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First exams 2025

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Atom Economy (SL IB Chemistry)

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Philippa Platt

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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

Atom space economy space equals fraction numerator molecular space mass space of space desired space product over denominator sum space of space molecular space masses space of space all space reactants end fraction cross 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 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
  • 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.

(Ar / Mr data: Fe2O3 = 159.6, CO = 28.0, Fe = 55.8, CO2 = 44.0)


Answer:

Step 1: Write the equation:

Atom space economy space equals space fraction numerator molecular space mass space of space desired space product over denominator sum space of space molecular space masses space of space ALL space reactants end fraction space cross times space 100

Step 2: Substitute values and evaluate:

Atom space economy space equals space fraction numerator 2 space cross times space 55.8 over denominator 159.6 space plus space left parenthesis 3 space cross times space 28.0 right parenthesis end fraction space cross times space 100 space equals space 45.8 percent sign

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.

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    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.