[kensher]

Hello.

I was wondering why H2O could form two hydrogen bonds per molecule, while NH3 and HF can only form one hydrogen bond per molecule. H2O has two lone pair electrons available and two H-atoms available for hydrogen bond formation; shouldn't this make it possible to form four hydrogen bonds (2 for the lone electrons + 2 for the hydrogen atoms)?.

Why can't NH3 and HF form more than one hydrogen bond per molecule?

There's obviously something here which I don't get, I just can't put my finger on it.

Source: http://www.slideshare.net/chungkin81/hydrogen-bonding (page 10)

[thetada]

What you can't fathom is the bizarre prevalence of the fallacy that water molecules can't form more than 2 hydrogen bonds!

   
"Warning:  It has been pointed out to me that some sources (including one of the UK A level Exam Boards) count the number of hydrogen bonds formed by water, say, differently. They say that water forms 2 hydrogen bonds, not 4. That is often accompanied by a diagram of ice next to this statement clearly showing 4 hydrogen bonds!" http://www.chemguide.co.uk/atoms/bonding/hbond.html

[kensher]

Thanks for your reply!

The reason I started thinking about it was that I had this one practice problem that I found really difficult:

«make a drawing which illustrates how hydrogen bonds get made between NH3 and H2O»

According to my book this is the correct answer: http://tinypic.com/r/t5myhl/9

I don't get why there is only two H2O molecules. Why not four? The NH3 has afterall one lone electron pair and three available hydrogen atoms. And why is NH3 the central atom, and not H2O? Is there anyway to figure it out from the Lewis structure?

[Burner]

I was wondering why H2O could form two hydrogen bonds per molecule, while NH3 and HF can only form one hydrogen bond per molecule. H2O has two lone pair electrons available and two H-atoms available for hydrogen bond formation; shouldn't this make it possible to form four hydrogen bonds (2 for the lone electrons + 2 for the hydrogen atoms)?.

For water, hydrogen bond is formed between an -OH 'group'(I can't think of other expressions to say this) AND a lone pair of electrons from another H₂O molecules.

In each water molecule, there are 2 'OH group' and two lone pair of electrons, or 2x('OH group + lone pair of electrons), so on average, each water molecules can form two hydrogen bonds with each other.

Consider this diagram, you may think that a water molecule form 4 hydrogen bonds with other molecules. However, strictly speaking, not all the hydrogen bonds are 'form' the same water molecule, they actually come from other H₂O molecules.

Perhaps my wordings are not so specific and clear, but this is how I understand this problem.

Edit:
1. Resized image
2. The second time I opened your second link I can't see the picture. Can you post it again, using the 'attach' function in the 'Additional Options' under the posting area?

[thetada]

It wouldn't necessarily be incorrect to show a different proportion of ammonia and water molecules. The important point is what the diagrams are used to show. What is different about the role of each water molecule in the diagram you linked?

[kensher]

Thanks for the replies!

Here. I have attached the photo.

[Burner]

Here. I have attached the photo.

I don't get why there is only two H2O molecules. Why not four? The NH3 has afterall one lone electron pair and three available hydrogen atoms. And why is NH3 the central atom, and not H2O? Is there anyway to figure it out from the Lewis structure?

Well the number of molecules drawn and the position of each molecule is not so important as long as you show the requried key point. This diagram is sufficient to illustrate the formation of hydrogen bond between the molecules. Drawing 30 water molecules and 15 NH₃ molecules would make no difference except wasting your time.

[kensher]

Thanks for the reply. So could I have drawn two NH3 molecules attach to one H2O particle and still be correct?

[Burner]

«make a drawing which illustrates how hydrogen bonds get made between NH3 and H2O»

Thanks for the reply. So could I have drawn two NH3 molecules attach to one H2O particle and still be correct?

I think it's ok in this case.