Question

Why is it advantageous to use one reagent in excess when you do a reaction?

-Ideally what kind of property must the reagent used in excess have in comparison to the properties of the product in order to be used in excess?

Answer 1

Let us take an example of a chemical reaction expressed below,

2 Na + Cl₂ = 2 NaCl,

From the balanced equation, for 2 sodium atom, 1 mole or 2 atoms of chlorine is required. But in reality excess of chlorine is used so that the reaction goes to complete.

In that case Na will be called the limiting reagent by definition. This means it is completely used up.

On the other hand some of Cl2 will remain unreacted which is called excess reagent.

Use of excess reagent can speed up the process, make the separation easier by finishing the limiting reagent and over all economy of the chemical process.

The excess of any reagent is generally chosen based on the following criteria-

1. When there is a reactivity difference among the reagents

2. The excess reagent should not be harmful to the product formed

3. One of the limiting reagent is expensive and needs to be completely converted into product.

4. If the reaction is revesible in nature then one of the reagent could be in excess

5. The excess reagent should easily be separable from the product mixture.