Determine which molecule has stronger intermolecular force?

SiH4 and CH4

The only intermolecular force they both have is London Dispersion forces

Strength of LDF is determined by molar mass

molar mass of SiH4 = 32.132

molar mass of CH4 = 48.42

Therefore shouldn't CH4 have a stronger LDF??

The answer says that SiH4 has a greater intermolecular force...why is that?

• Only problem is that you have the wrong molar mass for ch4.
– Maz
Apr 2 '17 at 21:49

2 Answers

Look at the boiling points. Interaction is based on geometry (surface area) and polarizability, among other things - and the phase: solid, liquid, vapor. Molecular weight is not diagnostic. SiH4 is "fluffy" versus methane being "hard." Compare the melting and boiling points of decane and isodecane versus adamantane, all being $\ce{C_{10}}$. Compare the melting and boiling points of n-octane versus 2,2,3,3-tetramethylbutane, both $\ce{C8H_{18}}$. Complicated.

You might check your MW for methane. Bells and whistles should have gone off, $\ce{CH4}$ versus $\ce{SiH4}$.

The molar mass of SiH4 is ~32, however the molar mass of CH4 is ~16. This explains the textbook's answer. Uncle Al, however, provides a more detailed answer.