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I am given to understand that the energy is stored in the molecule in the form of changes to bond angles. With water, the bond angle between the hydrogen atoms changes by a degree or so during melting.

Here, the bond angle is described as well as the dipole moment of torque on being rotated by an electromagnetic wave. Fun fact: Apparently the moment is calculated using the Shrodinger equation. Whatever that is:

hyperphysics.phy-astr.gsu.edu/hbase/molecule/rotrig.html

But the problem is that the rotation of the molecule is described as being not so different than typical oscillation which shows up as sensible heat.

So what's the deal? Is the rapid rotation generating enough centrifugal force to deform the bond angle? I have a hard time with this. It seems more likely that the energy of heat is absorbed directly by the molecule's bonds, thus changing the angles and resulting in no sensible heat or oscillation.

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  • $\begingroup$ @Poutnik your comment is a good answer. So why not post it? $\endgroup$ – Bob D Sep 9 '19 at 18:42
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Majority of latent heat, both for melting and evaporation, is due releasing of intermolecular bonding.

It is either van der Waals-like force for non polar molecules, or dipole-dipole bonding, donor-acceptor electron pair bonding, or hydrogen bonds of highly polar molecules.

E.g. It is estimated for water, if there had not been hydrogen bonds, it would have melted at -160 Deg C and boiled at -120 Deg C.


About bond angles:

  • 109.5 ideal tetraedr

  • 104.5 water ( due stronger repulsion of 2 oxygen free electron pairs, compared to O-H bonds )

  • 106.6 ice ( stronger intermolecular bonding brings angles closer to the tetraedr )


Influence of intermolecular bonding strength on melting and boiling point of polar molecules is straightforward:

Molecules to partially ( melting ) or fully ( evaporating ) free themselves from intermolecular bonding need to overcome a potential energy hole.

They need sufficient kinetic energy for that which rises with temperature.

Due large scale energy unification in solids, the melting is sharp transition. ( There are always exceptions allowing sublimation, that is more temperature dependent than evaporation ).

As there is already molecule energy distribution in liquids, evaporation occurs at nearly any temperature, growing about exponentially. As there is always a molecule fraction with enough kinetic energy to leave the liquid.

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  • $\begingroup$ Thanks for reminding me about the intermolecular aspect. That makes sense, but there's still the bond angles that I'm interested in. I'm wondering how far down this little rabbit hole I'm going to have to go to scratch my itch. LOL $\endgroup$ – Tom Russell Sep 10 '19 at 1:14
  • $\begingroup$ I'd love to see some elaboration on the high polarity of water being responsible for its relatively high melting and boiling temperatures. There's something fascinating in that but I'm not immediately grokking it. $\endgroup$ – Tom Russell Sep 10 '19 at 1:23
  • $\begingroup$ @TomRussell What is "grokking"? $\endgroup$ – Bob D Sep 10 '19 at 1:37
  • $\begingroup$ @Bob D grokking: To understand thoroughly and intuitively / definition from Dictionary.com: dictionary.com/browse/grokking $\endgroup$ – Poutnik Sep 10 '19 at 4:40
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Latent heat is the energy required to move molecules away from a surface: solid to liquid or liquid to vapor. The forces involved are discussed by Mr. Russell. Only the faster moving molecules can break away; leaving the slower ones behind. The material cools. In the reverse process, molecules are accelerated while approaching the surface and come in with extra energy.

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