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Atom Economy (AQA A Level Chemistry)

Revision Note

Stewart Hird

Last updated

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 = fraction numerator bold molecular bold space bold mass bold space bold of bold space bold desired bold space bold product over denominator bold sum bold space bold of bold space bold molecular bold space bold masses bold space bold of bold space bold all bold space bold reactants end fraction bold cross times bold 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 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:

  1. Write the equation:

    • Atom economy = fraction numerator bold molecular bold space bold mass bold space bold of bold space bold desired bold space bold product over denominator bold sum bold space bold of bold space bold molecular bold space bold masses bold space bold of bold space bold all bold space bold reactants end fraction bold cross times bold 100

  2. 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

  3. Substitute values and evaluate:

    • Atom economy = fraction numerator 2 cross times 55.8 over denominator 159.6 plus open parentheses 3 cross times 28.0 close parentheses end fraction x 100

    • Atom economy = 45.8 %

Examiner Tips and Tricks

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.

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    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.