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Heat of Transformation
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) = l12
- 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: l12 = (h2
- h1)p = (Tds)p. The entropy
change associated with the phase transition is therefore: ds = (l12
/ T)p . For Water at One Atmosphere:
Heat
of Melting (fusion) = 333 kJ/kg; Heat of vaporization (condensation)
= 2260 kJ/kg
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