The intermolecular force(s) responsible for the fact that CH4 has the lowest boiling point in the set CH4, SiH4, GeH4, SnH4 is/are: a) Mainly dispersion forces but also dipole-dipole interactions b) Hydrogen bonding c) London dispersion forces d) Dipole-dipole interaction e) Mainly hydrogen bonding but also London dispersion force
The Correct Answer and Explanation is:
Correct Answer: c) London dispersion forces
Explanation:
The boiling point of a substance is largely influenced by the strength of the intermolecular forces acting between its molecules. In the case of CH₄ (methane), SiH₄ (silane), GeH₄ (germane), and SnH₄ (stannane), all these compounds are nonpolar tetrahedral molecules made up of a central Group 14 element bonded to hydrogen atoms.
Since these molecules are nonpolar, they do not have dipole-dipole interactions. Furthermore, they also cannot form hydrogen bonds because hydrogen bonding only occurs when hydrogen is bonded to highly electronegative elements such as fluorine, oxygen, or nitrogen. None of these Group 14 hydrides meet that requirement.
Therefore, the only type of intermolecular force present in CH₄, SiH₄, GeH₄, and SnH₄ is London dispersion forces. These are weak intermolecular forces that arise from temporary fluctuations in the electron distribution within molecules, creating instantaneous dipoles that induce dipoles in neighboring molecules.
Among the set, CH₄ has the lowest molar mass and therefore the smallest number of electrons and smallest molecular size. Because of this, the London dispersion forces between CH₄ molecules are weaker compared to those in SiH₄, GeH₄, and SnH₄. As a result, less energy is needed to separate CH₄ molecules from each other, and this explains why CH₄ has the lowest boiling point among the group.
As the molecular weight increases from CH₄ to SnH₄, the number of electrons and polarizability of the molecules also increase, strengthening the London dispersion forces and thus raising the boiling points of the other compounds.
In conclusion, the correct choice is London dispersion forces, and these are the only intermolecular forces acting in these nonpolar molecules.
