why does warer boil at a higher temp then nh3 when they are both dipole dipole?
Oxygen has a higher electronegativity than N; therefore, the water dipole is stronger than the NH3 dipole. O is about 3.5 and N about 3.0.
To continue or add on to DrBob's explanation, hydrogen bonding due to O-H groups is stronger than hydrogen bonding involving N-H groups. That is probably related to the electronegativity differences mentioned by DrBob.Hydrogen bonding is the main intermolecular force in both H2O and NH3. The higher boiling point of water is due to stronger hydrogen bonding in water.
Water (H2O) boils at a higher temperature than ammonia (NH3) because of the differences in their molecular structures and the intermolecular forces present between their molecules. Although both water and ammonia are polar molecules with dipole-dipole interactions, there are several factors that contribute to the difference in their boiling points.
Firstly, water molecules have two hydrogen atoms bonded to a central oxygen atom, resulting in a bent V-shaped molecular geometry. On the other hand, ammonia molecules have three hydrogen atoms bonded to a central nitrogen atom, resulting in a trigonal pyramidal molecular geometry. This difference in molecular shape affects the strength and arrangement of intermolecular forces.
In water, hydrogen bonding occurs between the partially positive hydrogen atoms of one molecule and the partially negative oxygen atom of a neighboring molecule. Hydrogen bonds are relatively strong intermolecular forces that require a significant amount of energy to break, leading to higher boiling points. The presence of hydrogen bonding in water adds an additional attractive force between its molecules, contributing to its higher boiling point.
In contrast, ammonia molecules also exhibit dipole-dipole interactions between the partially positive hydrogen atoms and the partially negative nitrogen atom. However, the strength of these intermolecular forces is weaker than hydrogen bonding. While ammonia molecules can form hydrogen bonds to a lesser extent due to the presence of lone pairs on the nitrogen atom, it is not as strong as the hydrogen bonding in water. As a result, ammonia has a lower boiling point compared to water.
So, in summary, the difference in boiling points between water and ammonia can be attributed to the stronger intermolecular forces (hydrogen bonding) present in water compared to the relatively weaker dipole-dipole interactions in ammonia.