5.14C - Calculating atom economy
Atom economy is a percentage that shows how much of the mass in a balanced reaction equation becomes the desired product. A high atom economy means more of the atoms from the reactants are present in the product you wanted. In this lesson, the focus is the calculation: choosing the correct formula masses, using coefficients, and giving a clear percentage answer.
What the Calculation Measures
Atom economy is worked out from the balanced chemical equation, not from the mass actually collected in a practical. It asks what percentage of the total reactant mass enters the desired product; atom masses matter, rather than just the number of atoms.
Atom economy
The percentage of the total mass of reactants that becomes the desired product in a chemical reaction.
If all the atoms from the reactants become the desired product, the atom economy is 100 percent. If some atoms form other products, such as waste products or by-products, the atom economy is less than 100 percent.
For this calculation, "desired product" means the product named in the question. A reaction can make more than one product, so you must not assume that every product is desired.
Formula and Method
Use the relative formula mass of the desired product and compare it with the total relative formula mass of all products or all reactants in the balanced equation. For a balanced equation, the total relative formula mass of the reactants equals the total relative formula mass of the products.
Percentage atom economy
Some questions use the total relative formula mass of the reactants as the denominator instead. That gives the same answer if the equation is balanced.
A reliable method is:
| Step | What to do | Check |
|---|---|---|
| 1 | Identify the desired product. | It is usually named in the question. |
| 2 | Calculate the relative formula mass of the desired product. | Multiply by the coefficient if there is one. |
| 3 | Calculate the total relative formula mass of all products or all reactants. | Include every substance and every coefficient. |
| 4 | Divide, multiply by 100, and give a percentage. | Atom economy has no mass unit. |
The numerator is only the desired product; the denominator represents the total mass shown by the balanced equation.
Worked Example
Calcium carbonate decomposes when heated:
The desired product is calcium oxide, CaO.
Relative atomic masses: Ca = 40, C = 12, O = 16
| Substance | Relative formula mass |
|---|---|
| CaO | 40 + 16 = 56 |
| CO2 | 12 + (2 x 16) = 44 |
| Total products | 56 + 44 = 100 |
Now substitute into the formula:
So, the atom economy for making calcium oxide in this reaction is 56 percent. The carbon dioxide is a product, but it is not the desired product, so it is counted in the denominator but not the numerator.
Continue this lesson.
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