Why do molecules with dipole-dipole force have higher melting and boiling points than molecules that only exhibit London Dispersion force?

Because dipole - dipole force is stronger than london dispersion force

Explanation:

Intermolecular forces determines a wide range of physical properties of matter of which melting point is one of them.

Both london dispersion forces and dipole- dipole forces are vanderwaals force.

But diople - dipole forces exists between polar molecules which are molecules with permanent dipoles.

In london dispersion forces the dipole is temporary

describe matter a earth

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .