PNG 520
Phase Relations in Reservoir Engineering

Functions of State or State Functions


A function of state is one in which the differential change is determined only by the end states and not by intervening states. Most thermodynamic variables are state functions and hence property changes are determined by the end states and not by the process path. Notable exceptions are work and heat.

The most common thermodynamic state functions with which we shall deal include

  • Internal Energy (U),
  • Entropy (S),
  • Enthalpy (H),
  • Helmotz Energy (A), and
  • Gibbs Energy (G).