The boiling points of the following compounds increase in the order in which they are listed below:
CH4 < H2S < NH3
Discuss the theoretical considerations involved and use them to account for this order.
To explain why the boiling points of the compounds CH4, H2S, and NH3 increase in the order listed, we need to consider the intermolecular forces present in each compound.
The boiling point of a substance is determined by the strength of the intermolecular forces between its molecules. The stronger the intermolecular forces, the higher the boiling point.
CH4 is a non-polar molecule. It consists of four hydrogen atoms bonded to a central carbon atom. The carbon atom shares its electrons equally with the hydrogen atoms, resulting in a symmetrical tetrahedral shape and no permanent dipole moment. As a result, CH4 experiences only weak London dispersion forces between its molecules. London dispersion forces arise due to temporary fluctuations in electron distribution and are present in all molecules, regardless of polarity. These forces are generally weaker than other intermolecular forces.
H2S is a polar molecule with a bent shape. It consists of two hydrogen atoms bonded to a central sulfur atom. Since sulfur is more electronegative than hydrogen, it pulls the shared electrons closer to itself and generates a permanent dipole moment. This dipole-dipole interaction between H2S molecules contributes to stronger intermolecular forces compared to CH4.
NH3 is also a polar molecule with a pyramidal shape. It consists of three hydrogen atoms bonded to a central nitrogen atom. Nitrogen is more electronegative than hydrogen and generates a permanent dipole moment, similar to H2S. However, the NH3 molecule has one lone pair of electrons on the central nitrogen atom. This lone pair enhances the attractive forces between NH3 molecules through hydrogen bonding, which is a stronger intermolecular force than dipole-dipole interactions.
Therefore, based on the intermolecular forces analysis, we can conclude that the boiling points increase in the order CH4 < H2S < NH3. CH4 has the weakest intermolecular forces (only London dispersion forces), H2S has stronger intermolecular forces (dipole-dipole interactions), and NH3 has the strongest intermolecular forces (hydrogen bonding). Consequently, higher temperatures are required to break the intermolecular attractions and convert the substances from a liquid to a gas, resulting in the observed order of boiling points.