Hydrogen Bonding--chemistry

[XPFMember]

Assalamoalaikum!!

well i'm confused why b.p of H2O is greater than HF and why HF is greater than NH3

Can sumbody plz explain insimple words in terms of hydrogen bonding!!

Jazak Allah Khair!!

 

[libra94]

Walaikumusalam!
isn't the b.p. of HF more than H2O???
becuz, fluorine is the most electronegative element in the Periodic Table...so the hydrogen bond between the H and F are stronger than the hydrogen bonds in H and O (becuz oxygen is 2nd electronegative element). The b.p. of NH3 is the lowest cuz N is the least electronegative element (as compared to O and F) so the hydrogen bonds between H and N are weak therefore low boiling point.

hydrogen bonding exists between hydrogen atom and an electronegetive element, which can be either F,O or N

 

[XPFMember]

but HF has lower bp than H2O :!:

 

[libra94]

howcome??? i dont agree
where does it say that??

 

[XPFMember]

check this...http://www.chemguide.co.uk/atoms/bonding/hbond.html i hav e an exam tom. so dont have time to read the full story if u find the reason plz help Jazak Allah Khair!!

 

[DragonCub]

Well I think in this problem the intensity of H-bond is not the only factor to be concerned.
It is true that the H...F bond in hydrogen fluoride is stronger than the H...O in water, yet we ought to realise that between each two molecules in HF and H2O respectively, there are TWO H...O bonds yet ONLY ONE H...F bond! The total strength of O...H may then outweigh that of H...F. This has made the "unexpected" yet correct fact that the b.p. of HF is lower than that of H2O.

This is only my thoughts... I'm not certain of its reliability Just hope it may help you understand.

By the way, speaking the hydrogen bond, I want to make sure whether, among H2O, HF and those polar molecules, the H-bond is a kind of (permanent) dipole-dipole force. Could somebody tell me that? Thanks.

 

[soumayya]

hydrogen bonding is a form of permanent dipole-dipole force...that's wat i learnt..

 

[hassam]

Notice that each water molecule can potentially form four hydrogen bonds with surrounding water molecules. There are exactly the right numbers of hydrogens and lone pairs so that every one of them can be involved in hydrogen bonding.

This is why the boiling point of water is higher than that of ammonia or hydrogen fluoride. In the case of ammonia, the amount of hydrogen bonding is limited by the fact that each nitrogen only has one lone pair. In a group of ammonia molecules, there aren't enough lone pairs to go around to satisfy all the hydrogens.

In hydrogen fluoride, the problem is a shortage of hydrogens. In water, there are exactly the right number of each. Water could be considered as the "perfect" hydrogen bonded system.
this is a NASWER I got from another forum

 

[usman]

check this...http://www.chemguide.co.uk/atoms/bonding/hbond.html i hav e an exam tom. so dont have time to read the full story if u find the reason plz help Jazak Allah Khair!!

It is present on this page under the heading:- "Water as a "perfect" example of hydrogen bonding"

 

[MAVtKnmJ]

H2O has a stronger hydrogen bond than NH3 or HF because of it's geometry/how the hydrogen bonds are formed. In H2O each Oxygen atom is attached with two Hydrogens which is not the case with HF and NH3 which are attached to a single Hydrogen. You might think that NH3 is attached with 3 Hydrogens, but that is not the case. Since Hydrogen Bonding depends on the "Inter Molecular" bonds, the bond between NH3-NH3 molecule forms only one Hydrogen Bond.

Now HF also forms 1 Hydrogen bond, but this bond is more electronegative, thus it is more stronger than NH3.

Have a look here for more information:

http://en.wikipedia.org/wiki/Hydrogen_bond

Hope that helps