States and State Properties

The system of our choice will usually prevail in a certain macroscopic state, which is not under the influence of external forces. In equilibrium, the state can be characterized by state properties such as pressure (P) and temperature (T), which are called intensive properties. Equally, the state can be characterized by extensive properties such as volume (V), internal energy (U), enthalpy (H), entropy (S), Gibbs energy (G), and Helmholtz energy (A). 

These properties are called extensive because they relate to the amount of mass considered once related to a unit amount of mass, they also become intensive properties. 

The equilibrium state does not change with time, but it may change with location as in a flowing system where P, T, and other state properties can gradually change with position. Then we speak of a steady state. If the state temporarily changes with time, as in the startup of a plant, we call it a transient state. 

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TABLES OF THERMODYNAMIC DATA 7.5a Reference States and State Properties... 

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