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Bonding of HCL and HF

  1. Aug 2, 2007 #1
    1. The problem statement, all variables and given/known data
    Explain why hydrogen fluoride (HF) has a higher boiling temperature than hydrogen chloride (HCL) even though HF has a lower molecular weight.


    Attempt
    OK so from the looks of it I think that the bonding that bonds the H to the Cl and the H to the F is polar covalent. So why would it have the higher boiling point if it has a lower molecular weight?? I mean it must have something to do with bonding because a higher boiling point means that more thermal energy is required to separate the molecules.

    Any help would be greatly appreciated.

    Thanks
     
  2. jcsd
  3. Aug 2, 2007 #2

    chemisttree

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    The same rationale can be used to explain the difference in boiling point between water (MW = 18) and methane (MW = 16). Why the big difference in these two compounds of almost identical molecular weight?

    I think you were on to it when you wrote, "I mean it must have something to do with bonding...".
     
  4. Aug 2, 2007 #3
    Well I've been doing some reading and I think I might have it, but I'm not entirely sure. From the looks of it there are the intramolecular bonding forces and the intermolecular bonding forces. So the intramolecular forces are polar covalent because they are responsible for the bonding of the H to the Cl and the H to the F. However, the bonding forces between the various HCl molecules (intermolecular) are weak van der Waals forces. Then for HF we have hydrogen bonding which is stronger. Is this on the right track?
     
  5. Aug 2, 2007 #4
    Doesn't F have a way higher electronegativity than Cl, implying that the van der Waals forces of attraction would be significantly greater?
     
  6. Aug 2, 2007 #5

    chemisttree

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    Absolutely on the right track.
     
  7. Aug 2, 2007 #6
    ahh cool. :)

    Thanks!
     
  8. Aug 2, 2007 #7

    chemisttree

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    This statement isn't quite right. There is significant hydrogen bonding in HCl.
     
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