What Is Thermodynamic Equilibrium?

Regardless of whether the system is completely isolated from the environment, if the properties in the system do not change with time, the system is in a state of thermodynamic equilibrium, and this equilibrium is macroscopically static. At the microscopic level, each molecule is constantly moving, so it is also called dynamic equilibrium or thermodynamic equilibrium.

The thermodynamic equilibrium state refers to a thermodynamic state in which the system can be maintained for a long time without changing under the condition of no external influence. In other words, under the condition of not being affected by the outside world, the state of the macroscopic thermal properties of the system does not change with time. "Freedom from external influences" refers to the absence of heat transfer and work exchange with the outside world. The thermal system in equilibrium should have uniform temperature, pressure and other parameters everywhere.

If there is a potential difference in the internal force of the thermal system, macroscopic displacements will occur within the thermal system. At this time, the system will not maintain the same state, but will continue to produce state changes until the force and potential difference is eliminated and a balance is established. This balance Called force balance. Therefore, the force difference is the unbalanced potential difference that drives the state of the system, and the disappearance of the force potential difference is a necessary and sufficient condition for the system to establish a force balance.

If there is a temperature difference between the various parts of the thermal system, heat transfer will occur between different parts of the system, and the state of the system will continue to change until the temperature difference disappears and reaches an equilibrium. This equilibrium is called thermal equilibrium. Therefore, the temperature difference is the imbalance potential that drives the heat transfer, and the disappearance of the temperature difference is a necessary and sufficient condition for the system to establish thermal equilibrium. Similarly, for a system with a phase change or chemical reaction, because the phase change or chemical reaction is driven by an unbalanced chemical potential difference, the disappearance of the chemical potential difference is a necessary and sufficient condition for establishing a systematic chemical or physical chemical equilibrium. In summary, the disappearance of various imbalance potentials within and between the system and the outside world is a necessary and sufficient condition for the system to achieve a state of thermal equilibrium.

Thermodynamic equilibrium is one of the most basic and important concepts in classical thermodynamic theory. It is the basis for describing and analyzing the state and thermal process of a system. What the basic theory of thermodynamics describes is actually the equilibrium characteristic of the system. In principle, the states mentioned in classical thermodynamics refer to thermodynamic equilibrium states; the thermodynamic processes refer to processes composed of a series of equilibrium states. The thermodynamic state parameters used to describe and analyze states and processes in thermodynamics are also defined only in equilibrium. ^[2]

Due to some external action, when the system is in an unbalanced state, there is an imbalance potential difference between the parts. At this time, as long as there is no constraint, there is no need for external action. It changes until it reaches equilibrium again. Therefore, if the system is originally in an equilibrium state, its state will not change if there is no external action; but if the system is in an unbalanced state, its state will change even if there is no external action. . All systems will spontaneously approach equilibrium, which is a universal law in nature.

The course of change of the system state is necessarily: the original balance is broken-an imbalance-a new balance is established. As the system is in a thermodynamic equilibrium state, all imbalance potential differences between the parts have disappeared and no interaction will occur, so it is no longer possible for the state to change itself. Once the system has reached an equilibrium state, its state change can only depend on external effects.

Because the external effect on the system (work or heat transfer) always occurs on the boundary, and then gradually expands to the interior of the system. As long as the interaction between the system and the outside world is carried out at a certain speed under the action of a finite potential difference, the result of the propagation of external effects will definitely cause a certain imbalance potential difference between the various parts of the system. Only after the external effects are over, as time goes by, the various parts reach a new balance through interaction.

Thermal balance is

Thermal equilibrium: The temperature of the system and the environment is equal and constant. The sides of the insulation wall can vary.

Force balance: the various forces of the system and the environment are equal and unchanged. Rigid walls can vary from side to side.

Phase Equilibrium: Phase changes reach equilibrium, and the composition and quantity of each phase do not change with time.

Chemical Equilibrium: The chemical reaction reaches equilibrium, and the quantity and composition of each reaction substance remain unchanged.