Chemistry-only: atom economy follows the balanced reaction
| English | 中文 | Pinyin |
|---|---|---|
| by-product/baɪ ˈprɒdʌkt/ | 副产物 | fù chǎn wù |
| atom economy | 原子经济性 | yuán zi jīng jì xìng |
What would explain this observation?
- A reaction can recover all the product it theoretically makes while directing many starting atoms into an unwanted by-product 副产物. Yield and atom economy 原子经济性 measure different things.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Atom economy measures the share of starting-material mass that appears in the desired product according to the balanced equation. Percentage atom economy equals the relative formula mass contribution of the desired product divided by the sum of all reactant relative mass contributions, multiplied by 100. Include balancing coefficients in both contributions where needed.
- atom economy: The fraction of starting-material mass directed into the stated desired product by the balanced equation; by-product: A product other than the stated desired product of a reaction.
Which quantity is needed for atom economy?
For CaCO₃ → CaO + CO₂ with relative masses 100, 56 and 44, choosing CaO as desired product gives 56/100×100=56%. Choosing CO₂ as the desired product for a different purpose gives 44%. Conservation still accounts for all 100 mass units. A high atom economy can reduce waste and raw-material cost, but it does not alone establish safety, energy demand or commercial suitability.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For CaCO₃ → CaO + CO₂ with relative masses 100, 56 and 44, choosing CaO as desired product gives 56/100×100=56%. Choosing CO₂ as the desired product for a different purpose gives 44%. Conservation still accounts for all 100 mass units. A high atom economy can reduce waste and raw-material cost, but it does not alone establish safety, energy demand or commercial suitability.
- Balance the equation, identify the desired product explicitly, and write every coefficient-weighted relative mass. For 2H₂ + O₂ → 2H₂O, desired water contribution is 2×18=36 and reactant total is 2×2+32=36, so atom economy is 100%. This uses relative mass bookkeeping rather than an experimental product weighing. Both-tier calculations are distinct from Higher-only comparison of complete reaction pathways.
Which two habits make the investigation or model in this case more defensible?
Balance the equation, identify the desired product explicitly, and write every coefficient-weighted relative mass. For 2H₂ + O₂ → 2H₂O, desired water contribution is 2×18=36 and reactant total is 2×2+32=36, so atom economy is 100%. This uses relative mass bookkeeping rather than an experimental product weighing. Both-tier calculations are distinct from Higher-only comparison of complete reaction pathways.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a supplied balanced reaction uses total reactant relative-mass contribution 120 and produces desired-product contribution 90 plus by-product contribution 30. Atom economy=90/120×100=75%. An experiment could still recover only half its theoretical desired product, giving 50% yield while the balanced reaction retains 75% atom economy.
Desired-product relative-mass contribution is 84 and total reactant contribution is 120. Find atom economy. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Desired-product relative-mass contribution is 84 and total reactant contribution is 120. Find atom economy.
The result is 70 %. Known: a supplied balanced reaction uses total reactant relative-mass contribution 120 and produces desired-product contribution 90 plus by-product contribution 30. Atom economy=90/120×100=75%. An experiment could still recover only half its theoretical desired product, giving 50% yield while the balanced reaction retains 75% atom economy.
Check the conclusion and its limits
- Do not use recovered product mass in the atom-economy formula. Do not omit a reactant just because it is a gas. High yield cannot remove a stoichiometric by-product. If a by-product has a use, that may improve the pathway’s practical value, but keep the stated desired-product definition explicit.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A reaction with 100% yield necessarily has 100% atom economy. This claim is false: Do not use recovered product mass in the atom-economy formula. Do not omit a reactant just because it is a gas. High yield cannot remove a stoichiometric by-product. If a by-product has a use, that may improve the pathway’s practical value, but keep the stated desired-product definition explicit.
Chemistry-only: atom economy follows the balanced reaction: For CaCO₃ → CaO + CO₂ with relative masses 100, 56 and 44, choosing CaO as desired product gives 56/100×100=56%. Choosing CO₂ as the desired product for a different purpose gives 44%. Conservation still accounts for all 100 mass units. A high atom economy can reduce waste and raw-material cost, but it does not alone establish safety, energy demand or commercial suitability.
A reaction with 100% yield necessarily has 100% atom economy.
Do not use recovered product mass in the atom-economy formula. Do not omit a reactant just because it is a gas. High yield cannot remove a stoichiometric by-product. If a by-product has a use, that may improve the pathway’s practical value, but keep the stated desired-product definition explicit.
The fraction of starting-material mass directed into the stated desired product by the balanced equation: write the technical term.
atom economy means The fraction of starting-material mass directed into the stated desired product by the balanced equation.