Hydrogen bond vs dipole-dipole?

  1. I saw a video on youtube saying water molecules were held together by hydrogen bonding, not dipole dipole. Why is that?
     
  2. jcsd
  3. mfb

    Staff: Mentor

    Hydrogen bonds are special (stronger) version of dipole-dipole interactions.
     
  4. I have heard this statement before but I have no idea what it means. I think I know what dipole-dipole interactions are but I'm not sure about their relation to hydrogen bonds and conversely. Could you please elaborate?
     
  5. mfb

    Staff: Mentor

    Hydrogen bonds are bonds where one side of molecule A has a positively charged region (hydrogen, usually connected to oxygen) and one side of molecule B has a negatively charged region (usually oxygen). As both molecules are uncharged (otherwise, it would be an ionic bond), they have to be dipoles.
     
  6. But two H atoms or two H2 molecules undergo dipole-dipole dispersive interactions without having permanent dipoles. Likewise for two orientation-averaged H2O molecules. What's the magnitude of a typical hydrogen bond? Can it be measured in any or all phases of H2O?
     
  7. hydrogen bonds are in the 0.5 eV range.

    for comparison, covalent bonds are in the single eVs while van der Waals forces are in the 0.1 eV range.

    Of course in chemistry we more often talk about kJ/mol but those are hard numbers to compare when visualizing single bonds. eV are easy to compare to other things since we know the ionization energy of H atoms in eV.

    You might notice, hey, hydrogen bonds and van der Waals forces don't seem all that different. However in the regime of low energies, even a tiny difference is huge.
     
  8. mfb

    Staff: Mentor

    Van der Waals

    Their orientations are correlated, which gives an effective attraction even if the average orientation of each atom is random.

    Ice is like many frozen hydrogen bonds, and the high melting/boiling point is a result of hydrogen bonds in ice and water. As gas, interactions between atoms are weak as always.
     
  9. DrDu

    DrDu 4,249
    Science Advisor

    Although I don't remember the percentages, in hydrogen bonds there is also a considerable transfer of charge between the bonded molecules and other effects which can't be subsumed under dipole-dipole interaction.
     
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