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Atom Economy (DP IB Chemistry): Revision Note

Philippa Platt

Last updated

DP Chemistry: HLDP Chemistry: SL

Atom Economy

  • Atom economy is a measure of how efficiently a chemical reaction converts reactants into the desired product

  • It shows the proportion of the total mass of reactants that ends up in the useful product

  • A higher atom economy means:

    • Less waste

    • A more sustainable process

Calculating atom economy

  • The formula for atom economy is:

atom economy = 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 x 100

  • Alternatively, you can calculate atom economy using masses:

atom economy = fraction numerator space mass space of space desired space product over denominator total space mass space of space all space products end fraction x 100

Examiner Tips and Tricks

Atom economy is calculated from the balanced chemical equation, not from experimental data

100% atom economy

  • In addition reactions, atom economy is always 100%

    • This is because all of the atoms are used to make the desired product

  • For example, in the reaction between ethene and bromine:

CH2=CH2 + Br2 → CH2BrCH2Br

Atom economy and industrial waste

  • A low atom economy indicates that a significant portion of the reactants becomes waste or by-products

  • In industry, this means that:

    • More resources are needed

    • More waste must be managed

    • Leading to increased environmental and economic costs

  • 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

Comparing two methods to make ethanol

Ethanol can be produced by:

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 economy = 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 x 100

Step 2: Substitute values and evaluate:

atom economy = fraction numerator 2 cross times 55.8 over denominator 159.6 plus left parenthesis 3 cross times 28.0 right parenthesis end fraction x 100 = 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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    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry Content Creator

    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