## Thermodynamic Equilibrium

### Definition :

Thermodynamic Equilibrium is defined as a state of a macroscopic system in which all average properties do not change with time.

- Thermal equilibrium:[1] Suppose you have some water at 60°C in a container. If this container is left to itself, it will gradually cool down to room temperature. This means that the container and water interact with the surroundings, and the water's temperature decreases with time. Once the system attains room (surroundings) temperature, no further change occurs. We then say that the container and water have attained thermal equilibrium with the surroundings

- Mechanical equilibrium: If within the system there are variations in pressure or elastic stress, its parts may move/expand/contract. When these movements cease, the system will be in mechanical equilibrium.

- Chemical equilibrium:[2] when a system contains substances that can react chemically. After a sufficient time, when all chemical reactions show no tendency for a chemical change to occur, the system is said to be in chemical equilibrium.

## Notes

1. https://web.mit.edu/16.unified/www/FALL/thermodynamics/

2. 4

   Demirel, Y., & Gerbaud, V. (2019). Fundamentals of Equilibrium Thermodynamics. In Nonequilibrium Thermodynamics (pp. 1–85). Elsevier.