Thermodynamic Equilibrium

As we discussed earlier that thermodynamic is only concerned with equilibrium states, this means that there is no driving force within the system (or between the system and the surrounding). 
"When all the properties of the system remain the same throughout and do not change with time, we can say that our system will be in a state of equilibrium". 
Equilibrium is a term that refers to a steady state conditions (or static condition), the absence of change. Equilibrium means the state of balanced of a system with itself or surrounding. In the state of equilibrium, no driving force is present so the properties of the system no longer change. And the equilibrium in thermodynamics is known as "Thermodynamic Equilibrium". 
☛ No driving force means no temperature difference, no mass difference, no energy different, etc.
☛ No driving force means no exchange of any property between two bodies or between two systems or between system and surrounding.
☛ The state of equilibrium can be achieved in two ways;
(1) When the system and the surrounding are not interacting.
(2) When the system and the surrounding are interacting.

Driving Forces
"A driving force is a sign that tells us that there is a difference in property between two bodies, or between two systems, or between the system and the surroundings". 
For Example- 
Temperature difference, Forces difference, Pressure difference, Concentration difference, Chemical potential difference, etc. 
         
Different types of driving forces bring about different types of changes. 
For Example- 
1. An imbalance of mechanical forces such as pressure on a piston causes energy transfer in the form of work results in mechanical equilibrium, 
2. A temperature difference causes a flow of heat results in thermal equilibrium, 
3. A gradient in chemical potential causes substances to move from one place to another results in chemical equilibrium.

Thermodynamic Equilibrium
There are mainly three types of equilibrium in thermodynamics;
(A) Thermal Equilibrium
"A system will be in thermal equilibrium with its surroundings when there is no difference in temperature between that system and its surroundings". In thermal equilibrium, there is no heat transfer from the system to the surroundings as no temperature gradient exists between them.
(b) Mechanical Equilibrium 
"A system will be in mechanical equilibrium when no mechanical driving force of any kind exists between the system and its surroundings". 
For Example-
Pressure is a mechanical driving force Or we can say that if the pressure force inside the system is the same at all the points and does not change with time, then the system is said to be in mechanical equilibrium.
(c) Chemical Equilibrium
"A system will be in chemical equilibrium when there is no change in the chemical composition (such as concentration) within the system over time".
For Example- 
1) Water kept in a bottle is an example of chemical equilibrium as the chemical composition remains H₂O only in the entire volume of water. 
2) A reaction is said to be in chemical equilibrium if the rate of forward reaction and rate of the reverse reaction is equal.

What is thermodynamic equilibrium?
It's very simple, it is a combination of all the three equilibriums thermal, mechanical and chemical mentioned above. Thus,
"A system will be in thermodynamic equilibrium with its surroundings when that system and its surroundings are in simultaneously thermal, mechanical and chemical equilibrium". 
Or
"If two systems are in thermal, mechanical and chemical equilibrium with each other, then that systems are in thermodynamic equilibrium with each other".
The above figure of water bottle at room temperature is the best example of thermodynamic equilibrium.
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".

Hope you have found this article helpful!!
Let me know what you think about THERMODYNAMICS EQUILIBRIUM. Feel free to comment if you have any queries.!!

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