The boiling point of CH4 is lower than the boiling point of SiH4, whereas the boiling point of NH3 is higher than the boiling point of PH3. The lower boiling point of CH4 compared to SiH4 is expected because of its lower molecular weight.
is expected because of the hydrogen bonding in SiH4.
cannot be explained.
is expected because of the stronger induced dipole forces in CH4.
is expected because of the stronger dipole forces in SiH4.
The Correct Answer and Explanation is:
The correct answer is:
is expected because of its lower molecular weight.
Explanation
Boiling point trends in covalent molecules can often be explained by examining the intermolecular forces (IMFs) acting between the molecules. These include London dispersion forces (induced dipole-induced dipole), dipole-dipole interactions, and hydrogen bonding.
Methane (CH₄) and silane (SiH₄) are both group 14 hydrides with tetrahedral geometry and are nonpolar molecules. Since they lack a permanent dipole and do not engage in hydrogen bonding (hydrogen bonding requires highly electronegative atoms like N, O, or F), the dominant intermolecular force in both is London dispersion forces.
London dispersion forces increase with molecular size and mass because larger molecules have more electrons and are more polarizable. CH₄ has a molar mass of about 16 g/mol, while SiH₄ has a molar mass of about 32 g/mol. Because SiH₄ is heavier and has more electrons than CH₄, its dispersion forces are stronger, leading to a higher boiling point.
This makes the lower boiling point of CH₄ compared to SiH₄ expected due to its lower molecular weight and weaker dispersion forces.
Regarding the incorrect options:
- “Is expected because of the hydrogen bonding in SiH₄” is incorrect: SiH₄ does not exhibit hydrogen bonding, as silicon is not electronegative enough.
- “Cannot be explained” is incorrect: the difference can be clearly explained by London dispersion forces and molecular weight.
- “Is expected because of the stronger induced dipole forces in CH₄” is incorrect: CH₄ has weaker, not stronger, induced dipole (dispersion) forces than SiH₄.
- “Is expected because of the stronger dipole forces in SiH₄” is also incorrect: SiH₄ is nonpolar, so dipole-dipole forces are not relevant.
In conclusion, the boiling point trend between CH₄ and SiH₄ is best explained by their molecular weights and corresponding London dispersion forces.
