safety together.
Route selection
- Route selection Choosing a route with a higher atom economy gives less waste for the same mass of product.
Section 5 of 6Worked Example
Question
- Calculate the atom economy for making iron: Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g).
- (Ar Fe = 55.85;
- Mr CO₂ = 44.01)
- Mass of desired product = 2 × 55.85 = 111.70.
- Total mass of products = 111.70 + 3 × 44.01 = 243.73.
- Atom economy = 111.70 ÷ 243.73 × 100 = 45.8%.
Answer Atom economy = 45.8%
Section 6 of 6Exam Checklist & Tips
5.1.7 – Key Points for the Exam
- Atom economy The percentage of reactant mass converted into the desired product, calculated from the balanced equation.
- By-product A product of a reaction other than the desired one.
- Green chemistry The design of chemical processes which reduce waste and hazards.
- Addition reaction A reaction in which two reactants combine into a single product.
Common Mistakes
- Confusing atom economy with percentage yield.
- Forgetting to multiply molar masses by the coefficients in the equation.
- Using the mass of only one product in the denominator when the equation forms several products.
- Assuming that an atom economy of 100% guarantees a 100% yield.
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