Atom economy
Modern industrial processes are designed to be sustainableAn activity which does not consume or destroy resources or the environment.. The aim is that the process can meet current demands without having a long-term impact on the environment. This is sometimes known as green chemistry.
Atom economy
atom economyA measure of how many reactant atoms form a desired product. is a way to measure the amount of useful productA substance formed in a chemical reaction. made in a reaction. It is a theoretical value that provides another way to evaluate a reaction process, alongside yieldThe mass of product made in a chemical reaction. The percentage yield is a measure of the yield obtained compared to the maximum possible yield. and efficiency.
The higher the atom economy, the 'greener' the process. An atom economy of 100% means that all the atomThe smallest part of an element that can exist. in the reactantA substance that reacts together with another substance to form products during a chemical reaction. have been converted to the desired product.
Calculating atom economy
Atom economy can be calculated using this equation:
atom economy = \(\frac{mass~of~atoms~in~the~desired~product}{total~mass~of~atoms~in~reactants}\) x 100
Worked example
Question
Calculate the atom economy for making hydrogen by reacting zinc with hydrochloric acid.
Zn + 2HCl → ZnCl2 + H2
(relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom.: H = 1.0, Cl = 35.5, Zn = 65.4)
First, work out the total relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula. or relative atomic mass of the desired product. The desired product is hydrogen, H2:
relative formula mass of H2 = (2 × 1.0) = 2.0
Next, work out the total relative formula mass or relative atomic mass of the reactants.
relative atomic mass of Zn = 65.4
relative formula mass of HCl = 1.0 + 35.5 = 36.5
total relative formula mass of reactants = 65.4 + (2 × 36.5) = 138.4
atom economy = \(\frac{2.0}{138.4}\) x 100
= 1.45% (to three significant figures)
Industrial processes need as high an atom economy as possible, because this:
- reduces the production of unwanted products
- makes the process more sustainable
Question
Hydrogen can also be made from methane and steam: CH4 + 2H2O \(\rightarrow\) CO2 + 4H2
Calculate the atom economy for this process.
(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)
total relative formula mass of desired product = 4 × (2 × 1.0) = 8.0
relative formula mass of CH4 = 12.0 + (4 × 1.0) = 16.0
relative formula mass of H2O = (2 × 1.0) + 16.0 = 18.0
total relative formula mass of the reactants = 16.0 + (2 × 18.0) = 52.0
atom economy = \(\frac{8.0}{52.0}\) x 100
= 15.4% (to three significant figures)
