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 Mr of the desired product
percentage atom economy =
- 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 Mr
- 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
Worked example
Qualitative atom economy
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:
- The hydration of ethene has a higher atom economy
- The atom economy is 100 %
- This is 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.
Answer:
- Write the equation:
- Atom economy =
- Calculate the relevant atomic / molecular masses:
- Fe2O3 = (2 x 55.8) + (3 x 16.0) = 159.6
- CO = (1 x 12.0) + (1 x 16.0) = 28.0
- Fe = 55.8
- The Mr of CO2 is not required as it is not the desired product
- Substitute values and evaluate:
- Atom economy = x 100
- Atom economy = 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.