Thermodynamic State:

A thermodynamic state is defined as the condition of a system at a specific point in time. These condition of a system at a specific point is determined by variables called state variables.

Temperature, pressure, volume, internal energy, enthalpy, entropy are some of the common Thermodynamic state variables.

In other words, the thermodynamic state is essentially a snapshot of the system’s characteristics, such as temperature, pressure, volume, and other relevant properties, which together describe its energy, phase, and behavior.

The state of a system is fully described when all its state properties are specified.

In a graphical representation, these states are often depicted as points on a PV, PT, or TS diagram.

Types of Thermodynamic states:

Thermodynamic states are of two types:

Equilibrium State.
Non-Equilibrium State.

Equilibrium State :

Thermodynamic equilibrium is a condition where all the macroscopic properties of a system do not change with time.

The following are the types of thermodynamic Equilibrium.

Thermal Equilibrium:

In Thermal equilibrium, the temperature is uniform through out the system. In other words, a system is said to be in thermal equilibrium when two systems or parts of a system are at the same temperature, and there is no net transfer of heat between them.

In case If two objects are in thermal contact, heat will flow from the hotter object to the cooler one until they reach the same temperature, at which point thermal equilibrium is achieved.

Mechanical Equilibrium:

A system is said to be in Mechanical equilibrium when the system’s pressure is uniform throughout and there are no unbalanced forces within the system or between the system and its surroundings.

For example, if a gas in a container is not moving and there is no net force acting on the system (such as gravity or pressure differences), then it is in mechanical equilibrium since there is no unbalanced force, and the system is not undergoing any acceleration.

Chemical Equilibrium :

A system is said to be in chemical equilibrium if there is no net chemical reactions are occurring within the system or between the system and its surroundings.

This equilibrium occurs in a system where the rates of the forward and reverse chemical reactions are equal, resulting in no net change in the concentrations of reactants and products over time.

For example, in a reversible reaction such as:

A+B⇌C+D

chemical equilibrium is achieved when the rate of formation of C and D equals the rate of formation of A and B.

Conditions for Thermodynamic Equilibrium:

A system is in thermodynamic equilibrium when all three conditions (thermal, mechanical, and chemical) are satisfied simultaneously.Specifically, the system must also meet the following conditions:

Reversibility of Processes: At equilibrium, any process or reaction should be reversible. For instance, if the system is disturbed slightly, it should be able to return to its equilibrium state without irreversible changes (e.g., without energy dissipation like friction or unbalanced chemical reactions).

No Net Energy Flow: There should be no net heat, work, or mass transfer into or out of the system. The system’s energy content must be stable.

Uniformity of Properties: The macroscopic properties such as temperature, pressure, and chemical potential must be spatially uniform across the system. If these properties are not uniform, the system will undergo changes until equilibrium is restored.

Apart from the above listed equilibrium, there are other equilibrium types such as

Stable Equilibrium(System returns to equilibrium).
Unstable Equilibrium(System moves further from equilibrium).
Neutral Equilibrium(System remains at new position i.e.. neither stable nor unstable)

Non-Equilibrium State:

A state where there are gradients of temperature, pressure, or other properties, leading to changes in the system over time. A system in a non-equilibrium state may undergo processes until it reaches equilibrium.

Equation of State :

An equation of state is a mathematical equation that describes the relationship between state variables.

The most common example is the Ideal Gas Law:

PV=nRT

where, P: Pressure.

V: Volume.

n: Number of moles of gas.

R: Universal gas constant (8.31446261815324 J⋅K⁻¹⋅mol⁻¹)

T: Absolute temperature (Kelvin).

Other more complex equations of state account for non-ideal behaviors of real gases and liquids, such as the Van der Waals equation, Redlich-Kwong Equation , and Peng-Robinson Equation.

Importance of Equation of State and Equilibrium:

Understanding the state and equilibrium of a system is crucial for analyzing thermodynamic processes. By knowing the initial and final states of a system, one can determine the work done, heat transferred, and changes in internal energy, enthalpy, and entropy. This understanding is essential for the design and optimization of engines, refrigerators, heat pumps, and other thermodynamic systems.

Thermodynamic Equilibrium and Equation of States

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