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Enthalpy equations with pressure/volume assumptions

  1. May 28, 2012 #1
    I'm trying to learn the various equations of enthalpy with certain assumptions, but I couldn't understand where they came from. So, I tried to derive them myself, but now I'm lost beyond reason.

    I'm getting confused between the various forms of the enthalpy equation, depending on constant pressure, constant volume, or constant pressure and constant volume assumptions. And, I'm approaching this chemically, so the only work is expansion work (no non-mechanical types of work).

    The no-assumption equation is:

    ΔH = ΔU + PΔV + VΔP
    Now, via the 1st law of thermodynamics,
    ΔH = ΔQ + ΔW + PΔV + VΔP

    From this equation, I can have heat transfer in or out, work in or out, volume change, or pressure change, and then calculate change in enthalpy. Wikipedia, however, states there is a redundant term: ΔW = -PΔV. But then, this no-assumption equation can't deal with work in or out or volume change...but that can't be right...

    Have I made an implicit assumption when I say ΔW = -PΔV?
     
  2. jcsd
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