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whys has H20 got a higher boil point that NH3

why has h20 got a higher boiling point than nh3 if nh3 has 3 hydrogen bonds and h20 has 2?

It's to do with the lone pairs of electrons per molecule. H2O has two lone pairs so can form two hydrogen bonds between H2O molecules whereas NH3 can only form one hydrogen bond between NH3 molecules. Because of this the intermolecular forces are higher in H2O so take more energy to break

Reply 2

upthegunners

Original post by Mr Zurkon

I didn't think lone pairs had anything to do with hydrogen bonds? :s-smilie:

Reply 3

Mr Zurkon

Lone pairs on the oxygen atom in H2O allow it to make hydrogen bonds with other H2O molecules, same with the lone pairs on the nitrogen and NH3 molecules

Reply 4

Booyah

Original post by upthegunners

I didn't think lone pairs had anything to do with hydrogen bonds? :s-smilie:

Lone pairs cause the extreme electronegativity that allow for Hydrogen bonds to delta +ve molecules.

Reply 5

FutureMedic1009

Original post by upthegunners

why has h20 got a higher boiling point than nh3 if nh3 has 3 hydrogen bonds and h20 has 2?

Nitrogen is less electronegative than Oxygen,thus meaning that O forms stronger hydrogen bonds than Nitrogen.Thus meaning that O requires a higher amount of energy to break up the hydrogen bonds(boiling point).

Posted from TSR Mobile

Reply 6

shengoc

Original post by upthegunners

why has h20 got a higher boiling point than nh3 if nh3 has 3 hydrogen bonds and h20 has 2?

Hydrogen bonds are intermolecular forces, also a type of chemical bonding. A chemical bond is nothing more but forces of attraction (the same in ionic, covalent, metallic, intermolecular forces)

Change of states of simple molecular structures/matter involves breaking the relatively weak intermolecular forces of attraction rather than the covalent bonds.

Oxygen being more electronegative leads to the compound formed with hydrogen, that is water, H2O having a more polar O-H bond, hence the intermolecular forces between H2O molecules are relatively stronger than that in NH3, where N-H has smaller difference in electronegativity compared with O-H bonds (so less polar)

Reply 7

Kyri

It's not simply the polarity of the O-H bond compared to the polarity of the N-H bond otherwise HF would have a higher boiling point than water because H-F has the largest difference in electronegativity. It's simply the number of hydrogen bonds which can be formed per molecule. Water has two lone pairs and 2 hydrogens so can form 4 per molecule. With only one lone pair, NH3 will have two hydrogens not contributing to the hydrogen bonding so overall forms just 2 hydrogen bonds per molecule. So water has a much stronger hydrogen bond network.

Reply 8

ps1265A

Original post by Kyri

That makes sense, but what about HF? It can only form 1 hydrogen bond yet it still has a higher boiling point that NH3.

Reply 9

Kyri

Original post by ps1265A

That makes sense, but what about HF? It can only form 1 hydrogen bond yet it still has a higher boiling point that NH3.

HF forms 2 hydrogen bonds per molecule, the same number as ammonia. One with the H and one with the F. The higher boiling point than ammonia comes from the higher electronegativity of F compared to N leading to stronger hydrogen bonds between molecules.