The boiling ponit of phophine is -85 oC, whilst of ammonia in the same group is -33.5 oC, how can we account for this difference in BP?
since PH3 has a higher molar mass, it would be expected that it would have a higher BP than NH3, but that's not the case because of hydrogen bonding in NH3 and not in PH3. it occurs when H+ is attracted to lone pairs of atoms like N, O, F in another molecules.
other molecules that exhibit H-bonding are H2O, HF, R-OH, etc.
The difference in boiling points between phosphine (-85 oC) and ammonia (-33.5 oC) can be accounted for by examining the chemical and physical properties of these substances.
1. Molecular Size and Shape: Phosphine (PH3) and ammonia (NH3) molecules have different sizes and shapes. Phosphine has larger and more complex molecules compared to ammonia, which may contribute to stronger intermolecular forces between phosphine molecules. These forces are responsible for holding the molecules together in the liquid and solid states and require more energy to overcome, resulting in a higher boiling point.
2. Polarity: Both phosphine and ammonia molecules are polar due to the presence of an electronegative atom (phosphorus or nitrogen) and hydrogen atoms. However, ammonia is more polar than phosphine due to the higher electronegativity difference between nitrogen and hydrogen. The stronger polarity of ammonia results in stronger dipole-dipole interactions, which requires more energy to break and therefore increases its boiling point.
3. Hydrogen Bonding: Ammonia molecules can undergo hydrogen bonding because the nitrogen atom has a lone pair of electrons that can form hydrogen bonds with hydrogen atoms of neighboring ammonia molecules. This additional intermolecular force of attraction increases the boiling point of ammonia further compared to phosphine, which cannot form hydrogen bonds to the same extent.
4. Molecular Weight: Phosphine has a higher molecular weight compared to ammonia, which also contributes to the difference in boiling points. Generally, substances with higher molecular weights tend to have higher boiling points due to stronger van der Waals forces of attraction between their molecules.
In summary, the differences in boiling points between phosphine and ammonia can be attributed to factors such as molecular size and shape, polarity, hydrogen bonding, and molecular weight.
The difference in boiling points of phosphine (-85 °C) and ammonia (-33.5 °C) can be accounted for by considering their molecular structures and intermolecular forces.
1. Molecular Structure:
Phosphine (PH3) consists of a central phosphorus atom bonded to three hydrogen atoms. Ammonia (NH3) consists of a central nitrogen atom bonded to three hydrogen atoms. The main difference in their molecular structures is in the central atom: phosphorus in phosphine and nitrogen in ammonia.
2. Intermolecular Forces:
The boiling point of a substance is determined by the strength of its intermolecular forces. Intermolecular forces are the attractive forces between molecules. There are three main types of intermolecular forces: London dispersion forces, dipole-dipole forces, and hydrogen bonding.
a) London Dispersion Forces:
London dispersion forces exist between all molecules, regardless of their polarity. These forces are caused by temporary fluctuations in electron distribution, resulting in an instantaneous dipole. In general, larger, more complex molecules experience stronger London dispersion forces due to increased electron interactions. Phosphine is a larger molecule than ammonia, so it is expected to have stronger London dispersion forces, which contribute to its higher boiling point.
b) Dipole-Dipole Forces:
Dipole-dipole forces exist between polar molecules, where there is a separation of positive and negative charges. Ammonia is a polar molecule due to the electronegativity difference between nitrogen and hydrogen. Phosphine, on the other hand, is a nonpolar molecule because the difference in electronegativity between phosphorus and hydrogen is relatively small. Dipole-dipole forces contribute to the boiling point but are not as significant as London dispersion forces in this case.
c) Hydrogen Bonding:
Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine). In ammonia, there is a hydrogen bond between the nitrogen atom and the hydrogen atoms. This additional hydrogen bonding in ammonia leads to stronger intermolecular attractions, resulting in a higher boiling point compared to phosphine.
In summary, the difference in boiling points between phosphine and ammonia can be attributed to their molecular structures and the strength of their intermolecular forces. Phosphine's larger size contributes to stronger London dispersion forces, while ammonia's ability to form hydrogen bonds leads to stronger intermolecular attractions and a higher boiling point.