Question

Which main intermolecular force must be overcome to sublime solid NH3?

Answer 1

Sublimation is the phase transition of a substance directly from the solid to the gaseous phase without passing through the liquid phase.

Here are the three forces that exist between two NH3 molecules:

1- Dispersion forces - these forces exist between ANY two molecules, regardless of anything else.

2- Dipole-dipole interactions - NH3 is a polar molecule with a permanent separation of charge. Each molecule behaves  like a tiny magnet and has + and - poles so 2 NH3 molecules attract each other. Dipole-dipole interactions are stronger than dispersion forces in small molecules.  

3- Hydrogen bonding - H is directly bonded to the extremely electro negative atoms N, O, or F, hence hydrogen atom is nearly stripped off electrons due to the large electro - negativity difference. Hydrogen being a small atom further reduces in size even allowing it to approach other molecules very closely. Here N,O,F gathers more partial negative charge.

Due to the close approach between the molecules and the strong partial charges, hydrogen bonding is generally more important than dipole-dipole interactions.

So the main intermolecular force must be hydrogen bonding.