Reversible Or Irreversible Reactions

Chemical reaction simply define as a reaction in which a substance (or a compound) is converted into a new one with different property. In a reaction, the substance (or a compound) which is converted into new substance (or changes their state) is called 'REACTANT' and a newly formed substance is called 'PRODUCT'. 
We see many chemical changes around us in our daily life. For example, conversion of milk into curd and milk into cheese by bacteria, burning of a paper, burning of a candle, melting of an ice, neutralization reactions, photosynthesis reaction, etc.

A chemical reaction is usually written by placing an arrow symbol between the reactants and the products. For example, if A and B are two reactants in a chemical reaction while C and D are two products, then this reaction can be represented as -
                 A  +  B        ➝       C   +   D
             (Reactants)           (Products) 

As we know that in a reaction, the reactants react together and get converted into a new form molecule called product, but sometimes these products get converted back into reactants on their own. Thus, a reaction can occur in both directions. (forward and backward directions)
On the basis of reaction pathways, chemical reaction are of two types: Reversible and Irreversible.........
In this article, we will learn about reversible and Irreversible reactions with the help of various examples.

Types Of Reation 
There are two main types of reactions based on their forward and backward pathways and these are namely;

REVERSIBLE  REACTION
......."A reversible reaction is defined as a reaction in which reactants are converted into products and products into reactants simultaneously. That means, a reversible reaction proceeds in both forward and backward direction"......... 
In other words, A chemical reaction in which an infinitesimally small change in one of the variables (pressure, temperature, concentration of the reactant or product) changes the direction of the reaction (forward or backward), is called a reversible reaction. Therefore, these changes are not permanent and can be reversed back.

Let us consider the reaction between compound A and compound B;

Here, A and B reversibly react to form the compound AB as product. The bidirectional arrow here indicates that this reaction is Reversible Means, product AB can be reverted back to reactant A and B again.

There exists a "Chemical Equilibrium" here. At this point, the rate of both the forward and reverse reaction is the same.
In chemical equilibrium, there is no change in the concentration of reactants and products. It means, they become constant not equal at this point. But it doesn't mean that the reaction has been stopped here.
Chemical equilibrium is dynamic in nature i.e. continuous in nature. It is present in case of reversible reaction and not in case of irreversible reactions.

For Example
Water to ice and ice back to water is the best example of a reversible reaction. Let's see more examples;
(1) Formation of ammonia from nitrogen and nitrogen; 
                   N₂ + 3H₂ ⇋ 2NH₃
          Forward path: N₂ + 3H₂ ➞ 2NH₃
         Backward path: 2NH₃ ➞ N₂ + 3H₂
(2) Dissolution of sugar in water is a reversible reaction.
(3)               CaO + CO₂ ⇋ CaCO₃
(4)          KNO₃ + NaCl ⇋ K-Cl + 2NaNO₃


IRREVERSIBLE REACTION
........."A reaction is called as an irreversible reaction if only reactants are converted into products (conversion of product into reactant is not possible). That means, an irreversible reaction proceeds in one direction only".......
(forward path only or no backward path)
In other words, The reaction in which the entire amount of reactant is converted into a product and there is no reaction from the side of the product is called irreversible reaction. The chemical changes involved here are permanent changes.

Let us consider the reaction between compound A and compound B;
The unidirectional arrow here indicates that this reaction is Irreversible. In other words, product AB can not be reverted back to reactant A and B again.

For Example  
The burning of paper/wood is the best example of irreversible reactions as we cannot convert the ashes into paper/wood again. Let's see more examples;
(1) Combustion of propane is an example of Irreversible reaction.
(2)             C + O₂ ➞ CO₂
(3)     Ag-NO₃ + Na-Cl ➞ Ag-Cl + NaNO₃
(4)      BaCl₂+ Na₂SO₄ ➞ BaSO₄ + 2NaCl
(5)   Pb(NO₃)₂ + 2 NaI ➞ Pb-I₂ + 2NaNO₃

Here, NaNO₃, BaSO₄ and PbI₂ are solid precipitates which do not give back the reactants.
More examples;


Difference Between Reversible And Irreversible Reactions
  • Reversible reactions are those reactions in which reactants can be converted into products and products back to reactants whereas irreversible reactions are those reactions in which reactants can be converted into products but products can't be convert back into reactants.
  • Reversible reactions can achieve equilibrium but irreversible reactions can not.
  • Reversible reactions proceed in both the directions whereas irreversible reactions proceed only in forward direction.
  • Reversible reactions operate in both directions while irreversible reactions operate in only one direction. 
  • Reversible reactions are relatively slow while irreversible reactions occur at a faster rate.
  • In reversible reactions, the reactants can't be completely converted into products while in irreversible reactions, the reactants can be completely converted into products.


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