The boiling point of phosphine, PH3(-88C) is lower than that of ammonia, NH3(-33C) even though phosphine has twice th molar mass of NH3. Why?
Because of hydrogen bonding in NH3. Hydrogen bonding occurs in molecules containing N, O, and F atoms with associated H atoms. (N has EN of 3.0, P is about 2.0, F is 4.0 and O is 3.5.)
To understand why the boiling point of a substance is lower despite having a higher molar mass, we need to consider the intermolecular forces present in the substances.
Boiling point is a measure of the strength of intermolecular forces. The higher the boiling point, the stronger the forces holding the molecules together. The two most significant intermolecular forces to consider in this case are London dispersion forces and hydrogen bonding.
Ammonia (NH3) can form hydrogen bonds due to the presence of a highly electronegative nitrogen atom and the ability to donate a hydrogen atom to another electronegative atom. Hydrogen bonding is a relatively strong intermolecular force, leading to a higher boiling point.
On the other hand, phosphine (PH3) does not form hydrogen bonds. It only exhibits London dispersion forces, which are weaker intermolecular forces resulting from temporary fluctuations in electron distribution. Although phosphine has a higher molar mass than ammonia, the absence of hydrogen bonding makes its intermolecular forces weaker, resulting in a lower boiling point.
In summary, the lower boiling point of phosphine compared to ammonia, despite having a higher molar mass, is due to the absence of hydrogen bonding in phosphine and the presence of hydrogen bonding in ammonia.
The boiling point of a substance depends on various factors, including intermolecular forces and molecular mass. In the case of ammonia (NH3) and phosphine (PH3), although the molar mass of phosphine is twice that of ammonia, phosphine has a lower boiling point.
The reason for this difference lies in the intermolecular forces present in each compound. Ammonia molecules are held together by hydrogen bonding, which is a strong intermolecular force. Hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (in this case, nitrogen), creating a positive hydrogen atom that attracts the lone pairs of electrons on neighboring molecules. The presence of hydrogen bonding in ammonia makes it more difficult for the molecules to escape from the liquid phase and into the gas phase, resulting in a higher boiling point.
On the other hand, phosphine molecules do not experience hydrogen bonding. Phosphorus is less electronegative compared to nitrogen, and therefore, the positive hydrogen atom in phosphine does not create a strong enough attraction for neighboring molecules. Consequently, phosphine molecules are held together primarily by weaker van der Waals forces, which are easier to overcome, resulting in a lower boiling point.
So, even though phosphine has a greater molar mass, the absence of strong intermolecular forces such as hydrogen bonding leads to a lower boiling point compared to ammonia.