Water, the life giver.

Discussion in 'Chemistry' started by DRZion, Sep 4, 2009.

  1. Nasor Valued Senior Member

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    It's somewhat hard to describe this with text. But if water were an sp3 hybrid, the two non-bonding lone pairs would be equal in energy. That would lead us to expect two peaks in water's XPS spectrum; one from the two equivalent hydrogen 1s-oxygen sp3 sigma bonds, and one from the two equivalent oxygen sp3 lone pairs. In fact, the XPS spectrum shows three peaks; one for two equivalent hydrogen 1s-oxygen 2p sigma bonds, one from a non-bonding oxygen 3p orbital (one of the lone pairs) and one from the non-bonding oxygen 2s orbital (the other lone pair) that's almost 5 eV lower in energy than the peak from the sigma bonding orbitals. This indicates that the 2s orbital on the oxygen is not actually hybridized with the 2p orbitals.

    The fact that you get approximately the correct geometry for the molecule if you consider it an sp3 hybrid is just a coincidence.

    Edit: If you want to really discuss the electronic structure and bond nature in water you would need to discuss its molecular orbital diagram, but the MO diagram just moves it even further from being an sp3 hybrid.
     
    Last edited: Sep 18, 2009
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  3. DRZion Theoretical Experimentalist Valued Senior Member

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    The laws of physical chemistry yes, but you get very different chemistry at different temperatures and pressures. For instance liquid water can exist above 100 degrees - up to 370 degrees. This is the case in hot springs under the oceans known as black smokers.

    http://blogs.myspace.com/index.cfm?fuseaction=blog.view&friendId=9539926&blogId=421723847

    Indeed, water has among the highest heat capacities of all known substances. It is largely due to hydrogen bonds.

    Yes it is.

    My biochemistry textbook started off with an entire section on water and the forces in it.

    The expected configuration would be 120 degrees as would be present in a tetrahedral sp3 orbital - something like a triangular pyramid. Only two points of this pyramid are occupied by atoms in H20 - by the hydrogens, of course.

    http://andromeda.rutgers.edu/~huskey/images/methane_td.jpg

    The other two points still exist, but they are composed only of oxygen electrons and act as non-bonding lone pairs. It is these lone pairs that interact with the H - O sp3 bond, compressing the H-O-H angle from 120 to 105 degrees. It is also the negative electron orbitals that attract the slightly positive hydrogen nuclei from other H20 molecules, wa la, hydrogen bonding.
     
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  5. Bishadi Banned Banned

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    now we talking in reality.................. thanks!

    which is one way of observing the mass (Water) is not the 'carrier' of heat; the associated environment is hugenormous to observe (all cases)

    yes and no.........as the environment is what is so important to what that structure can maintain


    how are electrons 'compressing' anything?

    this is where the 'heat' and its causally observed environment divert from mechanics

    you are standing on it, but for some reason not seeing it!


    'heat' has nothing to do with electrons, but the environment does!
     
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  7. Nasor Valued Senior Member

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    If water were an sp3 hybrid (which it isn't), the ideal bond angle would be 109.5 degrees. People usually explain the deviation from what's predicted by an sp3 geometry by saying that the lone pairs are repelling each other slightly and so squeezing the oxygen-proton bonds closer together. That all seems to make sense, but it isn't true - in fact, the ideal bond angle for water based on its electronic structure is 90 degree. But the protons can't get that close together, because it would put them well inside each others bohr radius. Rather than the usual explanation of lone pair-lone pair repulsion moving the ideal bond angle from 109.5 degrees to 104.5, it's actually proton-proton repulsion that moves the ideal bond angle from 90 to 104.5.

    H2S, which has the exact same electronic structure as H2O, has a bond angle of 90 degree (because the sulfur atom is large enough for the protons to be at their preferred 90 degree angle without bumping into each other).
     
    Last edited: Sep 20, 2009
  8. DRZion Theoretical Experimentalist Valued Senior Member

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    You are at least partially right. Indeed the ideal bond angle would be 109.5 and not 120 , but it becomes 104.5 due to the two non-bonding electron groups. That is at least the standard description in most textbooks. I do not see why the ideal bond angle would be 90 degrees.
    A deceptively thorough page about water:
    http://www.chem1.com/acad/sci/aboutwater.html
     
  9. Nasor Valued Senior Member

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    Unfortunately most low-level chemistry textbooks are wrong about this. More advanced inorganic chemistry textbooks usually have correct discussions of water's hybridization and electronic structure, but such books are drowned out in the vast sea of highschool and college freshman-level textbooks that inaccurately claim that water is an sp3 hybrid.

    The fact that you can use sp3 hybridization and VESPR theory to correctly predict the approximate geometry of a water molecule is merely a coincidence.

    The bonding in water can be most accurately described (at least, most accurately described without resorting to an MO diagram) as a sigma bond that forms between the hydrogen's 1s orbital and a 2p orbital on the oxygen. Since the p orbitals are all 90 degrees from each other, that would put the protons 90 degrees from each other. The oxygen then has one lone pair in the third non-bonding p orbital, and another lone pair at much lower energy in the non-bonding 2s orbital. As I said before, this is easy to confirm with xps spectroscopy. The xps spectrum of water shows that the oxygen's 2s orbital is not hybridizing with the 2p orbitals.
     

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