Atom economy shows how much of the reactants ends up useful
Definition
Atom economy
The percentage of the total mass of reactants that becomes the desired product.
- The atom economy measures how much of the mass of the reactants ends up in the product you actually want.
- The rest of the mass leaves the process as unwanted by-products.
Key Idea
A reaction with only one product has an atom economy of 100%, because every atom of the reactants ends up in the wanted product.
Calculating atom economy from formula masses
Definition
Balanced equation
A chemical equation containing the same number of atoms of each element on both sides.
Definition
Coefficient
A large whole number placed before a chemical formula to show the relative number of particles or moles in a balanced equation.
- The formula is atom economy =Mr of the desired producttotal Mr of all reactants×100= \dfrac{M_r \text{ of the desired product}}{\text{total } M_r \text{ of all reactants}} \times 100=total Mr of all reactantsMr of the desired product×100.
- Multiply each MrM_rMr by its balancing number in the equation before adding, so every atom is counted.
Example
The total MrM_rMr of the reactants is the same as the total MrM_rMr of the products, because mass is conserved, so you can use whichever side is easier to add up.
High atom economy means less waste
- A high atom economy is better for sustainability, because fewer raw materials are turned into waste.
- It is also cheaper, because less money is spent making by-products and disposing of them.
Exam technique
- Do not confuse atom economy with percentage yield, as yield is about how much you make and atom economy is about how much of the reactant is wasted.
- Include the balancing numbers when you add up formula masses, or the total will be too small.
Example
Making calcium oxide by heating a carbonate
- In CaCO3→CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2CaCO3→CaO+CO2, the desired product is calcium oxide.
- The reactant CaCO3\text{CaCO}_3CaCO3 has Mr=100M_r = 100Mr=100 and the desired product CaO\text{CaO}CaO has Mr=56M_r = 56Mr=56.
- Atom economy =56100×100=56%= \dfrac{56}{100} \times 100 = 56\%=10056×100=56%, so the carbon dioxide by-product carries away the other 44%.
Self review
- What does atom economy measure?
- Write down the formula for atom economy.
- Why does a reaction with a single product have an atom economy of 100%?
- Give two reasons a high atom economy is desirable.
