Whenever we are asked a question about boiling point, the first thing we have to take into consideration is intermolecular forces. Intermolecular forces are the forces of attraction between molecules that hold them together and are therefore directly related to the boiling points of substances. The stronger the intermolecular forces are, the harder it is to separate the molecules from one another and the higher the melting and boiling points are.
Hydrogen bonding is one of the stronger intermolecular forces. These forces are present in any molecules that have NH, FH, or OH. Therefore, when looking at the two formulas, NH3 and PH3, we can notice that ammonia possesses Hydrogen bonding while phosphine does now. Phosphine has dipole-dipole forces since it is a polar molecule. Because Hydrogen bonding is stronger, NH3 has stronger intermolecular forces and therefore a higher boiling point.
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