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Different experiments show that replacing atoms with isotopes can change intermolecular forces. For example, heavy water has a higher melting point than normal water, and D2 has a higher boiling point than H2. But the trend is not always "the heavier the higher", as C13-methanol and ethanol, have a "lower" boiling point than C12 ones.

I've already read this topic, in which it is mentioned that vibrational energy is the cause of it. heavier atoms vibrate slower, so they have a higher melting point. (which again, the methanol case can't be explained by it).

My argument is that the "temperature" is defined as the average kinetic energy due to vibrations. So when D2O and H2O reach 0. they both have the same average vibrational energy, regardless of how much energy they hold. But with the same kinetic energy, intermolecular bonds in H2O are broken, and in D2O are not. Can someone explain why it happens? or persuade me that D2O and H20 can have the same temperature with different vibrations?

Noora
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    Why should they cause the same melting/boiling points? – Poutnik Mar 27 '24 at 21:10
  • You change something, so it has to be different. It should rather be "how" than "why". – Mithoron Mar 27 '24 at 21:20
  • The different masses directly indicate that the vibrations will be different (consider the classic mass on a spring from freshman physics). In any event, by changing how the different parts move relative to each other you make the interactions slightly different, meaning they respond to adding energy differently. – Jon Custer Mar 28 '24 at 12:19
  • The argument about temperature and vibrations ignores big quantization steps for bond vibration energies, that spoil the simple proportionality rule of classical thermodynamics. – Poutnik Mar 28 '24 at 16:58
  • @Mithoron I have read this post. My question is, how the vibration is different at the same temperature? temperature is basically an average of vibrations. – Noora Mar 28 '24 at 16:59
  • @Poutnik can you explain more? also, here we are talking about intermolecular forces, not intramolecular bonds. – Noora Mar 28 '24 at 17:01
  • Vibrations are not limited to intramolecular forces. If they were, there would be no liquids nor solids - only gases.(excluding giant molecules like diamond structure or some polymers). – Poutnik Mar 28 '24 at 17:03
  • Quick search C-13 methanol has slightly higher BP slightly lower MP. Different sources? Temperature is not vibrations it is relative molecular velocities resulting in average KE. Vibrational states in molecules are populated by temperature. Kinetic isotope effects result from changing of vibrational states in the transition state making zeropoint energy difference between the isotopes important. The effect on total energy affecting MP and BP is much more subtle. The differences are profound for H2, D2, H2O, D2O, He[3] and He[4], less so, but real, for others needing careful analysis. – jimchmst Mar 28 '24 at 22:41
  • You’re confusing ‘vibrations’ with ‘translations’. Temperature is more related to the movement of molecules in space (translation), as opposed to the movement of atoms within a molecule (vibration). – isolated matrix Mar 29 '24 at 15:25

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