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Homework Help: Boiling points and hydrogen bondings

  1. May 15, 2008 #1
    1. The problem statement, all variables and given/known data

    Explain the boiling points of HF, HCl, HBr nd HI


    3. The attempt at a solution

    The boiling points are:

    HF: 20 C
    HCl: -85 C
    HBr: -67 C
    HI: -35 C

    The hydrogen bondings between H and F are stronger than other hydrogen bondings. Therefore HF has the highest boiling point. But then I'd expect the order to be HCl > HBr > HI; not the opposite. How can that be explained?
     
  2. jcsd
  3. May 15, 2008 #2
    do H-bonds really occur in all the hydrogen halides? what are the requirements for an H-bond to be formed?
     
  4. May 15, 2008 #3

    GCT

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    Homework Helper

    This is somewhat of a trick question you really need to observe the relations between the intermolecular and intramolecular-what's especially important here is the polarizability with additional regard to dipole moments due to electronegativity- attractions in this case.
     
  5. May 15, 2008 #4
    No idea. My book only says that H-bonds occur between molecules when a H-atom bonds with an electronegative atom.
     
  6. May 15, 2008 #5
    it should've been said 'a VERY elctronegative atom'. only N, O and F can form H-bonds.


    does this help you?!
     
  7. May 16, 2008 #6
    Yes, thanks.
     
  8. May 16, 2008 #7
    i was more expecting you to explain the trends in boiling point using this information than saying thanks.

    :)
     
  9. May 18, 2008 #8
    This is actually a very hard trick question. I know I had some problem with it in the first chem-course at my university. But, it is a very multifaceted question. And also a rather tricky one, because many factors play their part in a complex thing like boiling points. Dipole-momentum, electronegativity, bond-lengths and such. But one thing that often is overlooked and why this is a hard question is about the quantum number, the amount of shells and electrons in the atom.

    When a atom has a lot of electrons, they also increase the the amounts of van-der-waalforces in the molecule. And if the amount goes up for a given timeperiod, the strength of the bond also goes up.

    Which means that downward in the halide-group, you always get stronger bonds (van-der-waals) due to larger molecules. This is something you can generalize on the atomic table and periods. But remember, that it is a lot of exceptions to this rule. Which is why chemistry is a bit tricky.
     
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