Atom economy
No atomThe smallest part of an element that can exist. are gained or lost in a chemical reaction. However, some atoms in the reactantA substance that reacts together with another substance to form products during a chemical reaction. may not end up in the desired productA substance formed in a chemical reaction.. They instead form other products, and so are regarded as by-productSomething that is produced as a waste product..
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 the amount of starting materials that end up as useful products. It is important for sustainable developmentEconomic development which meets the needs of the current population without polluting the environment or depleting natural resources. and for economicRelating to money, wealth and the economy, and the production, distribution and consumption of goods and services. reasons to use reactions with high atom economy.
Calculating percentage atom economy
The percentage atom economy of a reaction is calculated using this equation:
The highest possible value of atom economy is 100%, when all the reactant atoms end up in the desired product. If the atom economy is 50%, for example, then half the reactant atoms end up in the desired product or products.
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. (Ar of H = 1, Ar of C = 12, Ar of O = 16)
Mr of CH4 = 12 + (4 Ă— 1) = 16
Mr of H2O = (2 Ă— 1) + 16 = 18
total Mr of reactants = 16 + 18 = 34
Ar of H2 = (2 Ă— 1) = 2
total Mr of desired product = 3 Ă— 2 = 6 (there are three H2 in the balanced equation)
atom economy = \(\frac{\textup{total M}_{r} \textup{ \ of the desired product}}{\textup{total M}_{r} \textup{ \ of all reactants}}\) Ă— 100
atom economy = \(\frac{6}{34} \times 100\)
atom economy = 17.6% (to 3 significant figures)
Question
Ethanol, CH3CH2OH, can be produced by the fermentation of glucose, C6H12O6:
C6H12O6(aq) → 2CH3CH2OH(aq) + 2CO2(g)
Calculate the atom economy for the reaction. (Mr of C6H12O6 = 180, Mr of CH3CH2OH = 46)
atom economy = \(\frac{total~M_{r}~of~the~desired~product}{total~M_{r}~of~all~reactants} \times 100\)
atom economy = \(\frac{2 \times 46}{180} \times 100\)
atom economy = 51.1% (to 3 significant figures)