2012 Destroyer GC #114

[drill-and-fill]

The questions asks which one of the following would have the highest surface tension at a given temp...

C2H6
CF4
CI4
CBr4
CH4

The answer is CI4, due to greatest molecular weight. However, I thought the substance with the highest intermolecular forces would have the highest surface tension, and the order of intermolecular forces go from H-bonding > Dipole-Dipole > London Dispersion Forces. So wouldn't CF4 have a greater IM force than CI4 due to greater dipole-dipole force? F is more polar than I, right? Or do all halogens have the same amount of polarity therefore making the dipole-dipole force difference between them negligible? 😕

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[njdoc327]

large intermolecular forces indiciate high surface tension. since in this example, there's no hydrogen bonding or any other major IM force, go with molecular weight. the molecule with the most weight will have the largest amount of van der waals or london dispersion forces.

 

[njdoc327]

oh also, draw out the lewis structure of CF4. you'll see that theres no net dipole as the molecular geometry is tetrahedral and all of the dipoles cancel each other out. hope that helps!

 

[drill-and-fill]

Ahhh I see now. Thank youuuu! 🙂