During the phase transition, the temperature of the system remains con-
stant: phase transitions are isothermalprocesses. Only when all of one phase
has completely changed to another phase will the heat act to change the tem-
perature of the system. Because each chemical component requires a charac-
teristic amount of heat for a fusion (or melting), vaporization, or sublimation
process, we can define heats of fusion,fusH, heats of vaporization,vapH, and
heats of sublimation,subH, for pure compounds. Since these processes usu-
ally occur under conditions of constant pressure, these “heats”are in fact en-
thalpies of fusion, vaporization, or sublimation. Many of these changes are ac-
companied by a change in volume, which can be large for transitions involving
a gas phase.
Enthalpies of phase transitions are formally defined for the endothermic
process. Hence they are all positive numbers. But, since each process above
occurs under the same conditions except for the direction of heat flow, these
enthalpies of phase transition also apply to phase transitions in the opposite
direction. That is, the heat of fusion is used for the freezing process as well as
the melting process. A heat of vaporization can be used for a vaporization or
the reverse condensation process, and so on. For the exothermic processes, the
negative of the enthalpy is used, as Hess’s law requires us to negate the enthalpy
change when we consider the reverse process.
For a phase transition, the amount of heat absorbed or given off is given by
the well-known expression
qmtransH (6.5)
where mis the mass of the component in the system. We are using the “trans”
label to stand for any phase transition: fusion, vaporization, or sublimation.
Typically, it is the problem solver’s responsibility to understand the inherent
direction of heat flow, that is, exothermic or endothermic, and use the appro-
priate sign on transH.
In terms of moles, equation 6.5 is written as
qntransH
The units on the enthalpy of phase transition are typically kJ/mol or kJ/g.
A short table of enthalpies of phase transition is given in Table 6.2. Note
their units listed in the footnote, and be sure to express the amounts of the
components in the appropriate units when working problems.
We must remember that phase transitions themselves are inherently
isothermal.Furthermore, we have already established that at the melting point
or the boiling point of a substance,
phase1phase2
This implies that for a system where the amount of material is constant and
both phases exist in equilibrium,
transG 0 (6.6)
This is applicable only to the isothermal phase transition. If the temperature
changes from the normal melting or boiling point of the substance, equation
6.6 does not apply. For example, for the isothermal phase transition
H 2 O (, 100°C) →H 2 O (g, 100°C)
the Gvalue is zero. However, for the nonisothermal process
H 2 O (,99°C) →H 2 O (g, 101°C)146 CHAPTER 6 Equilibria in Single-Component Systems