Atom economy
No atomThe smallest part of an element that can exist. are created or destroyed in a chemical reaction. However, the atoms in the reactantA substance that reacts together with another substance to form products during a chemical reaction. may not become the desired productA substance formed in a chemical reaction.. They may instead end up forming other products, which are regarded as waste productsIn chemistry, these are by-products in a chemical reaction formed as well as the desired product. They are described as waste products if no use can be found for them. In general terms, they can be unwanted or unusable material, including household rubbish, hazardous materials (eg toxic waste) or bodily materials (eg urine or faeces)., also called by-products.
For example, hydrogen can be manufactured by reacting methane with steam:
methane + steam → hydrogen + carbon monoxide
CH4(g) + H2O(g) → 3H2(g) + CO(g)
In this reaction, carbon and oxygen atoms in the reactants do not form the useful product. Carbon monoxide is a waste gas.
The atom economyA measure of how many reactant atoms form a desired product. of a reaction is a measure of how many reactant atoms form a desired product.
Calculating atom economy
The atom economy of a reaction is calculated using this equation:
\(atom\ economy\ =\ \frac{total \mathit{M}_{r}\ of\ the\ desired\ product}{total\ \mathit{M}_{r}\ of\ all\ reactants}\ \times\ 100\)
Worked example
Hydrogen can be manufactured by reacting methane with steam:
CH4(g) + H2O(g) → 3H2(g) + CO(g)
Calculate the atom economy for the reaction. (Relative atomic masses: H = 1.0, C = 12.0, O = 16.0)
Mr of CH4 = 12.0 + (4 Ă— 1.0) = 16.0
Mr of H2O = (2 Ă— 1.0) + 16.0 = 18.0
total Mr of reactants = 16.0 + 18.0 = 34.0
Ar of H2 = (2 Ă— 1.0) = 2.0
total Mr of desired product = 3 Ă— 2.0 = 6.0 (there are three H2 in the balanced equation)
\(atom\ economy\ =\ \frac{total \mathit{M}_{r}\ of\ the\ desired\ product}{total\ \mathit{M}_{r}\ of\ all\ reactants}\ \times\ 100\)
\(atom\ economy\ =\ \frac{6.0}{34.0}\ \times\ 100\)
atom economy = 17.6% (to 3 significant figures)
Question
Ethanol, C2H5OH, can be produced by the fermentation of glucose, C6H12O6:
C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g)
Calculate the atom economy for the reaction. (Relative formula masses: C6H12O6 = 180.0, C2H5OH = 46.0)
\(atom\ economy\ =\ \frac{total \mathit{M}_{r}\ of\ the\ desired\ product}{total\ \mathit{M}_{r}\ of\ all\ reactants}\ \times\ 100\)
\(atom\ economy\ =\ \frac{2\ \times\ 46.0}{180.0}\ \times\ 100\)
atom economy = 51.1% (to 3 significant figures)