Boiling points and hydrogen bondings

[kasse]

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?

 

[Kushal]

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

 

[GCT]

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.

 

[kasse]

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.

 

[Kushal]

it should've been said 'a VERY elctronegative atom'. only N, O and F can form H-bonds. does this help you?!

 

[kasse]

Yes, thanks.

 

[Kushal]

i was more expecting you to explain the trends in boiling point using this information than saying thanks.

:)

 

[Fearless]

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.