What is thermodynamic equilibrium?

A thermodynamic system is said to have equilibrium when there is no change in mechanical, chemical and thermal properties.

When only temperature and pressure are under no change then it is called metastable equilibrium.

For a system under thermodynamic equilibrium with given energy, the entropy is greater than that of any other state with the same energy. For a thermodynamic equilibrium state with given pressure and temperature, the Gibbs free energy is smaller than that of any other state with the same pressure and temperature

For sustaining thermodynamic equilibrium main three types of equilibrium must be achieved:

1. Mechanical equilibrium: There should not be any disbalance in mechanical properties within system. Under mechanical equilibrium pressure should be same at all points of system and also should not change with time.
2. Thermal equilibrium: System under thermal equilibrium has same temperature all over system and does not changes with respect to time. There should not be any heat exchange between system and surrounding.
3. Chemical equilibrium: In chemical equilibrium composition should be same all over system and no chemical reactions should take place in system with respect to time.