3.3.2a Atom economy
Atom economy shows how much of the reactants ends up useful
Atom economy
The percentage of the total mass of reactants that becomes the desired product.
- Separate chemistry only: atom economy is assessed in GCSE Chemistry, not in Combined Science.
- 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.
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
Balanced equation
A chemical equation containing the same number of atoms of each element on both sides.
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.
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.
- 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.
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%.
- 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.
3.3.2b Choosing a reaction pathway
A reaction pathway is the route chosen to make a product
Reaction pathway
A sequence of one or more reactions used to convert starting materials into a specified product.
Specified product
The desired product named in the question as the substance that the process must produce.
- There is often more than one reaction pathway that makes the same specified product.
- Industry compares the routes and picks the one that makes the specified product most efficiently.
No single factor decides the best pathway; the choice weighs several against one another.
Yield and atom economy shape the choice
Atom economy
The percentage of the total mass of reactants that becomes the desired product.
- A high percentage yield is preferred because it makes more product from each batch of reactants.
- A high atom economy is preferred because less of the reactant mass is wasted as by-products.
A route with a high yield but a low atom economy still throws away a lot of material, so both figures have to be judged together.
By-products, rate and equilibrium also count
By-product
A substance formed alongside the desired product in a chemical process.
Equilibrium position
The relative amounts of reactants and products present when a reversible reaction reaches equilibrium in a closed system.
- A useful by-product that can be sold improves the economics of a pathway.
- A faster rate of reaction makes more product in the same time.
- In a reversible reaction, the equilibrium position limits how much product can form.
- In an evaluate question, give both a strength and a weakness of each pathway before you reach a decision.
- Support your final choice with the specific figures given, such as the yields or atom economies quoted.
Weighing up the whole pathway
- The cost and availability of the raw materials affect whether a route is practical.
- The energy needed and the environmental impact of any waste also weigh on the decision.
- The best pathway is the one that balances all of these against a good yield and a high atom economy.
- What is meant by a reaction pathway?
- Give four factors used to compare different reaction pathways.
- Why can a useful by-product make a pathway more attractive?
- Why is no single factor enough to choose the best pathway?
