Short Answer

Thermodynamic equilibrium is a condition in which a system’s properties like temperature, pressure, and chemical composition remain constant with time. In this state, there is no net flow of energy or matter within the system or between the system and its surroundings.

A system in thermodynamic equilibrium has mechanical, thermal, and chemical equilibrium at the same time. This means there is no temperature difference, no pressure difference, and no chemical reaction happening spontaneously inside the system. Everything remains balanced and stable.

Detailed Explanation :

Thermodynamic Equilibrium

Thermodynamic equilibrium is a stable condition of a system in which all measurable properties remain constant with time, and there is no tendency for any spontaneous change. It is one of the most important concepts in thermodynamics because it tells us when a system has reached complete balance. In this condition, energy is evenly distributed, and no natural process starts on its own.

For a system to be in thermodynamic equilibrium, three types of equilibrium must exist at the same time:

1. Thermal equilibrium – no temperature difference within the system.
2. Mechanical equilibrium – no pressure difference inside or outside.
3. Chemical equilibrium – no chemical reaction or change in composition.

When all these conditions are satisfied, the system becomes perfectly stable, and no further changes occur unless an external force disturbs it.

Conditions for Thermodynamic Equilibrium

To understand thermodynamic equilibrium deeply, we must examine the three essential types of equilibrium:

1. Thermal Equilibrium

A system is in thermal equilibrium when its temperature is uniform throughout.
If different parts of the system have different temperatures, heat will flow from the hotter part to the cooler part. When temperature becomes equal everywhere, thermal equilibrium is reached.

Example:
A cup of hot tea cools down when left in a room. When the temperature of tea and air become equal, thermal equilibrium is achieved.

2. Mechanical Equilibrium

A system is in mechanical equilibrium when pressure is the same throughout.
If one part has higher pressure, particles will move, or the system will change shape until the pressure becomes balanced.

Example:
A gas in a piston reaches mechanical equilibrium when the force inside the piston equals the force outside.

3. Chemical Equilibrium

A system is in chemical equilibrium when no chemical reaction is happening spontaneously.
Reactants and products remain in balance, and their concentrations stay constant.

Example:
Inside a closed container of water and steam, both phases remain in balance when chemical equilibrium is reached.

Features of Thermodynamic Equilibrium

Thermodynamic equilibrium has several important features:

1. No Net Energy or Matter Flow

Since temperature and pressure are equal everywhere, no heat or matter moves from one place to another.

2. Properties Remain Constant with Time

Quantities such as temperature, pressure, volume, and chemical composition remain unchanged unless the system is disturbed.

3. System is Completely Stable

A small disturbance does not change the state of the system unless it is strong enough to push it into a new equilibrium.

4. Equilibrium State is a Minimum Energy State

Systems naturally try to reach a state where their total energy is minimum.
Thermodynamic equilibrium represents this minimum-energy condition.

Types of Systems That Reach Thermodynamic Equilibrium

Different systems reach equilibrium in different ways:

1. Isolated System

No exchange of heat, work, or matter occurs with surroundings.
These systems naturally move towards equilibrium.
Example: A closed thermos bottle containing hot water.

2. Closed System

Heat and work can be exchanged, but matter cannot.
If heat exchange continues until temperatures equalize, equilibrium is reached.

3. Open System

Heat, work, and matter can be exchanged.
These systems usually do not remain in equilibrium for long unless controlled.

Examples of Thermodynamic Equilibrium

1. Ice–Water Mixture in an Insulated Container

At 0°C, both ice and water coexist without changing amounts.
Temperature and pressure remain constant, so the system is in equilibrium.

2. Gas in a Closed Container

If temperature and pressure become uniform throughout the container, the gas is in mechanical and thermal equilibrium.

3. Human Body is Not in Thermodynamic Equilibrium

Humans constantly exchange heat and matter with surroundings, so they are never in thermodynamic equilibrium.

Importance of Thermodynamic Equilibrium

Thermodynamic equilibrium is crucial in many scientific fields:

1. Foundation of Thermodynamic Laws

The concept helps define thermodynamic properties like pressure, temperature, and entropy.

2. Industrial Processes

Many chemical industries rely on equilibrium principles to control reactions efficiently.

3. Predicting Natural Processes

Thermodynamic equilibrium explains why heat flows, why gases expand, and why chemical reactions stop.

4. Heat Engines and Refrigerators

These machines work by disturbing and restoring equilibrium in controlled ways.

Conclusion

Thermodynamic equilibrium is a state where a system has no temperature, pressure, or chemical differences within it, and all its properties remain constant with time. It is the combined result of thermal, mechanical, and chemical equilibrium. In this state, the system is completely stable, with no natural processes occurring inside it. Understanding thermodynamic equilibrium helps explain energy transfer, reaction balance, and the working of machines and natural systems.