Reversible and Irreversible Reactions Explained Simply

In this article, I will describe reversible and irreversible reactions, highlighting their definitions, main characteristics, and simple examples. By the end, you will clearly understand how these two types of reactions differ and why they are important in both chemistry and daily life.

In most of the reaction two or more substances react to form the products which react among themselves to give back the original substances. For example, A and B may react to form C and D, which react together to form A and B, Such reactions are denoted by a pair of arrows.

A+ B ⇌ C+D

Reversible Reaction.

The Reactions, which go in the forward and the backward direction simultaneously under the similar conditions are called reversible reactions.

Characteristics:

• Represented with a double arrow (⇌).
• Do not go to completion, instead reach a state of equilibrium.
• Forward and backward reactions occur simultaneously.
• Amount of reactants and products becomes constant at equilibrium.

Example:

Some common examples of reversible reactions are shown below

 \text{H}_{2} + \text{I}_{2} \;\rightleftharpoons\; 2\text{HI} 

\\[1.5em]

 \text{N}_{2} + 3\text{H}_{2} \;\xrightleftharpoons[\;]{\text{Fe}/450^\circ \text{C}}\; 2\text{NH}_{3} 

\\[1.5em]

 2\text{NO}_{2} \;\xrightleftharpoons[\;]{\text{high pressure}}\; \text{N}_{2}\text{O}_{4} 

\\[1.5em]

 \text{PCl}_{5} \;\rightleftharpoons\; \text{PCl}_{3} + \text{Cl}_{2} 

Applications of Reversible Reactions

• Industrial:
  • Haber Process (NH₃ production)
  • Contact Process (H₂SO₄ production)
• Biological:
  • Oxygen binding with hemoglobin (Hb + O₂ ⇌ HbO₂)
  • Carbonic acid formation in blood (CO₂ + H₂O ⇌ H₂CO₃)

Irreversible Reactions:

Those chemical reactions which proceed only in the forward direction to give products are called irreversible reaction.

Characteristics:

• Reactants are fully converted into products.
• The reaction goes to completion.
• Cannot be reversed easily.
• Usually associated with large energy changes.

Examples:

• Combustion of methane:
  • CH₄​ + 2O₂​ → CO_2​ + 2H₂​O + heat
• Neutralization of a strong acid and base:
  • HCl + NaOH → NaCl + H₂​O

Characteristics of Irreversible Reactions.

• Represented with a single arrow (→).
• Go to completion, reactants are fully converted into products.
• Only the forward reaction takes place.
• Products formed do not revert back to reactants.

Irreversible Reactions in Daily Life

• Cooking of food
• Rusting of iron
• Burning of fuel

Difference between reversible and irreversible reactions:

Reversible Reaction.

1. The reactions in which products formed are converted into reactants are known as reversible reactions.
2. These reactions proceed only in one direction.
3. These are represented by a pair of oppositely directed half headed arrows.
4. Example:

 \text{N}_{2} + 3\text{H}_{2} \;\xrightleftharpoons[\;]{\text{Fe}/450^\circ \text{C}}\; 2\text{NH}_{3} 

\\[1.5em]

 \text{PCl}_{5(g)} \;\xrightleftharpoons[\;]{\text{high pressure}}\; \text{PCl}_{3(g)}+{Cl}_{2(g)} 

Irreversible reaction.

1. The reactions in which products formed are not reconverted into reactants are known as irreversible reactions.
2. These reactions proceed only in one direction.
3. These reactions are represented by a full headed arrow.
4. Example.
   • 2Na_(s)+2H₂O₍₁₎ -> 2NaOH_(aq) +H2_(g)
   • 2H₂ + O₂ -> 2H₂O

Equilibrium in Reversible Reactions.

• Dynamic equilibrium is reached when forward reaction = rate of backward reaction, keeping the amounts of reactants and products constant.
• At equilibrium:
  • Concentrations of reactants and products remain constant.
  • Reaction does not stop; both processes continue at equal rates.