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
The atom economy formula is:

Atom economy = $\frac{total M_{r} of desired product}{total M_{r} of all products} \times 100$

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 M_r of all products:
- Total M_r = CO + 3H₂
- Total M_r = (12 + 16) + (3 x 2 x 1) = 34
Step 2: Calculate the M_r of the desired product:
- M_r of 3H₂ = (3 x 2 x 1) = 6
Step 3: Substitute the values into the percentage atom economy equation:
- Atom economy = $\frac{total M_{r} of desired product}{total M_{r} of all products} \times 100$
- Atom economy = $\frac{6}{34} \times 100$ = 17.6%
Therefore, in this process 82.4 % of reactant material is wasted
- 100 - 17.6 = 82.4 %

Examiner Tip

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.

Atom Economy (Edexcel GCSE Chemistry)

Revision Note

Atom Economy

Worked example

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Author: Stewart