March 28, 2024, 08:06:49 PM
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Topic: nonpolar molecules that have smaller molecular mass but have higher boiling pts?  (Read 1616 times)

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Offline castromatic

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As I learned in class, if a molecule is non polar, it only has london dispersion forces. And when two non polar molecules are compared, the one with the stronger london dispersion force automatically has the higher boiling point. And to find which one has the stronger dispersion force, one needs to simply get the molar mass of each molecule and see which is heavier. But when I compared CO2 (molar mass = 44) and CF4 (molar mass = 88), I found that CO2 actually has a higher boiling point (-78 degrees celsius for CO2 and -127 degrees Celsius for CF4). Why is this so?

Offline Borek

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As I learned in class, if a molecule is non polar, it only has london dispersion forces.

That's only an approximation.

CO2 - while non-polar on the whole - is linear and made of two canceling dipoles. Imagine putting two such molecules side by side and think how they can interact.

CF4 is made of four canceling dipoles. Think how they are arranged in space.
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Offline Enthalpy

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[...] one simply needs to [...] see which is heavier. But [...]

Welcome in a complicated world.

Pb(C2H5)4 is a nice example. Bp +85°C @15mmHg despite 323g/mol.
https://en.wikipedia.org/wiki/Tetraethyllead
Or UF6, sublimes at +57°C despite 352g/mol
https://en.wikipedia.org/wiki/Uranium_hexafluoride
and UCl6, boiling at +75°C with 451g/mol
https://en.wikipedia.org/wiki/Uranium_hexachloride

and ZrCl4 sublimes at +331°C but ZrO2 boils at +4300°C
https://en.wikipedia.org/wiki/Zirconium(IV)_chloride
https://en.wikipedia.org/wiki/Zirconium_dioxide
Ti, Hf, Si, Ge... behave like Zr.

Melting points are even less predictable.

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