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Number of hydrogen bonds possible - why?

I'm revising for my end-of-year exams and I came across this question:

Explain the difference in boiling points of water (373K), ammonia (245K) and hydrogen fluoride (295K).
As answer, our teacher gave us this:
1. HF can form only 1 hydrogen bond per molecule as it has only 1 hydrogen atom.
2. H2O can form 2 hydrogen bonds per molecule as it has 2 hydrogen atoms and 2 lone pairs of electrons.
3. NH3 can only form 1 hydrogen bond per molecule as it has only 1 lone pair of electrons.
Hence water has the highest boiling point.

I understand that the more hydrogen bonds there is, the more the boiling will be. But what about the hydrogen bonds? Does the number of hydrogen bonds that a molecule can form depend on the number of hydrogen atoms or the number of lone pair of electrons contained in that molecule?

P.S. HF has 3 lone pairs of e and 1 H, H2O has 2 lone pairs of e and 2 H, NH3 has 1 lone pair of e and 3 H!
Reply 1
Avatar for oxymoron
oxymoron
7
NotScientific
I'm revising for my end-of-year exams and I came across this question:

Explain the difference in boiling points of water (373K), ammonia (245K) and hydrogen fluoride (295K).
As answer, our teacher gave us this:
1. HF can form only 1 hydrogen bond per molecule as it has only 1 hydrogen atom.
2. H2O can form 2 hydrogen bonds per molecule as it has 2 hydrogen atoms and 2 lone pairs of electrons.
3. NH3 can only form 1 hydrogen bond per molecule as it has only 1 lone pair of electrons.
Hence water has the highest boiling point.

I understand that the more hydrogen bonds there is, the more the boiling will be. But what about the hydrogen bonds? Does the number of hydrogen bonds that a molecule can form depend on the number of hydrogen atoms or the number of lone pair of electrons contained in that molecule?

P.S. HF has 3 lone pairs of e and 1 H, H2O has 2 lone pairs of e and 2 H, NH3 has 1 lone pair of e and 3 H!


In order to form a hydrogen bond you need a lone pair AND a hydrogen. So if a molecule has 2 of each, it can form two H-bonds, but if it has one of EITHER, then it can only form 1 H-bond.
Reply 2
Avatar for Da Mouse
Da Mouse
3
I thought it was also to do with something about having a high electronegativity. F, N2 and O2 have a high electronegativity so the F/N/O would attract the electrons more towards its side so there would be a higher elsectron density onto one side wihch would expose the proton in Hydrogen so a greater dipole is created. This would make the hydrogen bond stronger. Plus, Oxygen is more elctronegative than Nitrogen. Hope that makes sense lol ( We both have mauritian flags - yay! :wink: :cool: :p: lol)
Reply 3
Avatar for snmichaels
snmichaels
1
Careful, N, O, and F are highly electronegative, not O2 and N2. The elements, not the molecules.
Reply 4
Avatar for samd
samd
10
Our teacher told us to remember Hydrogen Bonding as:

Hydrogen FONding, after F, O, and N.

Not the best way to remember it but it certainly works.
Reply 5
Avatar for Da Mouse
Da Mouse
3
whoopsy - yeah, F,O,N not N2 and O2 thanx for correcting me.
Reply 6
Avatar for snmichaels
snmichaels
1
:biggrin: :wink:

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