Percentage yield, atom economy and gas calculations - EdexcelAtom economy
The percentage yield shows how much product is obtained compared to the maximum possible mass. The atom economy of a reaction gives the percentage of atoms in reactants that form a desired product.
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 instead end up forming 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 percentage atom economy
The percentage atom economy of a reaction is calculated using this equation:
The atom economy of a reaction is 100% if there are no unwanted by-products in the reaction. Usually the atom economy is less than 100%. The more atoms that end up in the by-products, the lower the atom economy.
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 × 1) = 16
Mr of H2O = (2 × 1) + 16 = 18
Total Mr of reactants = 16 + 18 = 34
Ar of H2 = (2 × 1) = 2
Total Mr of desired product = 3 × 2 = 6 (there are three H2 in the balanced equation)
Atom economy = \(\frac{\textup{total M}_{r} \textup{of the desired product}}{\textup{total M}_{r} \textup{of all reactants}}\) × 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 = \(\frac{{total~M_{r}~of~the~desired~product}}{{total~M_{r}~of~all~reactants}}\times100\)
Atom economy = \(\frac{\textup{(2\times46)}}{\textup{180}} \times 100\)
