Atom Economy

The atom economy of a reaction shows how many of the atoms used in the reaction become the desired product
- The rest of the atoms or mass is wasted
It is found directly from the balanced equation by calculating the M_r of the desired product

$Atom economy = \frac{molecular mass of desired product}{sum of molecular masses of all reactants} \times 100$

In addition reactions, the atom economy will always be 100%, because all of the atoms are used to make the desired product
- Whenever there is only one product, the atom economy will always be 100%
For example, in the reaction between ethene and bromine:

CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

The atom economy could also be calculated using mass, instead or M_r
In this case, you would divide the mass of the desired product formed by the total mass of all reactants, and then multiply by 100
Efficient processes have high atom economies and are important to sustainable development
- They use fewer resources
- Create less waste
As well as atom economy and percentage yield there are other factors that can be used to gauge the efficiency of a chemical process
- Rate
- Quantities of reagents such as catalysts and solvents
- Energy uses
- Economic efficiency

Worked example

Ethanol can be produced by various reactions, such as:

Hydration of ethene: C₂H₄ + H₂O → C₂H₅OH

Substitution of bromoethane: C₂H₅Br + NaOH → C₂H₅OH + NaBr

Explain which reaction has a higher atom economy.

Answer:

Hydration of ethene has a higher atom economy (of 100%) because all of the reactants are converted into products, whereas the substitution of bromoethane produces NaBr as a waste product

Worked example

Quantitative atom economy

The blast furnace uses carbon monoxide to reduce iron(III) oxide to iron.

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Calculate the atom economy for this reaction, assuming that iron is the desired product.

(A_r / M_r data: Fe₂O₃ = 159.6, CO = 28.0, Fe = 55.8, CO₂ = 44.0)

Answer:

Step 1: Write the equation:

$Atom economy = \frac{molecular mass of desired product}{sum of molecular masses of ALL reactants} \times 100$

Step 2: Substitute values and evaluate:

$Atom economy = \frac{2 \times 55.8}{159.6 + (3 \times 28.0)} \times 100 = 45.8 %$

Examiner Tip

Careful: Sometimes a question may ask you to show your working when calculating atom economy.

In this case, even if it is an addition reaction and it is obvious that the atom economy is 100%, you will still need to show your working.

Atom Economy (HL IB Chemistry)

Revision Note

Atom Economy

Worked example

Worked example

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Author: Philippa