Chemistry / Unit 8 Chem 1373 worked example, 60 to 90 seconds
Ranking Boiling Points by Dispersion-Force Strength
The idea
Ranking the boiling points of a series of nonpolar molecules by dispersion-force strength means comparing their molar masses and shapes, since larger and more elongated molecules hold more polarizable electrons and offer more surface contact for the temporary dipoles to act across.
In practice
For CH4, SiH4, GeH4, and SnH4 — all nonpolar group-14 hydrides — molar mass rises from about 16 to 123 g/mol across the series, so dispersion forces and boiling points rise the same way, giving the order CH4 < SiH4 < GeH4 < SnH4. Among C5H12 isomers, elongated n-pentane's larger contact area gives it a higher boiling point than compact, spherical neopentane, despite both having the identical formula and molar mass.
Where people slip
Ranking by molar mass alone works only when shape doesn't also vary — same-formula isomers can still separate by boiling point through shape's effect on contact area.
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