Atom Economy

Atom economy is a measure of how efficient a chemical reaction is, based on the amount of useful product that is produced. It is the percentage of atoms in the reactants that end up in the useful product, and we can use it to compare different reactions to determine which one is more efficient.

  • The higher the atom economy, the more efficient the reaction is

Conservation of Atoms in Chemical Reactions

In a chemical reaction, the number of atoms of each element remains conserved; they are neither lost nor gained. This means that the number of atoms present in the reactants will always be present in the product, although they may form different compounds.

For example, let’s look at the reaction between methane and steam, which produces hydrogen gas and carbon monoxide gas:

Methane + SteamHydrogen + Carbon monoxide

CH4 (g) + H2O (g)3H2 (g) + CO (g)

When balancing chemical equations, it is important to ensure that the same number of atoms is present on both sides of the equation. In this example, we can see that we have the same number of atoms before and after the reaction occurs:

  • 1 carbon atom (C)
  • 6 hydrogen atoms (H)
  • 1 oxygen atom (O)

This means that no atoms are lost or gained during the reaction. However, it is important to note that you may still lose some of these atoms to the environment if the experiment is not conducted in a closed system.

Also, not all products of a chemical reaction are useful. In the example above, if you were attempting to make pure hydrogen gas, you would have 1 mol of carbon monoxide gas for every 3 mols of useful hydrogen. In this case, carbon monoxide is a waste product.

Calculating Atom Economy

We can use the following equation to calculate the atom economy:

When all of the reactant atoms make up the desired product, the atom economy reaches its maximum value of 100%. This means that if the atom economy is 50%, only half of the reactant atoms make up the desired product(s).

Let’s look at an example to show the calculation of atom economy.

Example

We can obtain iron(II) chloride by reacting iron with copper chloride, according to the following balanced equation:

Iron + Copper chlorideiron(II) chloride + Copper

Fe + CuCl2FeCl2 + Cu

Calculate the atom economy of the reaction. (Ar of Fe = 56, Ar of Cu = 63.5 and Ar of Cl = 35.5)

The relative formula mass (Mr) of CuCl2 = (1 × Ar of Cu) + (2 × Ar of Cl)

= 63.5 + (2 × 35.5) = 134.5

The relative formula mass (Mr) of FeCl2 = (1 × Ar of Fe) + (2 × Ar of Cl)

= 56 + (2 × 35.5) = 127

Total Mr of all the reactants = Ar of Fe + Mr of CuCl2

= 56 + 134.5 = 190.5

The relative formula mass (Mr) of the desired product (FeCl2) is 127. Therefore:

Atom economy = (127 / 190.5) × 100

= 66.7% (1 decimal place)


Low vs High Atom Economy

A reaction with a low atom economy means that a low percentage of the reactants become useful products. In contrast, a high atom economy means that a high percentage of the reactants become useful products. Therefore, companies tend to prefer processes with higher atom economies, as they are more efficient and cost-effective.

However, there are factors that may influence the choice of a particular process, even if it has a lower atom economy. For example:

  • The availability of raw materials
  • Environmental impact
  • Safety concerns

Processes with low atom economies are also less sustainable because waste products often need to be stored or disposed of. This consumes energy and resources. However, waste products can sometimes be used in other ways, which can make a process more sustainable.

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