What is Thermodynamic Equilibrium? (With Best Example)

Before explaining thermodynamic equilibrium, let me tell you one thing.

The Thermodynamic Equilibrium includes three types of equilibrium.

1. Thermal equilibrium
2. Mechanical equilibrium and 
3. Chemical equilibrium

If two systems are in thermal, mechanical and chemical equilibrium with each other, then that systems are in thermodynamic equilibrium with each other.

Here, you will come to know everything about thermodynamic equilibrium. Don’t worry, I’ll also give you the definitions as well as examples of the same. (Scroll down to see images of real life examples)

Now let us discuss all these three types of equilibrium one by one.

What is Thermal equilibrium?

Let’s take an example to understand thermal equilibrium.

You can see that two blocks A (blue color) and B (green color) are kept apart from each other. Initially assume that the temperatures of body A and body B are 80 °C and 30 °C respectively.

Now what will happen if we bring these two bodies in contact with each other?

There will be heat transfer between these two bodies. Right? 

Because both these bodies are at different temperatures and you know that heat will start to flow from the higher temperature body to the lower temperature body. 

In other words,the hotter body will lose its heat and the colder body will gain heat.

Thus, after some time, let say the temperature of both the bodies becomes 50 °C.

What does this indicate?

This indicates that both the bodies (body A and body B) are in thermal equilibrium with each other.

There will be no heat transfer once both the bodies reach the same temperature.

Definition of thermal equilibrium:

“The condition under which two objects in physical contact with each other exchanges no heat energy is termed as thermal equilibrium. During this condition, the temperature of both the bodies remains same”

Now, I’ll show you some real life examples based on thermal equilibrium. 

1). Cold water, hot water and surrounding air

Have you understood from this picture?

If not, then read the explanation given below.

In the first image, the ice water is at 0 °C, the hot water is at 70 °C and the surrounding air is at 25 °C.

After few minutes, both these three bodies will come at the same temperature (i.e 25 °C). 

After this condition, there will be no heat transfer between them and this is known as thermal equilibrium. (In other words, all the three bodies are in thermal equilibrium with each other)

2). Vegetables and refrigerator air

If you keep vegetables and fruits overnight in your refrigerator, then all the vegetables and fruits will cool down and their temperature will be the same as that of the temperature of the air inside the refrigerator.

This indicates that all the vegetables, fruits as well as surrounding air of the refrigerator are in thermal equilibrium with each other.

For more examples like this, you can refer this article – “Examples of Zeroth law of thermodynamics“

Must read (these are amazing examples)

What is mechanical equilibrium?

“A system is said to be in mechanical equilibrium if the net force acting on the system is zero” 

Let me give you some examples for better understanding.

1). Constant pressure inside the bottle

For example, if the pressure force inside the system is same at all the points and does not change with time, then the system is said to be in mechanical equilibrium.

1). Rope wrestling

Here you can see that both the teams are pulling the rope towards them. 

But the rope is not moving on anyone’s side. Rope remains at rest only. That means rope is in the state of mechanical equilibrium.

2). The boxes are in mechanical equilibrium

Few boxes are kept over here and two boys are pushing them from their side. 

What you can see in the above picture?

The boxes are neither moving left nor right.

This indicates that the boxes are in mechanical equilibrium.

Now, let us move to the chemical equilibrium.

What is chemical equilibrium?

“A system is said to be in chemical equilibrium if its chemical composition does not changes with time and no chemical reaction takes place in the system”

Or 

“A reaction is said to be in chemical equilibrium if the rate of forward reaction and rate of reverse reaction is equal”

Let me give a simple example for the chemical equilibrium.

aA+bB ⇌ cC+dD

Look at this chemical reaction. 

A and B are the reactants and C and D are the products. As time passes, the amount of all the reactants and products remains the same. No changes occur in their quantity. Or in other words, the rate of forward reaction and rate of reverse reaction remains the same.

The chemical composition of reactants as well as products also remains the same with the passing of time. 

(In a single substance, the chemical composition of the entire substance remains the same. 

For example, in water, the chemical composition remains H₂O only in the entire volume of water. 

Thus, water kept in a bottle has the same chemical composition (H₂O). Thus, water kept in a bottle is an example of chemical equilibrium)

Thus a system in which its chemical composition does not change with time and no chemical reaction takes place in it is said to be in chemical equilibrium.

Thermodynamic Equilibrium

What is thermodynamic equilibrium?

It is very simple, it is a combination of all these above mentioned three equilibriums.

“Two systems are said to be in thermodynamic equilibrium when they are in thermal, mechanical and chemical equilibrium with each other”

Now let me give a best example of thermodynamic equilibrium for your better understanding.

This is the best example of thermodynamic equilibrium which you already know.

Explanation: 

1). The temperature of water is 25 °C and the temperature of surrounding air is also 25 °C. Thus the water is in thermal equilibrium with the surrounding air.

2). All the water molecules exerts equal pressure on each other as well as on the walls of the bottle. Also these molecules are in balanced condition and they are not moving at all. So the water molecules are said to be in mechanical equilibrium.

3). The chemical composition of all the water molecules is also the same in the entire bottle. The chemical composition is H₂O only in the entire volume of water. Thus the water is also in chemical equilibrium.

Now, as the water is in thermal, mechanical and chemical equilibrium, we can say that it is in the state of thermodynamic equilibrium.

I hope you have clearly understood all the concepts along with the examples.

If you have any doubts, feel free to comment below. Also let me know which example you like the most.

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Also read:

• What is thermodynamics? (35+ topics with definitions and examples)
• Zeroth law of thermodynamics
  • Examples of Zeroth Law of Thermodynamics
• What is First law of thermodynamics?
  • First law of thermodynamics definition/statement (In simple way)
  • Examples of First Law of Thermodynamics / Law of conservation of Energy
  • First law of thermodynamics equation (A practical explanation)
  • Limitations of First Law of Thermodynamics
• What is second law of thermodynamics? [8+ Best examples to remember the law]
  • Examples of Second Law of Thermodynamics (8+ best examples)
  • Second Law of Thermodynamics Definition/Statement (Next level explanation)
  • Second Law of Thermodynamics Equation [Practical explanation]
• What is third law of thermodynamics? (in simple terms)
• Laws of thermodynamics
• What is Carnot Cycle in Thermodynamics?
• What is the definition of entropy in thermodynamics?
• Heat capacity vs specific heat in thermodynamics
• Thermodynamic Process (With Examples)
• What is Thermodynamic System? – Open, Closed & Isolated system (With Examples)