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Bond Polarity vs Bond Length

  1. Feb 13, 2012 #1
    I have some confusion about the relationship between bond polarity and bond length.

    Generally strong bonds are short in length.
    Generally strong bonds are more polar. (Ionic bonds are stronger than covalent bonds).

    Therefore, the more polar bonds tend to be shorter in length?

    And yet, the dipole moment is the product of charge and distance, which implies that the longer the distance, the more the polarity?

    I thank you for your time to resolve my confusion.

    BiP
     
  2. jcsd
  3. Feb 13, 2012 #2
    This is true.

    Are you sure about that? There is overlap, but generally, covalent bonds are stronger.
     
  4. Feb 13, 2012 #3
    My guess is that because ionic bonds are more polar, the electrostatic attractions are stronger. I see no reason why covalent bonds should be stronger than ionic bonds. I suppose it probably differs from compound to compound.

    But let's forget I said anything about ionic compounds.
    Within covalent compounds, does polarity lead to bond strength?
     
    Last edited: Feb 13, 2012
  5. Feb 13, 2012 #4
    I don't think so. There are many interrelated factors, especially regarding acids-

    http://library.thinkquest.org/C006669/data/Chem/acidsbases/factors.html

    1. As polarity of X-H bonds increases, acidity increases, so as polarity goes up, bond stability goes down.

    2. As anion size increases, acidity increases. This may seem contrary to point #1, but that didn't take conjugate base stability into account.

    So, based on #2, you could say that increased polarity leads to increased bond strength, but that's not looking at the big picture. That said, X-H bonds are some of the most polar bonds and acids are defined by the weakness of those bonds.

    http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html (I'm particularly looking at the table in blue)

    The 5 strongest bonds, from strongest to weakest are- N*N (triple bond), C*C, C=C, H-F, and O=O. This tells a much different story and one that I would say is generally more applicable.

    If this post seems kind of sloppy, it's late and I've been throwing stuff in and editing this post as I've been reading more and more.
     
  6. Feb 14, 2012 #5
    YEs! That's the post I've been looking for! I never thought about it by considering what determines the acid dissociation constants of most acids. In my textbook too it says that higher polarity contributes to more acidity (thus more instability and less bond strength) and that higher bond strength contributes to less acidity (by virtue of its tight grip on its hydrogens).

    In fact, I sort of thought of looking at it a different way:
    Bond polarity is given by the net electric dipole which for a diatomic covalent compound is given by μ = qr.
    Bond strength is simply the Coloumb force so it is F = k(q1)(q2)/(r^2).

    IF you increase r, you will increase both μ and decrease F. But the decrease in F is more significant than the increase in μ because of the exponent of 'r' in the Coloumb law.
    Therefore, though the H-X bond sort of becomes more polar, it becomes significantly weaker in terms of electrostatic strength, hence the greater acidity!!!
    Thus both the effects that contribute to acidity are countering one another, but the bond strength (effect number 2) is more significant, so its effect is recognized.

    I thank you greatly for your time! I get it NOW!!!!

    With regards,
    BiP
     
  7. Feb 14, 2012 #6
    Glad I could help!
     
  8. Feb 14, 2012 #7
    Ionic bonds are based on electrostatic attraction (differences in electronegativity). Covalent bonds are usually stronger because electrons are shared between atoms.

    Ionic bonds>polar covalent bonds>nonpolar covalent bonds when it comes to the differences in electronegativity of atoms but this isn't necessarily correlated with bond length. Triple bonds< double bonds< single bonds in length but dipole moments (polarity) are determined by the difference in electronegativity of the atoms.
     
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