The state of a system in which properties have definite, unchanged values as long as external conditions are unchanged is called an equilibrium state. Thermodynamic Equilibrium takes place when the same temperature exists throughout the system. The states of dynamic equilibrium are dealt by Classical thermodynamics. At thermodynamic equilibrium, the state of the system is determined by the intensive parameter values. These parameters are pressure, temperature, etc. Thermalization is the process which leads to a thermodynamic equilibrium
The word equilibrium means a state of balance. In an equilibrium state, there are no unbalanced potentials (or driving forces) with the system. A system that is in equilibrium experiences no changes when it is isolated from its surroundings.
A system in thermodynamic equilibrium satisfies:
1. Mechanical equilibrium (no unbalanced forces): Two systems are said to be in thermal equilibrium if their temperatures are same.
2. Thermal equilibrium (no temperature differences): Two systems are are said to be in mechanical equilibrium if their pressures are same.
3. Chemical equilibrium: Chemical equilibrium is the state in which the chemical activities or concentrations of the reactants and products have no net change over time. Two systems are are said to be in diffusive equilibrium if their chemical potentials are same.
Thermal equilibrium is when temperature remains same and mechanical equilibrium is when pressure remains same.