HPP's for Phase Changes

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.