Stoichiometry and yield of balanced chemical reactions

Learning objectives

In this article the author has explained stoichiometry, limiting reactant, yield, and percentage yield.

What is Stoichiometry?

The branch of chemistry that deals with the study of the relationship between the quantities of reactions and products as given by a balanced chemical equation is called stoichiometry.

Definition of stoichiometry

Advantages and Limitations of a chemical equation

Consider the following reaction

Chemical reaction of Carbon and Oxygen
Chemical reaction of Carbon and Oxygen

This equation tells that 1 mole of Carbon reacts with 1 mole of Oxygen to give one mole CO2. Therefore, this equation can be used to study the quantitative relationship between reactants and products.

Limitations of a chemical equation

When we talk about the chemical equations these do not tell us about the following factors.

  • Conditions such as temperature and pressure
  • Rate of reaction
  • The physical state of reactants and products
  • Mechanism of reaction

The following types of relationship can be studied with the help of a balanced chemical equation.

simple chemical equation.png
simple chemical equation.png

The stocihiometry calculations are based on following assumptions.

i) Reactants should be completely converted into products

ii) No side reaction occurs

iii) The law of conservation of mass and the law of definite proportions must be obeyed.

According to the law of conservation of mass,

Mass can neither be created nor destroyed during a chemical reaction.

Definition of law of conservation of mass

It can also be defined as total mass of reactants is equal to the total mass of products.

Therefore, total number of atoms entering into a chemical reaction is equal to the total number of atoms in the products.

Law of conservation of mass
Law of conservation of mass

According to the law of definite proportions

A pure chemical compound always contains the same elements combined in the same ratio by weight.

Law of definite proportion
Law of definite proportions
Law of definite proportions

Sotchiometric relationships

Mass-Mass relationship

With the help of mass of given substance, mass of another substance can be calculated.

Mass-Mole relationship

With the help of mass of one substance volume of the other substance can be calculated and vice versa.

Mole-Mole relationship

With the help of moles of a given substance, mole of another substance can be calculated.

Also read: Avogadro’s number and molar volume

Limiting reactant

A limiting reactant is a reactant that controls the amount of the products formed in a chemical reaction due to being less than the required amount.

Definition of limiting reactant

It can also be defined as follow:

It is a reactant that produces the least number of moles of product.

It is consumed earlier in the reaction.

Identification of limiting reactant

  • To identify a limiting reactant, the following three steps are performed.
  • Calculated the number of moles from the given amount of reactants.
  • Calculated the number of moles of product formed from the given moles of each reactant.
  • Identify the reactant as limiting reactant which produces the least moles of product.

Example

reaction of hydrogen and oxygen
reaction of hydrogen and oxygen

When 2 moles of hydrogen (4g) react with 2 moles of oxygen (64g), then only 2 moles (36g) of water are produced. It is because 2 moles (4g) of hydrogen react with 1 mole 32g) of oxygen. Since less hydrogen is present than oxygen, so hydrogen is the limiting reactant.

Example 2

The burning of coal occurs in excess of oxygen. In this coal is the limiting reactant while oxygen is in excess.

Example 3

Rusting of iron occurs in excess of oxygen present in the air. In this, iron is the limiting reactant while oxygen is in excess.

Applications of deliberate use of limiting reactant

Complete consumption of expensive reactant

The other reactant are taken in large amount than the expensive reactants. It ensures that all of the expensive reactant is completely used up in the chemical reaction, so it becomes the limiting reactant.

To speed up the reaction

Rate of reaction is directly proportional to the amounts of reactants. Therefore a large quantity of one reactant may speed up the reaction. For example, a large quantity of oxygen burns things fater. Thus excess of oxygen is left behind at the end of reaction and burning things like paper, wood are the limiting reactants.

Yield

The amount of product formed during a chemical reaction is called yield.

Definition of yield

Types of yield

There are two types of yield

Theoretical yield

The amount of product calculated from balanced chemical equation is called theoretical yield.

Definition of theoretical yield

It is the maximum yield of product that can be produced by a given amount of reactant according to balanced chemical equation.

Example: H2 and O2 react together

Reaction of water and oxygen
Reaction of water and oxygen

According to this equation, 2 moles of H2 and 1 mole of O2 should form 2 moles of H2O. Therefore, 2 moles of water is the theoretical yield.

Actual yield

The amount of product actually obtained during a chemical reaction is called actual yield.

Definition of actual yield

Percentage yield

The percentage yield is also known as the efficiency of the reaction is given by the formula

Percentage yield formula
Percentage yield formula

Why actual yield is less than its theoretical yield?

Actual yield of a chemical reaction is always less than its theoretical yield because of following reasons:

  • Inexperienced workers waste a significant amount of products.
  • By-products may be formed due to side reactions.
  • The reversible reactions are never completed.
  • Product is lost during their separation and purification processes and experimental techniques such as processes like filtration, distillation, separation by separating funnel, washing, drying, and crystallization reduce the amount of product.

Difference between actual yield and theoretical yield

Following table will make it clear the difference between theoretical yield and actual yield.

difference between theoretical yield and actual yield
Difference between theoretical yield and actual yield

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