posted by andrew on .
Between HF and H2O, which one has the greater intermolecular force?
i know they both exhibit hydrogen-bonding, but which one is greater?
H and F are easily pulled apart (into ions) in solution, but H2O is not. That might provide a clue.
My take on this is that HF ionizing more than H2O is due to the reaction of
HF + H2O ==> H3O%+ + F^- versus
H2O + H2O ==> H3O^+ + OH^- as well as the strength of the HF versus the OH bonds. The HF bond distance is 91.7 pm and that of the HO bond is 95.8 which means that the HO in water is the weaker bond of the two. We would expect that since the electronegativity of F is much greater than that of O. But the H---F-H hydrogen bond for HF is 155 kJ/mol versus the H----O-H hydrogen bond in water is 21 kJ/mol. That tells me that the H bond for HF is stronger than the H bond in H2O. Again, that is to be expected because of the difference in EN between F and O. Other opinions are welcome. Here is the site from which I picked the H bonding numbers quoted above (155 vs 21).
I agree that the H-F bond is stronger. I was too lazy to do the bond strength numbers, and came up with some diofferent ones. I also looked at the bond strength for separating HF into H+ and and F- in the gas phase. The heat of that reaction
HF -> H+ + F-(from JANAF tables) is
-Hf (HF) + Hf(H+) + Hf(F-) = -65.1 +(365.2) + (-61.1) = 239 kcal/mol
[Hf denotes heat of formation at 0 K.]
For HF -> H + F, I get
18.4 + 51.6 -(-65.1) = 135 kcal/mol
For H2O -> H + OH, I get
51.6 + 9.3 - (-57.8) = 118.7 kcal/mole,
a strength (bond energy) that is not greatly different.
The bond strengths quoted in the Wikipedia reference cited by Dr Bob seem to be for hydrogen bonds in certain larger molecules, which is not the same thing as what I have calculated.
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