BOILING POINT QUESTION

[kfcman289]

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Can someone explain to me why h20 has a higher boiling point than hf?
someone told me it was because h2o has more hydrogen bonds, but if that is true, why doesnt ammonia have a higher boiling point than water?

 

[Oso]

I think it has to do with the ratio of lone pairs to hydrogens for hydrogen bonding in each substance. Water has two hydrogens and two lone pairs, so each water molecule can form 4 hydrogen bonds, so you end up having very extensive hydrogen bonding within water molecules. HF has a bunch of lone pairs, but only one available H per molecule, so there are less hydrogen bonds being formed between HF molecules. And NH3 has 3 H's, but only one lone pair (kinda the opposite of HF), so again, less hydrogen bonds happening between NH3 molecules. Water has 'the perfect' amount, allowing it to maximize the number of hydrogen bonds.

 

[1TB4RKSB4CK]

Think of lone pairs and H-bonding.

 

[Cmdr_Shepard]

Also water is smaller and able to pack closer together and form more efficient bonds.

But yeah, also the above stuff about H-bonds. Usually H-bonds dominate in comparisons of boiling points between substances, but if two substances have the same number of H-bonds, then the bulkier one has weaker bonds, and the heavier one has a higher BP.