Meaning of Equilibrium

In a general chemical reaction, the reactants react to produce products. When an arrow indicating chemical equilibrium is marked between the reactants and products that depicts that the said reaction is reversible in nature. In a reversible chemical reaction, as soon as the products are formed they react to produce back the original products. Chemical equilibrium is a dynamic process. When the reaction is at the equilibrium state, the two opposite reactions occur at equal rates or speed or velocities and no net change in concentration of the reactants or the products involved. At this stage a reaction is considered as completed. In simpler words, it means that maximum conversion of the reactants to form the products has been already attained and reached an equilibrium state. 

Chemical Equilibrium explained: 

In simple words, definition of a chemical equilibrium can be as a dynamic state of a system at which the concentration or amount or properties of the reactants and concentration or amount or properties of the products do not change with time (no net change in the amounts/concentration of the substances involved) and both reactions occurring at the same speed. The two opposite reactions that occur in a chemical reaction are termed as forward reaction if reactants react to produce the products and as backward reaction if products reverse back to produce the reactants involved. The rate of the forward reaction is equal to the rate of backward reaction in a chemical equilibrium system and such a system is said to be in a state of dynamic equilibrium.

When the rate of forward reaction and rate of the reverse reaction reaches an equilibrium stage i.e. a stage after which there is no further change in the number of molecules of any of the reactants that stage is called chemical equilibrium. This equilibrium stage can be attained with the same reactants under the same conditions anywhere therefore an equilibrium state is dynamic in nature. 

On the basis of phase of the reactants and products, there are two types of chemical equilibrium one is homogeneous equilibrium and other is heterogeneous equilibrium. 

When the reactants and products are in the same phase then the equilibrium is a homogeneous equilibrium . It is further classified into two types one in which the number of molecules of reactants and products are the same and other in which the number of molecules of products is not equal to the total number of reactant molecules.

Examples of homogeneous equilibrium reactions 

C2H2(aq) + 2Br2(aq) ↔ C2H2Br4(aq)

Heterogeneous Chemical Equilibrium are the reactions in which the reactants and the products are present in different phases. Below equations shows examples of heterogeneous equilibrium:

Br2(l) ↔ Br2(g)

There are many factors which affect a state of chemical equilibrium such as changes in concentration, temperature, pressure etc. 

General statement of Le Chatelier’s principle

It states that changes in the temperature, pressure, volume, or concentration of a system will result in predictable and opposing changes in the system in order to achieve a new equilibrium state.

According to Le-Chatelier’s principle, the system tries to reduce the effect or counteract the effects of various factors affecting the equilibrium conditions. Such that the effect of the overall transformation is reduced. 

Now we will discuss hereinbelow the various factors which affects a chemical equilibrium:

1. Effect of change in concentration: The composition of the product mixture in a chemical equilibrium changes if the concentration of the reactants or products is changed. 
2. Effect of pressure changes:. In a heterogeneous reaction, since volume is not dependent over pressure therefore in accordance with Le Chatelier’s principle the change of pressure in both solids and liquids can be nullified. 
3. Effect of temperature change: The Change in temperature in an equilibrium affects the equilibrium constant and the rate of a reaction. If temperature increases the equilibrium constant decreases in an exothermic reaction. While equilibrium constant increases with an increase in the temperature in endothermic reactions. In accordance with Le Chatelier’s principle, the equilibrium shifts towards the reactant side when the temperature increases in case of exothermic reactions, for endothermic reactions the equilibrium shifts towards the product side with an increase in temperature.
4. Effect of catalyst: Presence of a catalyst is known to increase the speed of a reaction (both forward and backward reaction) without participating in the reaction. In a catalysed reaction the equilibrium stage will be achieved faster than in a non catalysed reaction.
5. Effects of addition of an inert gas: Inert gases do not affect the chemical equilibrium state of a reaction. 

Some of the examples of chemical equilibrium reactions are given hereinbelow:

• N2O4 ⇌ 2NO2
• PCl5 ⇌ PCl3 + Cl2
• N2 +3 H2 ⇌ 2NH3

Chemical equilibrium reactions are very useful in production of various industrial applicable chemicals such as in the preparation of ammonia by using the Haber’s process. Another useful application of chemical equilibrium reaction is the preparation of sulphuric acid by the contacts process. 

• Preparation of ammonia by Haber’s process: Iron is used as a catalyst in this process. The reaction between the nitrogen and hydrogen gas to produce ammonia in the presence of iron as a catalyst is shown hereinbelow:
• N2 + 3H2 ⇌ 2NH3
• Preparation of sulphuric acid by contacts process: Oxidation of sulfur dioxide into sulfur trioxide produces sulphuric acid.

Conclusion:  

Chemical equilibrium is a state of reaction when the number of molecules of the reactants becomes equal to the number of molecules of products and there are no further changes in the concentration of the reactants or products. It is of two types on the bases of the phases of the reactants or products. If phases are the same they are referred to as homogeneous equilibrium and if different phases then they are heterogeneous chemical equilibrium. A chemical equilibrium system is affected by concentration, temperature, catalyst presence, pressure etc. There are many useful industrial applications of chemical equilibrium reactions.