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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: castromatic on June 24, 2018, 01:07:18 AM

Title: nonpolar molecules that have smaller molecular mass but have higher boiling pts?
Post by: castromatic on June 24, 2018, 01:07:18 AM

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?

Title: Re: nonpolar molecules that have smaller molecular mass but have higher boiling pts?
Post by: Borek on June 24, 2018, 03:37:55 AM

Quote from: castromatic on June 24, 2018, 01:07:18 AM

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.

Title: Re: nonpolar molecules that have smaller molecular mass but have higher boiling pts?
Post by: Enthalpy on June 25, 2018, 07:01:41 AM

Quote from: castromatic on June 24, 2018, 01:07:18 AM

[...] 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.