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# A New Thermodynamic State Function: Enthalpy

For an simple pure fluid, consider the physical meaning of the term alone;

Question: What are the units of ? What are the units of ?

Imagine that there is a completely homogeneous system that has been arbitrarilry divided into small volumes of size :

Consider what is left of the internal energy after we subtract off the mechanical'' or compression'' energy:

 (11-12)

The reason that the integration can be carried out is that both and are extensive variables and are thus additive for each subsystem.

So that we can define a new state function (that is also extensive),

 (11-13)

It is sensible to interpret as the thermal energy'' at constant pressure. In other words, we divide the internal energy for a simple fluid into two parts--one part corresponding to that stored as compressive energy ( ) and another part the thermal energy . Another way to see this is:

 (11-14)

In other words

 (11-15)

for an ideal gas:

 (11-16)

Next: Another State Function Up: Lecture_11_web Previous: Internal Energy of an
W. Craig Carter 2002-09-28