why do ammonia and water have such different bps?

[ilovemycat]

so the boiling point of water is 100 degrees Celsius, while boiling point of ammonia is minus 33 degrees Celsius, which makes 133 degrees difference. Now when we discuss value of boiling point, we also say that it depends on inter-molecular forces, and in case of both water and ammonia I can see two such forces: London dispersion forces and hydrogen bonds. Relative molecular mass of water is 18 amu, and relative molecular mass of ammonia is 17 amu, so unlikely we can explain the difference in boiling points by difference in London dispersion forces: as they depend on mass, the LDF should be about the same for two compounds. Speaking about hydrogen bonds, it looks as each molecule (water or ammonia) is able to form 4 hydrogen bonds. And in this way amount of hydrogen bonds again should be about the same. So what is an explanation for the difference in boiling point between water and ammonia? im really confused

[sweetiepi]

so the boiling point of water is 100 degrees Celsius, while boiling point of ammonia is minus 33 degrees Celsius, which makes 133 degrees difference. Now when we discuss value of boiling point, we also say that it depends on inter-molecular forces, and in case of both water and ammonia I can see two such forces: London dispersion forces and hydrogen bonds. Relative molecular mass of water is 18 amu, and relative molecular mass of ammonia is 17 amu, so unlikely we can explain the difference in boiling points by difference in London dispersion forces: as they depend on mass, the LDF should be about the same for two compounds. Speaking about hydrogen bonds, it looks as each molecule (water or ammonia) is able to form 4 hydrogen bonds. And in this way amount of hydrogen bonds again should be about the same. So what is an explanation for the difference in boiling point between water and ammonia? im really confused

Hey there!
Nitrogen (in the ammonia molecule \(\ce{NH3}\) ) has a lesser affinity to hydrogen bond in comparison to oxygen (from the water molecule \(\ce{H2O}\) ). This is due to oxygen being more electronegative than nitrogen.
Therefore the water takes more energy (heat) to break the hydrogen bonds, whereas the nitrogen-hydroden bonds take less energy (or heat) to break, thus \(\ce{NH3}\) has a lower boiling point.

Hope this helps.

(Disclaimer: This is coming from a combo of what I know from the VCE and uni )

[ilovemycat]

thanks for the reply but if thats the case, why is HF 's boiling point even lower at -88? it is the most electronegative element

[sweetiepi]

thanks for the reply but if thats the case, why is HF 's boiling point even lower at -88? it is the most electronegative element

\(\ce{H2O}\) can form more hydrogen bonds than HF. \(\ce{H2O}\) can have an equal number of H-bond donors (HBD) and H-bond acceptors (HBA). HF on the other hand only has 1 HBA and 3 HBDs. This leaves the HF molecule to have less stability, as there is an imbalance. Furthermore, the electrons on the fluorine atom are more weakly hydrogen bond donating, as it is stable, which could possibly mean that it's H-bonds are infact weaker than \(\ce{H2O}\).

(Apologies is this is a tad confusing, this actually stretched my brain a little and I'm not 100% sure if I'm correct here )