Atom economy
The atom economy of a chemical reaction is a measure of the percentage of reactants that become useful products.
Inefficient, wasteful processes have low atom economies. Efficient processes have high atom economies, and are important for sustainable development, as they use fewer natural resources and create less waste.
The atom economy of a reaction can be calculated:
\(\%\,atom\,economy=\frac{mass\,of\,desired\,product\,from\,equation}{total\,mass\,of\,products\,from\,equation} \times 100\)
Note that the total mass of reactants can be substituted for products in this equation because in every reaction the total mass of products equals the total mass of reactants. This is shown below:
\(\%\,atom\,economy=\frac{mass\,of\,desired\,product\,from\,equation}{total\,mass\,of\,\mathbf{reactants}\,from\,equation} \times 100\)
Example
What is the atom economy for making hydrogen by reacting coal with steam?
Write the balanced equation:
\(C(s) + 2H_{2}O (g)\rightarrow CO_{2} (g) + 2H_{2} (g)\)
The masses of all the chemicals involved can be written below each species in the balanced equation.
\(C(s) + 2H_{2}O (g)\rightarrow CO_{2} (g) + 2H_{2} (g)\)
\(\mathbf{12}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\mathbf{2}\times \mathbf{18}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\mathbf{44}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\mathbf{2}\times \mathbf{2}\)
The total mass of products = 44 + 4 = 48 g (note that this is also the same as the total mass of reactants (12 +36))
The mass of desired product (H2) = 4 g
\(\% \,atom\,economy = \frac{4}{{48}} \times 100 = 8.3\% \)
This process has a low atom economy and is therefore an inefficient way to make hydrogen. It also uses coal, which is a finite (non-renewable) resource.
It is possible for chemical reactions with a high percentage yield to have a low atom economy.
This problem arises when processes have large quantities of unwanted by-products produced.
