1.2.5Physical Chemistry
Atom Economy — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 12 of 102
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 6What Is 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 = (Mr of desired product ÷ sum of Mr of all products) × 100, using the balanced equation.
Balanced equation
- Balanced equation Use the coefficients in the equation, so each Mr is multiplied by the number of moles.
Addition reactions
- Addition reactions Addition reactions often have 100% atom economy because all reactant atoms end up in the single product.
Part 2 of 6Atom Economy and Yield
The two measures are different.
Atom Economy and Yield
Yield
- Yield Percentage yield measures how much of the possible product is actually obtained in an experiment.
Atom economy
- Atom economy Atom economy is a calculation about the reaction itself, showing how much of the reactant mass could become product.
High yield, low economy
- High yield, low economy A reaction can give a high yield but still waste atoms if it forms large amounts of by-products.
Link
- Link Both should be high for an efficient, sustainable process.
Part 3 of 6Why It Matters
1
Reactions with low atom economy produce large amounts of waste products that must be treated or disposed of.
2
Using atoms efficiently conserves raw materials, many of which are finite resources.
3
Less waste reduces the cost of disposal and lowers the environmental impact of manufacturing.
4
By-products that can be sold or used elsewhere improve the effective atom economy of the process.
5
Chemists therefore prefer routes with high atom economy, such as addition reactions, when designing new processes.
Part 4 of 6Comparing Routes
Ethanol by hydration
- Ethanol by hydration C₂H₄ + H₂O → C₂H₅OH has a single product, so its atom economy is 100%.
Ethanol by fermentation
- Ethanol by fermentation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ wastes mass as CO₂, giving an atom economy of only 51.1%.
Other factors
- Other factors Atom economy ignores renewable feedstocks, energy use and rate, so it is not the only factor to consider.
Decision
- Decision Industry weighs atom economy with yield, cost, hazards and the sustainability of the raw materials.
Part 5 of 6Worked Example
Question Calculate the atom economy for making ethanol by fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (Mr: C₆H₁₂O₆ = 180.0, C₂H₅OH = 46.0, CO₂ = 44.0).
- The desired product is ethanol2 × 46.0 = 92.0.
- Total mass of products = 92.0 + (2 × 44.0) = 180.0.
- Atom economy = 92.0 ÷ 180.0 × 100 = 51.1 %.
Answer Atom economy = 51.1 %
Part 6 of 6Exam Checklist & Tips
1.2.5 – Key Points for the Exam
- Atom economy The proportion of the mass of reactants that becomes the desired product, as a percentage.
- By-product A substance formed in addition to the desired product.
- Sustainability Meeting present needs without harming the ability of future generations to meet theirs.
- Addition reaction A reaction in which two molecules combine to form one product.
Common Mistakes
- Using the masses of the reactants used in an experiment instead of the Mr values from the equation.
- Forgetting to multiply the Mr of a product by its coefficient in the balanced equation.
- Confusing atom economy with percentage yield.
- Stating that a high yield means a high atom economy.
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