Atom economy
Key words:
Atom economy
Percentage yield
Learning objectives
• Define the atom economy of a reaction and describe the
benefits of developing chemical processes with a high atom
economy.
• Explain that addition reactions have an atom economy of
100%, whereas substitution reactions are less efficient.
• Carry out calculations to determine the atom economy of a
reaction.
• Explain that a reaction may have a high percentage yield but a
low atom economy.
Percentage yield is used to assess the efficiency of a
chemical reaction. Calculating the percentage yield
only tells part of the story. A reaction may not only
produce the desired product, but by products as well
What happens to the by-products?
• Considered as a waste and disposed of
– Costly
– Potential environmental problems
• May be sold on or used elsewhere in the plant
– Preserves Earth’s resources
– Minimises waste
Equation
𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑒𝑠𝑖𝑟𝑒𝑑 𝑝𝑟𝑜𝑑𝑢𝑐𝑡
𝑎𝑡𝑜𝑚 𝑒𝑐𝑜𝑛𝑜𝑚𝑦 =
𝑥 100
𝑠𝑢𝑚 𝑜𝑓 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑚𝑎𝑠𝑠𝑒𝑠 𝑜𝑓 𝑎𝑙𝑙 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠
• Atom economy considers not only the desired
product, but also all the by-products of a chemical
reaction. It describes the efficiency of a reaction in
terms of ALL the atoms involved . A reaction with a
high atom economy uses atoms with minimal waste
• We are now much more aware of our environment.
By using processes with a higher atom economy,
chemical companies can reduce the amount of waste
produced. This is good news, especially as we are
running out of landfill sites. It has also been
suggested that about 5-10% of the total expenditure
of a chemical company goes on waste treatment
1. Calculate the molecular mass of the desired product
2. Divide this by the sum of the molecular masses of all products
The type of reaction used for a chemical process is a
major factor in achieving a higher atom economy.
• Addition reactions have an atom economy of 100%
• Reactions involving substitution or elimination have
an atom economy less than 100%
This doesn’t mean that we should never carry out
substitution or elimination reactions but we need to
find a use for all the products to improve their atom
economy. This can be problematic when the undesired
products are toxic