When
a material changes phase heat is either liberated or must be supplied.
The amount of heat associated with the transformation of unit mass of the
material is the Heat of Transformation,
l12
= Q/m.
In
general, there will also be a volume change associated with this process
and so work is done against the ambient pressure: w = p(v2
- v1). From
the First Law, the change in the specific internal energy of the system
is given by:
du
= (u2 - u1)
= l21 - p(v2
- v1).
The
phase transition process occurs under conditions of constant temperature
and pressure, and hence the enthalpy change is: dh = du + pdv. Substituting
into the expression for the heat of transformation gives:
l21
= (h2 - h1)p
= (Tds)p.
The entropy change associated with the phase transition is therefore: ds
= (l21
/ T)p . |