# HPP's for Phase Changes

In the previous lesson, we used

#### hypothetical process paths

to make it easier to determine the change in the properties of an ideal gas, liquids and solids for which is constant, and real substances for which we have thermodynamic tables.
BUT only in one phase !
PHASE CHANGE
Initial State
Final State
In this lesson, we will learn how to use

#### hypothetical process paths

to determine the change in properties associated with a

#### phase change

.
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### Ch 3, Lesson E, Page 1 - HPP's for Phase Changes

• In the last lesson, we learned how to calculate the change in internal energy and enthalpy for a process involving almost any pure substance.
• We always assumed that the substance remained in the same single phase.
• For example, if the liquid in our system boils during a process, we cannot use the techniques discussed in the previous lesson to calculate the changes in U and H.
• Why not ?
• Well, to begin with, we know that sat’d vapor and sat’d liquid have different volumes, right ?
• Different properties means that sat’d liq’d and sat’d vapor are different STATES.
• What do you have to do to a system to cause the phase to change from liquid to vapor at constant pressure ?
• Well, I am sure you know that an energy input is required to boil a liquid.
• If you put energy into the system, then you change its internal energy and enthalpy.
• We can safely conclude that the internal energy of a sat’d vapor is higher than the internal energy of the sat’d liquid at the same temperature and pressure.
• Enthalpy, volume and most other intensive properties are also different.
• We will consider boiling and condensing, melting and fusion and sublimation and de-sublimation, but as far as our HPP goes, all phase changes are handled in pretty much the same way.
• So, we will focus on boiling and condensation because they are central to this course.
• This lesson is about how we can use HPP’s to include the affects of phase changes when we are analyzing a process.