[habbababba]

I've read many sources on the web stating that the main reason for the higher boiling point of water than the boiling points of ammonia and hydrogen fluoride is due to the higher number of hydrogen bonds that water molecules can make with each other in comparison to the number of hydrogen bonds NH₃ and HF molecules can make. But it seems to me that for the same number of water and ammonia molecules, both can make equal number of hydrogen bonds (4 at max).

Could anyone explain how water can form more hydrogen bonds than ammonia?

[Corribus]

Quality matters as much as quantity. The hydrogen bond enthalpy can vary substantially; that of OH---O is about 21 kJ/mol and that between NH---N is about 13 kJ/mol.

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

[mikasaur]

I think also that H₂O has two hydrogen bond acceptors (the two H atoms) and two hydrogen bond donors (the two unpaired e^- on O). Because they're paired like this you don't have a mismatch of pairs acceptors and donors like you do with HF (too many donors) and NH₃ (too many acceptors).

[habbababba]

I think also that H₂O has two hydrogen bond acceptors (the two H atoms) and two hydrogen bond donors (the two unpaired e^- on O). Because they're paired like this you don't have a mismatch of pairs acceptors and donors like you do with HF (too many donors) and NH₃ (too many acceptors).

But regardless of how many bond acceptors and donors a molecule has got, it can eventually make the same maximum number of hydrogen bonds provided that there are enough molecules around it. Although N has one lone pair, it can form 1 H bond using its lone pair and 3 other H bonds using the 3 hydrogen atoms for a total of 4 H bonds.
Water has 2 lone pairs that can make 2 H bonds and 2 H atoms that can form 2 other H bonds, for a total of also 4. Am I wrong?

[mjc123]

But can every molecule form one donor bond and 3 acceptor bonds? Think about it.

[habbababba]

But can every molecule form one donor bond and 3 acceptor bonds? Think about it.

That's what I've been trying to find out. So I also wonder can every water molecule make 2 donor bonds and 2 acceptor bonds? And the only way that this can happen is if the H-O--H angle is not equal to 180°.

If I may put my reasoning in other terms: a water molecule requires 4 other water molecules to make the maximum number (4) of H bonds. An ammonia molecule requires 4 other ammonia molecules to make the maximum number (4) of H bonds.

[mjc123]

A mole of water contains 2N lone pairs and 2N hydrogens. In theory these could all pair up (probably in reality that never quite happens) to give 2N hydrogen bonds, an average of 4 per molecule (4 rather than 2 because each bond is shared between two molecules).
A mole of ammonia contains N lone pairs and 3N hydrogens. You can't get more than N hydrogen bonds out of that, an average of two per molecule. An individual molecule might have four, but that means others must have less.

[habbababba]

A mole of water contains 2N lone pairs and 2N hydrogens. In theory these could all pair up (probably in reality that never quite happens) to give 2N hydrogen bonds, an average of 4 per molecule (4 rather than 2 because each bond is shared between two molecules).
A mole of ammonia contains N lone pairs and 3N hydrogens. You can't get more than N hydrogen bonds out of that, an average of two per molecule. An individual molecule might have four, but that means others must have less.

I get it now. Thank you very much.