Types of Equilibrium: Chemical reactions occur when the reactants undergo effective collision to form new products. Most of the chemical reactions are unidirectional, but some are bidirectional too. Such bidirectional reactions are also known as reversible reactions. Chemical equilibrium is a state of reversible chemical reaction in which the rate of the forward reaction is equal to or the same as the rate of backward reaction. Thus, we cannot see any change in the concentration of the products or the reactants at equilibrium though the reaction doesn’t stop because the concentration change of a chemical reaction is constant; this stage may also be called dynamic equilibrium.

Chemical equilibrium is of two types: Homogeneous chemical equilibrium, in which both reactants and products are in the same state, and Heterogeneous chemical equilibrium, in which both reactants and products are in a different state. In chemistry, there are several factors that affect the chemical equilibrium of a reaction, such as a change in concentration, change in pressure, change in temperature, and catalyst. All these factors play a very important or crucial role in achieving and maintaining the chemical equilibrium of a chemical reaction. Let’s learn about equilibrium in detail.

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Equilibrium in Chemistry

Equilibrium in chemistry refers to the state of a system in which there is no change in the concentration of the reactant and the concentration of the products with time, and as a result, the system does not experience any further change in the properties.

The equilibrium stage is a stage at which the rate of the forward reaction is equal to the rate of backward reaction. At this stage, the number of reactant molecules that get converted into products and product molecules that get converted into reactants are the same. If similar conditions are maintained, the same equilibrium can be carried out anywhere, followed by continuous interchanging of molecules. As a result, chemical equilibrium is also known as dynamic equilibrium.

The above graph shows the variation of concentration of reactants and products with time and once, both the concentration becomes the same, the state of equilibrium is achieved.

Types of Equilibrium in Chemical Processes

There are two types of equilibrium in chemistry:

1. Homogeneous Equilibrium
2. Heterogeneous Equilibrium

1. Homogeneous Equilibrium

When the reactants and the products of chemical equilibrium are all in the same phase, such a condition is known as a homogeneous chemical equilibrium. For example, in the reaction given below, the reactants and the products all are in the gaseous phase.

\({{\text{H}}_{_2}}\left( {\text{g}} \right) + {{\text{I}}_2}\left( {\text{g}} \right) \rightleftharpoons 2{\text{HI}}\left( {\text{g}} \right)\)

\(2{\text{S}}{{\text{O}}_2}\left( {\text{g}} \right) + {{\text{O}}_2}\left( {\text{g}} \right) \rightleftharpoons 2{\text{S}}{{\text{O}}_3}\left( {\text{g}} \right)\)

2. Heterogeneous Equilibrium

When the reactants and the products of chemical equilibrium are all in a different phase, such a condition is known as a heterogeneous chemical equilibrium. For example, in the reaction given below, the reactants and the products are in the gaseous and solid phases.

\({\text{C}}{{\text{O}}_2}\left( {\text{g}} \right) + {\text{C}}\left( {\text{s}} \right) \rightleftharpoons 2{\text{CO}}\left( {\text{g}} \right)\)

\({\text{CaC}}{{\text{O}}_3}\left( {\text{s}} \right) \rightleftharpoons {\text{CaO}}\left( {\text{s}} \right) +{\text{C}}{{\text{O}}_2}\left( {\text{g}} \right)\)

Equilibrium in Physical Processes

Like chemical reactions, there exists an equilibrium in physical processes too. This refers to the equilibrium between different phases of a substance, i.e., solid, liquid and gas. Physical equilibrium can be achieved in a closed system. Phase transformation can occur like:

\({\text{Solid}} \rightleftharpoons {\text{liquid}}\)

\({\text{liquid}} \rightleftharpoons {\text{gas}}\)

\({\text{Solid}} \rightleftharpoons {\text{gas}}\)

Solid-Liquid Equilibrium

When ice melts into water, a stage comes when the transformation of solid (ice) to liquid (water) comes to equilibrium as mass and temperature remain constant.

Liquid-Vapour Equilibrium

In a closed system, water boils at \({\rm{100}}\,{\rm{^\circ C}}\) to convert from liquid to water vapour at atmospheric pressure. Under this condition, a state of equilibrium is achieved.

Solid-Vapour Equilibrium

In a closed jar, iodine gets sublimed into violet vapours, and this is a good example of solid-vapour equilibrium. With time, we may observe an increase in the intensity of the violet colour, and a time comes when the intensity remains constant, this is the stage of equilibrium.

Formula of Equilibrium

The equilibrium of a chemical reaction is calculated by the equilibrium constant. The equilibrium constant is defined as the ratio between the amount of reactant and the amount of product that is used to determine chemical behaviour in a chemical reaction. Thus, at equilibrium,

\({\mathbf{Rate}}{\text{ }}{\mathbf{of}}{\text{ }}{\mathbf{the}}{\text{ }}{\mathbf{forward}}{\text{ }}{\mathbf{reaction}}{\text{ }} = {\text{ }}{\mathbf{Rate}}{\text{ }}{\mathbf{of}}{\text{ }}{\mathbf{the}}{\text{ }}{\mathbf{backward}}{\text{ }}{\mathbf{reaction}}\)

In the chemical reaction given below:

\({\text{aA}} + {\text{bB}} \rightleftharpoons {\text{cC}} + {\text{dD}}\)

Equilibrium constant \(\left( {{{\text{K}}_{{\text{eq}}}}} \right)\):

\({{\text{K}}_{{\text{eq}}}} = \frac{{{\text{Rate}}\,{\text{constant}}\,{\text{of}}\,{\text{forward}}\,{\text{reaction}}}}{{{\text{Rate}}\,{\text{constant}}\,{\text{of}}\,{\text{backward}}\,{\text{reaction}}}}\)

\({{\text{K}}_{{\text{eq}}}} = \frac{{{{\text{K}}_{\text{f}}}}}{{{{\text{K}}_{\text{b}}}}}\)

Thus,

\({{\text{K}}_{{\text{eq}}}} = \frac{{{{\left[ {\text{C}} \right]}^{\text{c}}}\,{{\left[ {\text{D}} \right]}^{\text{d}}}}}{{{{\left[ {\text{A}} \right]}^{\text{a}}}{{\left[ {\text{B}} \right]}^{\text{b}}}}}\,\)

Types of Equilibrium Theory

Le Chatelier′s Principle

According to the Le Chatelier′s Principle, at equilibrium, when stress is applied to a chemical system, the equilibrium will shift to relieve the stress. In other words, Le Chatelier′s Principle can be used to estimate the direction of a chemical reaction in response to any change in various conditions like temperature, pressure, concentration, volume.

Factors Affecting Equilibrium

1. Change in Pressure – As we know, that pressure is directly proportional to the volume; a change in pressure occurs due to the change in the volume. Change in pressure can also affect the gaseous reactions because the total number of gaseous reactants and products are different at this stage.
2. Change in Temperature – The sign of \(\vartriangle {\text{H}}\) of the reaction shows the effect of temperature on chemical equilibrium, and it also follows Le-Chatelier’s Principle. In an exothermic reaction, when the temperature increases, the equilibrium constant decreases, and in the case of an endothermic reaction, the equilibrium constant increases along with the rise in temperature.Along with this, the rate of a chemical reaction is also affected by the change in temperature. According to Le Chatelier’s principle, in the case of exothermic reactions, the equilibrium shifts towards the reactant side when the temperature increases. On the other hand, in the case of endothermic reactions, the equilibrium shifts towards the product side as the temperature increases.
3. Effect of a Catalyst – A catalyst does not affect the chemical equilibrium of a reaction. It only increases the rate of a chemical reaction by lowering the activation energy.

Importance of Chemical Equilibrium

Chemical equilibrium is useful in many industrial processes like:

1. It is used in Haber’s process used in the preparation of ammonia. The yield of ammonia occurs more at low temperature, high pressure and in the presence of iron as a catalyst using nitrogen and hydrogen as reactants.
2. The Contact process used in the preparation of sulphuric acid by the oxidation of sulphur dioxide into sulphur trioxide is also governed by chemical equilibrium.

Summary

Chemical equilibrium is a state of chemical reaction in which both the reactants and the products have no further tendency to change with time. As a result, the system does not experience any further change in the properties. In this stage, the rate of the forward reaction is equal to the rate of the backward reaction. The number of reactant molecules that get converted into products and product molecules that get converted into reactants are equal. If similar conditions are maintained, the same equilibrium can be carried out anywhere, followed by a continuous exchange of molecules. Due to this reason, chemical equilibrium is also known as dynamic equilibrium.

Chemical equilibrium is of two types: homogeneous chemical equilibrium and heterogeneous chemical equilibrium. Equilibrium also occurs in physical processes in a closed system. Equilibrium systems work according to the Le Chatelier′s Principle, which states that at equilibrium, when stress is applied to a chemical system, the equilibrium will shift to relieve the stress. In other words, Le Chatelier′s Principle can be used to predict the direction of a chemical reaction in response to a change in various conditions like temperature, pressure, concentration, volume. Equilibrium is very important for various purposes, such as in the preparation of ammonia by Haber’s process in industries.

FAQs on Types of Equilibrium

Q.1. What are the two types of equilibrium?
Ans: There are two types of equilibrium in chemistry: Homogeneous Equilibrium and Heterogeneous Equilibrium. When the reactants and the products of chemical equilibrium are all in the same phase, such a condition is known as a homogeneous chemical equilibrium. When the reactants and the products of chemical equilibrium are all in a different phase, such a condition is known as a heterogeneous chemical equilibrium.

Q.2. What do you mean by equilibrium and its types?
Ans: Equilibrium refers to the state of a system in which there is no change in the concentration of the reactant and the concentration of the products with time, and as a result, the system does not experience any further change in the properties. It is a stage at which the rate of the forward reaction is equal to the rate of the backward reaction. At this point, the number of reactant molecules that get converted into products and product molecules that get converted into reactants are the same. There are two types of equilibrium in chemistry: Homogeneous equilibrium and Heterogeneous equilibrium.

Q.3. What are the factors that affect the change in equilibrium?
Ans: The various factors that affect equilibrium are pressure, volume, and temperature. As we know that pressure is directly proportional to the volume, a change in pressure occurs due to the change in the volume. Change in pressure can also affect the gaseous reactions because the total number of gaseous reactants and products are different at this stage.

The sign of \(\vartriangle {\text{H}}\) of the reaction shows the effect of temperature on chemical equilibrium, and it also follows Le-Chatelier’s Principle. In an exothermic reaction, when the temperature increases, the equilibrium constant decreases, and in the case of an endothermic reaction, the equilibrium constant increases with an increase in temperature. Along with this, the rate of a chemical reaction is also affected by the change in temperature.

Q.4. What is the formula of equilibrium?
Ans: For a chemical reaction: \({\text{aA}} + {\text{bB}} \rightleftharpoons {\text{cC}} + {\text{dD}}\) the equilibrium rate of the forward reaction is equal to the rate of backward reaction. Therefore, the formula of equilibrium is: \({{\text{K}}_{{\text{eq}}}} = \frac{{{{\left[ {\text{C}} \right]}^{\text{c}}}{{\left[ {\text{D}} \right]}^{\text{d}}}}}{{{{\left[ {\text{A}} \right]}^{\text{a}}}{{\left[ {\text{B}} \right]}^{\text{b}}}}}\)

Q.5. What are the examples of equilibrium?
Ans: A few examples of equilibrium are its use in Haber’s process used in the preparation of ammonia. The yield of ammonia occurs more at low temperature, high pressure and in the presence of iron as a catalyst using nitrogen and hydrogen as reactants. Another use is in the contact process used in the preparation of sulphuric acid by the oxidation of sulphur dioxide into sulphur trioxide is also governed by chemical equilibrium.

Learn About Homogeneous Equilibria Here

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