Introduction, Examples And Calculation Of An Excess Reactant. - Byju's

Chemical Reaction occurs in a perfect stoichiometry among reactants and products. However, reactants of a reaction are not necessarily in an ideal stoichiometric mixture. The reactant that is fully consumed or entirely reacted is known as Limiting Reactant, whereas the reactant that is not wholly consumed is known as an excess reactant.

• The quantity of limited reactants determines the amount of product formed.

This section will discuss the introduction, examples and calculation of Excess Reactant.

Table of Content

• What is an Excess Reactant?
• Examples of Excess Reactant
• How to find out Excess Reactant
• Frequently Asked Questions – FAQs

What is an Excess Reactant?

Chemical Reaction occurs in a perfect stoichiometry among reactants and products. An excess reactant is a substance that is not wholly consumed or entirely reacted in a chemical reaction. It is also known as an excess reagent.

Note: The amount of product formed is independent of the quantity of excess reactant.

Let us consider the following reaction of the formation of sodium chloride.

2Na(s)+Cl2(g)→2NaCl(s)

Sodium metal reacts with diatomic chlorine gas. The above equation indicates that 2 moles of sodium will react with 1 mole of chlorine. If we have equivalent moles of sodium and chlorine,

then sodium will be a limiting reactant, while chlorine will be an excess reactant.

Examples of Excess Reactant

• A combustion reaction occurs when a candle is burned in the air, i.e. oxygen. The amount of product formed is independent of oxygen. Thus oxygen is an excess reactant here.
• A combustion reaction occurs when methane gas is burned in the air, i.e. oxygen. The amount of product formed is independent of oxygen. Thus oxygen is an excess reactant here.

Here pink-coloured atoms indicate the excess reactants, while blue-coloured atoms are limiting reactants.

How to find out Excess Reactant?

Let us consider the following reaction of the formation of sodium chloride.

Na(s)+Cl2(g)→NaCl(s)

The following strategy is applied to find out excess reactants in a reaction.

• To calculate the excess reactant, firstly, we will balance the chemical reaction.

2Na(s)+Cl2(g)→2NaCl(s)

• Then we will calculate the molecular mass of each reactant.

For the above reaction,

Molecular mass of Na = 23g

Molecular mass of Cl2 = 2 x 35.5= 71g

This 23:71 is a standard or fixed ratio for the formation of sodium chloride. That means, for the 23 grams of sodium, we will require 71 grams of chlorine.

• Identification of Excess Reactant and mass calculation

Suppose we have 46 grams of sodium and 200 grams of chlorine.

We know that 23g of sodium requires 71 g of chlorine to form sodium chloride.

The unitary method calculates the amount of chlorine required for 46g of sodium.

We know,

23g of Na → 71 g of Cl2

46g of Na → [(71/23) x 46]g of Cl2

= 142g of Cl2 is required

From the above calculation, it is clear that 46g of sodium and 142 g of chlorine is required to form sodium chloride. However, we have 200g of chlorine present in the reaction. So, the extra chlorine will not get used up in the reaction, thus becoming the excess reactant.

Therefore, chlorine is the excess reactant here.

∴Amount of excess chlorine = [Total amount of Cl2 – Consumed Cl2] = 200g – 142g = 58g

The reaction will stop after this 46g of sodium gets consumed completely.

Frequently Asked Questions on Excess Reactant

Q1

What is an excess reactant?

An excess reactant is a substance that is not wholly consumed or entirely reacted in a chemical reaction. It is also known as an excess reagent.

The amount of product formed is independent of the quantity of excess reactant.

Q2

Are excess reactants present in all reactions?

No, an excess reactant is not present in all reactions. It is present in stoichiometrically imbalanced reactions.

Q3

What is a limiting reactant?

The reactant that is fully consumed or entirely reacted in a reaction is known as a Limiting Reactant.

Q4

Give examples of excess reactants?

• When a candle is burned in the air, i.e. oxygen, the amount of product formed is independent of oxygen. Thus oxygen is an excess reactant here.
• When methane gas is burned in the air, i.e. oxygen, the amount of product formed is independent of oxygen. Thus oxygen is an excess reactant here.

Q5

Ammonia reacts with oxygen to produce nitric oxide. Find out limiting reactants and excess reactants.

To calculate the limiting and excess reactant, firstly, we will write a balanced chemical equation.

4NH3 + 5O2 → 4NO + 6H2O

We can see from the equation that oxygen is a limiting reactant here, and ammonia is an excess reactant here.