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Do water molecules go from bent to linear on vaporization?

I heard that the bonding angle in water changes when that molecule goes from a liquid to a gas. I was told that this is one of the reasons that a water is able to escape the other water molecules. If the bond angle changed from aproximately 109 to 180 degrees then the molecule would not be polar any more and hydrogen bonding wouldn't exist. Previously, I thought that it was simply a matter of how much kinetic energy the molecule absorbed so that the more energy absorbed the more vibration, et cetera until it got enough energy to break away. Is there any truth to this 180 degree angle statement? If so, where can I find some information on this type of phenomena. I am a high school teacher who majored in chemistry so I do have some background but I also need you to keep it simple.
Lara O'Brady

Lara, I'm skeptical about explanations for melting and boiling in terms of specific molecular features (like dipole moments), particularly if the explanation doesn't include the term "free energy". Talking about phase transitions without mentioning thermodynamics is like writing a symphony without being able to read music. Maybe it can be done, but it's a serious handicap.

There are many other problems with this explanation, including:

  • As temperature rises, vibrational excursions from the equilibrium bond angle get larger. But the time-averaged bond angle is not going to be too far from the equilibrium bond angle. A stable 180° bond angle isn't achieved at higher temperature.
  • Opening the bond angle raises the energy of the molecule, because it pushes the bonding and nonbonding electron regions closer together, increasing electron-electron repulsion. Opening the bond angle from 104.5° to 180° requires a tremendous amount of energy, and if this much energy were required for water vaporization the boiling point would be much higher than what is actually observed.

    Oliver Smart has posted a typical bond angle vs. molecular potential energy curve as part of an online course on protein structure. He makes a quick estimate of the size of bond angle deviations from their equilibrium values for a particular temperature, using the equipartition theorem. While the estimate isn't terribly accurate, it's easy to see that typical bond angles aren't going open up during vibrations much more at 373 K than they do at 273 K.

  • The explanation implies that the permanent dipole moment of water should decrease rapidly with increasing temperature, and that water molecules in steam would have a dipole moment of zero. This isn't observed experimentally.
  • The premise that hydrogen bonding wouldn't exist if the bond angle was opened is incorrect. The OH bonds would still be individually polar, and the molecule would still be able to form hydrogen bonds.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Do water molecules go from bent to linear on vaporization?

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/liquids/faq/bond-angle-and-vaporization.shtml