Thermodynamic Equilibrium: Definition, Types, FAQ’s [PDF]


The study of thermodynamic equilibrium is important because it helps us understand the behavior of complex systems, such as chemical reactions, in a more comprehensive manner. Understanding thermodynamic equilibrium also helps us to predict the conditions under which a system will reach a state of balance. This is essential in a wide range of fields, including physics, chemistry, engineering, and materials science. In this article, we will explore the concept of thermodynamic equilibrium in detail.

Thermodynamic equilibrium definition

A system is said to be in thermodynamic equilibrium when no spontaneous change in any macroscopic property is observed. As the system is isolated from its surroundings it is known as thermodynamic Equilibrium.

Types of Thermodynamic Equilibrium

Following are the 3 types of thermodynamic equilibrium :

  • Chemical Equilibrium
  • Mechanical Equilibrium
  • Thermal Equilibrium

Chemical Equilibrium

What is Chemical Equilibrium?

A system is said to be in chemical equilibrium when no chemical reaction takes place in the system or between the system and its surrounding. The chemical composition will be the same throughout the system and this will not vary the system in chemical equilibrium. This is a reversible reaction.

Equations of Chemical Equilibrium

Chemical equilibrium is a state where the rate of the forward reaction is equal to the rate of the reverse reaction. At this point, there is no net change in the concentration of reactants and products. Equations of chemical equilibrium are used to represent the balance between the reactants and products in a chemical reaction.

The equilibrium constant ‘Kc’ is a measure of the degree of completion of a reaction at equilibrium. It is calculated by taking the ratio of the product concentrations to the reactant concentrations, with each concentration raised to the power of its stoichiometric coefficient. The value of Kc varies with temperature and pressure, and it provides important information about the behavior of chemical reactions at equilibrium.

Equilibrium Condition

Keq = Kc

Chemical Equilibrium Example

CaCO→ CaO + CO

When we heat Calcium Carbonate (CaCO3 ) at temperature 1073K will get CaO and CO

Mechanical Equilibrium

What is Mechanical Equilibrium?

Mechanical equilibrium refers to a state in which an object is not in motion and has no acceleration. This means that the net force acting on the object is zero, and there is no unbalanced force acting on it.

Mechanical equilibrium can be divided into two types:

  • Static Equilibrium : Static equilibrium is when an object is at rest and remains at rest unless acted upon by an external force.
  • Dynamic Equilibrium : Dynamic equilibrium is when an object is moving at a constant velocity in a straight line.

Equations of Mechanical Equilibrium

Mechanical equilibrium is a state in which the net force acting on an object is zero, and the object is either at rest or moving with a constant velocity. In order to analyze the equilibrium of an object, we use equations of mechanical equilibrium, which are derived from Newton’s laws of motion.

These equations are based on the principle of conservation of momentum and the principle of conservation of angular momentum. The equations of mechanical equilibrium include the sum of forces in the x-direction, the sum of forces in the y-direction, and the sum of moments about a point. By solving these equations simultaneously, we can determine the forces and moments acting on an object and whether it is in equilibrium or not.

Equilibrium Condition

grad p = ρ g

Mechanical Equilibrium Example

Treadmill: This is a gym instrument on which we run but we don’t move forward because the force that is trying to push you forward the same force is acting the opposite to push backward.

Thermal Equilibrium

What is Thermal Equilibrium?

Thermal equilibrium is a state in which two or more objects or systems are at the same temperature and there is no net transfer of heat between them. In simpler terms, it is a state of balance in which the rate of heat transfer between two objects is equal. This state can only be achieved when the objects are in thermal contact and they are able to exchange energy until their temperatures are equal. Once thermal equilibrium is reached, there is no longer any net transfer of heat between the objects, and they remain at the same temperature unless an external force is applied.

Thermodynamic Equilibrium pdf

Equations of Thermal Equilibrium

Thermal equilibrium occurs when two bodies in contact with each other reach the same temperature and stop exchanging heat. The equations that describe thermal equilibrium are based on the principles of energy conservation and the second law of thermodynamics. These equations provide a mathematical framework for understanding how heat is transferred between bodies and how it affects their temperature. The equations of thermal equilibrium are used in many fields, including engineering, physics, and chemistry.

Equilibrium Condition

F = U – TS

Thermal Equilibrium Example

A hot cup of tea has a higher temperature as compared to its surrounding so this is not in thermal equilibrium.

But when you keep this in an open environment for some time, the temperature starts changing due to the radiation process to the environment. Due to this the surrounding temperature and the temperature of the cup of tea become the same at that moment.


What is local Thermodynamic Equilibrium?

This refers to a state where the thermodynamic properties are uniform within a region of the system. It is commonly used to study different fields such as plasma physics, laser physics, etc. It can also be used to simplify calculations. It is an essential concept that can be used to understand the different thermodynamic properties of a system.

How many Laws of Thermodynamics are there?

There are three main laws of thermodynamics also known as laws of energy conservation. Let’s have a quick look at each law:
First Law of Thermodynamics – This states that energy cannot be created nor destroyed but can only be converted from one form to another.
Second Law of Thermodynamics – This states that in any spontaneous process, the total entropy of a closed system will increase over time.
Third Law of Thermodynamics – This states that it is impossible to reach absolute zero, the temperature at which all molecular motion will get ceased.

What is Thermal Equilibrium Equation?

The thermal equilibrium equation states that there is no net flow of heat between two objects that are in contact with each other. The thermal equilibrium equation is based on the zeroth law. Mathematically it is a very simple equation : T1 = T2 where T1 and T2 are temperatures of objects that are in thermal contact.

How to find thermal equilibrium of two substances?

In order to find the thermal equilibrium of two substances, it is important to understand the concept of heat transfer. When two substances are in contact with each other, heat will naturally flow from the substance with a higher temperature to the substance with a lower temperature. This process will continue until both substances reach the same temperature, at which point they are said to be in thermal equilibrium.

To determine when two substances have reached thermal equilibrium, it is necessary to take temperature readings of both substances at regular intervals. This can be done using a thermometer or other temperature sensing device. Once the temperature readings of both substances have stabilized and are no longer changing, it can be assumed that they have reached thermal equilibrium.


In conclusion, the thermodynamic equilibrium is a state where all thermodynamic variables are relevant. This state is achieved when there is no net flow of heat or energy. Understanding thermodynamic equilibrium is crucial for modeling and simulations. Overall it is a fundamental concept in the study of energy and matter.

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