Equilibrium State - Definition, Types and Application

In certain processes or reactions, no changes are observed. It maintains the balance between the reactants and products. An equilibrium state refers to the state in which there is no change in temperature, colour, concentration, etc. It is the state of a process in which the system does not show any change with the passage of time.

Table of Contents:

What is the Equilibrium State?
Physical Equilibrium
Characteristics of Physical Equilibrium
Chemical Equilibrium
Characteristics of Chemical Equilibrium
Types Of Chemical Equilibrium
Practice Problems On Equilibrium State

What is the Equilibrium State?

An equilibrium state is a balancing condition or a stable condition in which the opposite forces or reactions cancel out each other, and no changes are observed in the system. Equilibrium is a term used in many fields, such as physics, chemistry, economics, etc., with slightly different meanings; however, the concept of equilibrium is the same. Whenever an external and small induced force tends to oppose the displacement, the body or the particle returns to its equilibrium state.

The equilibrium associated with physical processes, such as the dissolution of salt, melting of solids, etc., is known as physical equilibrium. Similarly, the equilibrium associated with a chemical reaction is known as a chemical equilibrium.

Physical Equilibrium

In physical equilibrium, the opposing processes involve only physical changes. For example, the physical equilibrium state between vapour and water in the water evaporation process. Some of the other examples of physical equilibrium state are given below:

Solid ⇋ Liquid equilibrium

H₂O(solid) ⇋ H₂O(liquid)

Liquid ⇋ Gas equilibrium

H₂O(liquid) ⇋ H₂O(gas)

Solid ⇋ Gas equilibrium

I₂(solid) ⇋ I₂(vapour)

Characteristics of Physical Equilibrium

Physical equilibrium is possible only under a closed system at certain conditions such as temperature, pressure etc.
All measurable properties of the system always remain constant.
The backward and forward reactions take place at a constant rate.
The magnitude of quantities at any state shows the extent to which the physical process has proceeded until reaching the equilibrium state.

Chemical Equilibrium

In chemical equilibrium, the opposing processes involve only chemical changes. In a reversible reaction, a chemical reaction is a state where both forward and backward reactions take place at equal speeds. The concentration of reactants and products remains the same with time. The measurable quantities do not undergo any changes. For example, the equilibrium state between acetic acid, water and acetate.

Characteristics of Chemical Equilibrium

Chemical equilibrium can be achieved from any side or direction.
This type of equilibrium is dynamic in nature.
The products and the reactants remain constant at equilibrium.
Chemical equilibrium is achieved by the presence of a catalyst.

Ionic equilibrium is a type of chemical equilibrium that involves the ions in the aqueous solution.

Types Of Chemical Equilibrium

Chemical equilibrium is classified into two types:

Homogeneous Equilibrium

In Homogeneous equilibrium, the reactants and products of the chemical equilibrium are in the same phase. The number of molecules present in the product is equal to the number of molecules present in the reactant. For example:

H₂ (g) + I₂ (g) = 2HI (g)

N₂(g) + O₂ (g) = 2NO (g)

Heterogeneous Equilibrium

In Heterogeneous equilibrium, the reactants and products of the chemical equilibrium are in a different phase. For example:

2CO = CO₂(g) + C (s) (g)

CO₂ + CaCO₃ (s) + CaO (s) (g)

Equilibrium State Practice Problems

Q1. Which of the following remains constant at equilibrium in a chemical reaction

a. Products
b. Reactants
c. Both a and b
d. None of the above

Ans: c. Both a and b

The products and the reactants remain constant at chemical equilibrium.

Q2. The backward and forward reactions of physical equilibrium take place at _________ rate.

a. Maximum
b. Minimum
c. Constant
d. None of the above

Ans: c. constant

The backward and forward reactions take place at a constant rate.

Q4. Which of the following is correct about chemical equilibrium?

a. Weak
b. Fast
c. Dynamic
d. None of the above

Ans: c. Dynamic

Frequently Asked Questions

Q1. What is the pH formula?
Answer: pH is described as the negative of the logarithmic scale of molar hydronium-ion concentration. It is written as -log [H₃O⁺].

Q2. What are Kp and Kc in equilibrium?
Answer: Kp is referred to as an equilibrium constant under partial pressures. Kc is referred to as an equilibrium constant expressed in molarity concentration.

Q3. What are strong and weak electrolytes?
Answer: The completely ionised electrolytes are called strong electrolytes, while the electrolytes that partially ionise to a small extent are called weak electrolytes.