1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why is the enthalpy of a phase transition different from 0?

  1. Jun 1, 2014 #1
    Wikipedia states: Phase changes, such as melting or evaporation, are also isothermal processes.
    I am interested in calculating the enthalpy of a given phase transition.

    If the process is isothermal, I would immediately say that H is 0, according to the following equation:


    But I know that this is not true, why?
  2. jcsd
  3. Jun 1, 2014 #2


    User Avatar
    Science Advisor

    ##(\frac{\partial H}{\partial T})_p = C_p## is only true at constant pressure. During a phase transition neither the volume nor pressure of a substance need be constant.
  4. Jun 1, 2014 #3
    The equation you wrote applies to only a single phase at constant pressure. If you want to extend it to calculate the change in enthalpy for a phase transition, you need to include latent heat of melting or evaporation. During the phase transition at constant pressure, the temperature remains constant until the phase transition is complete. One way of integrating the above equation over the phase transition is to express the heat capacity in terms of the Dirac delta function δ(T), assuming you are familiar with this function. This gives:
    where L is the latent heat of vaporization, and Tv is the heat of vaporization. In this way, the heat capacity at the transition is Lδ(T-Tv).

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook