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Calculating Atom Economy (AQA GCSE Chemistry)

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Stewart Hird

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Atom economy

  • Along with the percentage yield, atom economy is used to analyse the efficiency of reactions

  • Most reactions produce more than one product and very often some of them are not useful

  • Atom economy studies the amount of reactants that get turned into useful products

  • It illustrates what percentage of the mass of reactants become useful products

  • It is used extensively in the analysis of systems and procedures in industries, in an effort to obtain sustainable development

  • It is also a very important analysis for economic reasons as companies prefer to use processes with higher atom economies

  • The higher the atom economy of a process then the more sustainable that process is

Atom economy formula

  • The atom economy formula is:

Atom economy = fraction numerator total space M subscript straight r space of space desired space product over denominator total space M subscript straight r space of space all space products end fraction cross times 100

Worked Example

Hydrogen gas is obtained from methane in a process called steam-methane reforming. The reaction is as follows:

CH4 (g) + H2O (g) → CO (g) + 3H2 (g)

Calculate the atom economy of this reaction.

Answer:

  • Step 1: Calculate the total Mr of all products:

    • Total Mr = CO + 3H2 

    • Total Mr = (12 + 16) + (3 x 2 x 1) = 34

  • Step 2: Calculate the Mr of the desired product:

    • Mr of 3H2 = (3 x 2 x 1) = 6

  • Step 3: Substitute the values into the percentage atom economy equation:

    • Atom economy = fraction numerator total space M subscript straight r space of space desired space product over denominator total space M subscript straight r space of space all space products end fraction cross times 100

    • Atom economy = 6 over 34 cross times 100 = 17.6%

  • Therefore, in this process 82.4 % of reactant material is wasted

    • 100 - 17.6 = 82.4 %

Examiner Tips and Tricks

Unwanted byproducts can sometimes be put to use so although a low atom economy is a sign that a process is not green (sustainable) it doesn’t necessarily imply that the process is not economically viable.

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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.