Atom Economy

Mole Calculations

What is atom economy?

A measure of the percentage of atoms from the reactants that end up in the desired product.

What does a high atom economy mean?

A high proportion of the reactant atoms end up in the desired product, so less waste is produced.

What does a low atom economy mean?

A large proportion of the reactant atoms end up in unwanted products or waste.

What equation is used to calculate percentage atom economy?

Atom economy = (molar mass of desired product ÷ sum of molar masses of all products) × 100.

What is the maximum possible atom economy?

100%.

Why is a high atom economy desirable?

It reduces waste and makes better use of raw materials.

Why is atom economy important in industry?

It can reduce waste, raw-material costs and the environmental impact of a process.

Who developed the idea of atom economy?

Barry Trost.

Why was the concept of atom economy developed?

To assess how many atoms from the reactants end up in the desired product.

What is the difference between percentage yield and atom economy?

Percentage yield measures how efficiently reactants are converted into products, whereas atom economy measures the percentage of reactant atoms that end up in the desired product.

Can a reaction have a high percentage yield but a low atom economy?

Yes. A reaction may produce nearly all of its theoretical product but also produce large amounts of unwanted products.

Does increasing percentage yield necessarily increase atom economy?

No. Atom economy depends on the balanced chemical equation and the products formed.

Which type of reaction usually has 100% atom economy?

Addition reactions.

Why do addition reactions usually have 100% atom economy?

They normally produce only one product, so all reactant atoms end up in the desired product.

What types of reactions usually have lower atom economies?

Elimination and substitution reactions.

Why do elimination and substitution reactions usually have lower atom economies?

They form additional unwanted products as well as the desired product.

Why may multistep reactions have particularly low atom economies?

Several unwanted products may be produced across the different steps.

How do you calculate atom economy when there is only one product?

The atom economy is 100% because all atoms end up in that product.

What must you account for when calculating atom economy from a balanced equation?

The coefficients in the equation as well as the molar masses of the products.

For CaCO₃ + 2NaCl → Na₂CO₃ + CaCl₂, what is the desired product in the Solvay-process example?

Na₂CO₃.

What is the relative formula mass of Na₂CO₃?

106.0.

What is the relative formula mass of CaCl₂?

111.1.

What is the atom economy for producing Na₂CO₃ from CaCO₃ + 2NaCl → Na₂CO₃ + CaCl₂?

[106.0 ÷ (106.0 + 111.1)] × 100 = 48.8%.

If 75.0 kg of CaCO₃ theoretically produces 79.4 kg of Na₂CO₃ but 76.5 kg is actually obtained, what is the percentage yield?

(76.5 ÷ 79.4) × 100 = 96.3%.

Can the Solvay-process example have a high percentage yield but relatively low atom economy?

Yes. Its percentage yield is 96.3%, but its atom economy is only 48.8%.

What is the equation for manufacturing hydrazine in the example?

2NH₃ + NaOCl → N₂H₄ + NaCl + H₂O.

What is the desired product in the hydrazine reaction?

N₂H₄.

What is the molar mass of N₂H₄?

32.0 g mol⁻¹.

What is the molar mass of NaCl?

58.5 g mol⁻¹.

What is the molar mass of H₂O?

18.0 g mol⁻¹.

What is the atom economy for producing hydrazine?

[32.0 ÷ (32.0 + 58.5 + 18.0)] × 100 = 29.5%.

Why does hydrazine production have a relatively low atom economy?

NaCl and H₂O are also formed, so many reactant atoms do not end up in the desired N₂H₄ product.

What is the equation for converting ethene directly into 1,2-dichloroethane?

H₂C=CH₂ + Cl₂ → ClCH₂CH₂Cl.

What is the atom economy of the direct addition reaction producing 1,2-dichloroethane?

100%.

Why does this reaction have 100% atom economy?

There is only one product, so all atoms from the reactants end up in the desired product.

What is the equation for hydration of ethene to produce ethanol?

C₂H₄ + H₂O → C₂H₅OH.

What is the atom economy for the hydration of ethene?

100%, because ethanol is the only product.

What is the equation for dehydration of ethanol to produce ethene?

C₂H₅OH → C₂H₄ + H₂O.

What is the atom economy for producing ethene by dehydration of ethanol?

(28 ÷ 46) × 100 = 60.9%.

Why can reactions producing several products be less economical?

Some reactant atoms end up in unwanted products instead of the desired product.

Why can having multiple products increase energy costs?

Energy may be required to separate the desired product from the other products.

What factors other than percentage yield and atom economy should be considered when choosing an industrial process?

Availability or scarcity of raw materials, cost of raw materials and the amount of energy required.