The boiling point of Phosphine, PH3 (-88 degrees C), is lower than that of ammonia, NH3 (-33 degrees C), even though PH3 has twice the molar mass of NH3. Why?
a. PH3 is polar while NH3 is nonpolar.
b. NH3 is polar while PH3 in nonpolar.
c. NH3 has hydrogen bonds while PH3 has dipole-dipole interactions.
d. PH3 has hydrogen bonds while NH3 has dipole-dipole interactions.
e. There are a number of possible expanations; more information is needed.
The correct answer and explanation is :
The correct answer is:
c. NH3 has hydrogen bonds while PH3 has dipole-dipole interactions.
Explanation:
The boiling point of a substance depends on the strength of intermolecular forces (IMFs) present between its molecules. Stronger IMFs lead to a higher boiling point because more energy is required to separate the molecules into the gas phase.
Intermolecular Forces in NH₃ and PH₃:
- Ammonia (NH₃)
- NH₃ is a polar molecule due to the significant difference in electronegativity between nitrogen (3.0) and hydrogen (2.1).
- The nitrogen atom has a lone pair, leading to a trigonal pyramidal molecular shape.
- NH₃ molecules can form hydrogen bonds because nitrogen is highly electronegative and can form strong dipole interactions with hydrogen atoms on adjacent molecules.
- Hydrogen bonding is the strongest type of dipole-dipole interaction, leading to a significantly higher boiling point (-33°C).
- Phosphine (PH₃)
- PH₃ is also a polar molecule, but phosphorus is much less electronegative (2.1) than nitrogen (3.0).
- Although PH₃ has a similar trigonal pyramidal shape, it does not form hydrogen bonds because phosphorus is not electronegative enough to create a strong dipole moment with hydrogen.
- Instead, PH₃ molecules experience only weaker dipole-dipole interactions, which are not as strong as hydrogen bonding.
- As a result, PH₃ has a much lower boiling point (-88°C).
Why Molar Mass is Not the Dominant Factor:
Although PH₃ has twice the molar mass of NH₃, which would typically suggest stronger London dispersion forces and a higher boiling point, the presence of hydrogen bonding in NH₃ dominates and leads to a much higher boiling point. This explains why NH₃ has a significantly higher boiling point than PH₃ despite the larger molecular weight of phosphine.