How would you describe the intermolecular forces between #H_2# molecules and #NH_3# molecules in the liquid phase?

1 Answer
anor277
Jul 24, 2017

Dispersion forces operate in liquid dihydrogen........

Explanation:

And the low magnitude of such forces result in an exceptionally low boiling point. This site quotes a normal boiling point of #-252.9# #""^@C#. On the other hand, ammonia, #NH_3#, is a heavier molecule, with more electrons, and thus should have an intrinsically higher boiling point.

And yet there is another more potent intermolecular force that operates in ammonia (as it does in water), that of hydrogen bonding. Because nitrogen is more electronegative than hydrogen, we could represent the resultant dipole as #""^(-delta)NH_3^(delta+)#. The dipoles line up, and this results in a normal boiling point of #-33.3# #""^@C#.

This boiling point is high for such a small molecule. The normal boiling point of water, #100# #""^@C#, is also anomalously high, and here hydrogen bonding also operates extensively, and elevates the boiling point.

And ammonia is thus a water like solvent, that is extensively used in inorganic chemistry. The reaction of sodium in liquid ammonia, to give an intensely blue solution, is something to see.

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