I have a question about the illustration from chem.libretexts.org:

Figure 15.8.9. Phase Changes

the ground states of the liquid and gas are offset from that of the previous state by the heats of fusion and vaporization, respectively.

Why does the change in enthalpy increase the ground state energy?

I would be very grateful if someone can help me with this problem. I have been struggling with it for five days, but Google could not help me.

P.S. My question mark does not work.


In order to make the phase transition, you need to supply your sample molecules with an additional amount of kinetic energy (i.e., the heat of fusion or vaporization) to overcome their intermolecular bonding. This however means that after the transition, your system contains more internal energy than before. That additional energy is encoded in the vertical offset on the energy scale.

Note that if you look at each phase on its own, you could set the their ground states as the zero point on your energy scale. However, if you want to compare the system among different phases, then you need to take that offset into account. Otherwise you'd assume that your system can change from solid to liquid to gas without any effort.

  • 1
    $\begingroup$ thank you for your answer. Does this mean the ground state energy itself does not change but because we need an increase in internal energy for having the possibiliy to occupy the new energy levels, we symbolically illustrating this by an elevation of the ground state. I wish you a merry Christmas from austria. $\endgroup$ – Anna Dapont Dec 25 '19 at 9:08
  • $\begingroup$ Well, the ground state energy itself does change... that's what the shift means. If you only look at one of the phases though, then all thermodynamics are in reference to its own ground state, so its absolute value doesn't matter much. Only once you take different phases into account. - Thanks, and Merry Christmas to you too! $\endgroup$ – Antimon Dec 26 '19 at 0:28
  • $\begingroup$ Well, where could I find a formula or a calculation instruction for translational ground state energy.I would like to get a feeling how much the Eg changes with enthalpy change and on which variables it depends, respectively, and especially why the Eg changes. $\endgroup$ – Anna Dapont Dec 26 '19 at 8:30
  • $\begingroup$ The "translational" part was mostly a qualitative explanation as to what's happening inside the sample. In reality it will be more complicated than that, to the point where you probably wouldn't be able to strictly tell apart which energy level "belongs to the liquid" and which one "to the solid". The way I see it, this whole picture is more conceptual than anything else, to give you a broad idea how the phases relate to each other energetically. - Also, the ground state energy change is essentially the heat of fusion/vaporization. $\endgroup$ – Antimon Dec 26 '19 at 19:54
  • $\begingroup$ This seems to be a complex topic. Thank you for your helpful answers! $\endgroup$ – Anna Dapont Dec 28 '19 at 9:33

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