Difference Between Atom Economy and Percentage Yield

Key Difference – Atom Economy vs Percentage Yield
 

Atom economy and percentage yield are used to determine the efficiency of chemical synthesis. Determination of the atom economy is very important because it gives details about how greener the process is. It also indicates the waste of atoms during the process. Percentage yield is the amount of product given by a chemical reaction with respect to the expected calculated amount (the theoretical amount). The key difference between atom economy and percentage yield is that atom economy is calculated by dividing the molar mass of the desired product by the molar mass of all the reactants whereas percentage yield is calculated by diving the actual yield of the product from the theoretical yield of the product.   

CONTENTS

1. Overview and Key Difference
2. What is Atom Economy 
3. What is Percentage Yield
4. Similarities Between Atom Economy and Percentage Yield
5. Side by Side Comparison – Atom Economy vs Percentage Yield in Tabular Form
6. Summary

What is Atom Economy?

Atom economy or atom efficiency is the determination of the efficiency of a chemical synthesis with respect to the atoms used during the synthesis reaction. If the atom economy is 100%, it means all the atoms that were involved in the process have been used during the process. This means all the atoms in the reactants have been converted into atoms of the product. If the atom economy of a process is high, it is called a green process.

Equation for Atom Economy Calculation

The determination of the atom economy can be done via the following equation.

Atom economy = (molar mass of desired product/molar mass of all reactants) x 100%

In an ideal process, all atoms in the reactants are used up by the process and are converted into reactants. Hence, no atom is wasted. But in actual processes, the atom economy is lower than 100%. This is due to the production of byproducts instead of giving only the desired product. This is a major concern when it comes to the process that consumes high-cost raw materials.

Example

Production of maleic anhydride from benzene. This is an oxidation reaction; the reactants involved in the process are benzene and molecular oxygen.

Benzene + 4.5oxygen   →   maleic anhydride + 2carbon dioxide + 2water

Molar mass of desired product                = (12×4) + (16×3) + (1×2)

                                                     = 98 g/mol

Molar mass of all reactants; benzene    = (12×6) + (1×6)

                                                            = 78 g/mol

Molecular oxygen                                     = 4.5(16×2)

 = 144 g/mol

The total mass of reactants                    = 78 + 144

= 222 g/mol

Atom economy                                       = (98/222) x 100%

= 44.14%

What is Percentage Yield?

Percentage yield (also called percent yield) is the actual yield obtained from a chemical synthesis reaction, with respect to the theoretical yield. The value is given as a percentage. The actual yield is what we obtain from the experiment whereas theoretical yield is the value calculated from the equation of the chemical reaction, considering the stoichiometry.

When calculating the theoretical yield, one should consider the limiting reagent. Limiting reagent or limiting reactant is the reactant that determines how much of the product has been made. The limiting reactant is consumed during the reaction where other reactants remain even after the completion of the reaction because they are excess reagents.

How to Find the Limiting Reagent?

Ex: Let us consider the reaction between Al (14 g) and Cl₂ (4.25 g) gas. The final product is AlCl₃.

2Al   +   3Cl₂     →     2AlCl₃

Thee moles of Al present = 14 /26.98 = 0.52 mol

The moles of Cl₂ present = 4.25 / 70.90 = 0.06 mol

Stoichiometric ratio between Al and Cl₂ = 2:3

Therefore, 2 moles of Al react with 3 moles of Cl₂. Then amount of Cl₂ required to react with 0.52 mol of Al is = (3/2) x 0.52 = 0.78 mol

But, only 0.06 mol is present. Therefore, Cl₂ is the limiting reagent here. Then the theoretical yield is calculated using the amount of Al present in the reaction mixture.

Theoretical yield = (2/3) x 0.06 x 133.3 = 5.33 g

If the actual yield obtained from the experiment is given as 4.33g, then the percentage yield can be calculated as follows.

Percentage yield = (4.33 / 5.33) x 100% = 81.24%

What are the Similarities Between Atom Economy and Percentage Yield?

• Both atom economy and percentage yield are percentages.
• Both atom economy and percentage yield are very useful to determine the efficiency of a chemical process.

What is the Difference Between Atom Economy and Percentage Yield?

Summary – Atom Economy vs Percentage Yield

Atom economy and percentage yield are percentage values that are calculated regarding chemical reactions. The key difference between atom economy and percentage yield is that atom economy is calculated by dividing the molar mass of the desired product by the molar mass of all the reactants whereas percentage yield is calculated by diving the actual yield of the product from the theoretical yield of the product.

Reference:

1. “Limiting reagents and percent yield.” Khan Academy, Available here.
2. “Atom economy.” Wikipedia, Wikimedia Foundation, 28 Feb. 2018, Available here.

Image Courtesy:

1.”Atom economy V3 en” By Astrid 91 – Own work (CC BY-SA 4.0) via Commons Wikimedia