Skip to main content

Thermodynamic equilibrium, state and conditions

Short answer

Thermodynamic equilibrium requires no unbalanced potentials: mechanical equilibrium (uniform pressure, no motion), thermal equilibrium (uniform temperature, no heat flow), and chemical equilibrium (no reactions or diffusion). Only then are properties single-valued.

Define thermodynamic equilibrium and thermodynamic state. Also explain conditions required for a thermodynamic equilibrium.

Thermodynamic state

The state of a system is its condition at an instant, described completely by the values of its thermodynamic properties (P, V, T, etc.). When all properties have fixed values, the system is in a definite state. For a simple compressible pure substance, two independent intensive properties fix the state.

Thermodynamic equilibrium

A system is in thermodynamic equilibrium when its properties do not change with time and there is no unbalanced potential (force, temperature or chemical difference) inside it or between it and the surroundings. No spontaneous change occurs if it is isolated.

Conditions for thermodynamic equilibrium

A system must satisfy all three together:

  1. Mechanical equilibrium: no unbalanced force, so pressure is uniform and there is no bulk motion within the system (and across the boundary).
  2. Thermal equilibrium: no temperature difference, so temperature is uniform and there is no heat flow.
  3. Chemical equilibrium: no chemical reaction or diffusion of species; chemical composition does not change with time.

If any one is not satisfied, the system is not in thermodynamic equilibrium. Properties of a system can be defined as single values for the whole system only when it is in equilibrium.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Fundamental of Thermodynamics & Heat Transfer chapter 1 questions →