Boiling points and hydrogen bondings

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Discussion Overview

The discussion revolves around the boiling points of hydrogen halides (HF, HCl, HBr, and HI) and the role of hydrogen bonding in determining these properties. Participants explore the factors influencing boiling points, including intermolecular forces, electronegativity, and molecular size.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant notes that HF has the highest boiling point due to stronger hydrogen bonding compared to other hydrogen halides, but expresses confusion over the expected order of boiling points.
  • Another participant questions whether hydrogen bonds occur in all hydrogen halides and seeks clarification on the requirements for hydrogen bond formation.
  • A later reply emphasizes the importance of polarizability and dipole moments in understanding boiling point trends, suggesting that these factors complicate the explanation.
  • One participant asserts that only highly electronegative atoms (N, O, F) can form hydrogen bonds, which may clarify the earlier confusion.
  • Another participant highlights the complexity of the question, mentioning various factors such as dipole moments, electronegativity, bond lengths, and the influence of atomic structure on van der Waals forces.

Areas of Agreement / Disagreement

Participants express differing views on the factors influencing boiling points and the role of hydrogen bonding, indicating that multiple competing perspectives remain. The discussion does not reach a consensus on the explanation of boiling point trends.

Contextual Notes

Participants acknowledge the complexity of the topic, noting that various factors interplay in determining boiling points, and that there are exceptions to general trends in chemistry.

kasse
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Homework Statement



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


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?
 
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do H-bonds really occur in all the hydrogen halides? what are the requirements for an H-bond to be formed?
 
kasse said:

Homework Statement



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


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?

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.
 
Kushal said:
what are the requirements for an H-bond to be formed?

No idea. My book only says that H-bonds occur between molecules when a H-atom bonds with an electronegative atom.
 
it should've been said 'a VERY elctronegative atom'. only N, O and F can form H-bonds. does this help you?!
 
Yes, thanks.
 
i was more expecting you to explain the trends in boiling point using this information than saying thanks.

:)
 
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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