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which of the following has the highest boiling point?

a)C2Cl6
b)C2Br6
c)C2H6
d)C2F6
e)C2I6
i know its e, but why is that the answer? are all questions about boiling points similar? or they they have to do with intermolecular forces? i have a test soon on boiling point problems...

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2 answers

  1. Intermolecular forces play a role in boiling points, for sure, but in this case it's a matter of molecular weight. Notice C2 is the same for all answers with the other molecules being H, F, Cl, Br, I. So the one with the highest molar mass will be the highest boiling point.

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  2. It all depends on the type and strength of the intermolecular forces between the molecules. Consider these 3 main types: Permanent Dipole-Dipole forces, Hydrogen bonds, and Dispersion forces. Firstly, dipole-dipole forces. All examples above have polar bonds but the shape of the molecule is symmetrical and the atoms are identical so, *All Molecules are non-polar*. This is very important. A molecule may contain polar bonds but they cancel each other and the overall molecule becomes non-polar. Since the molecules are proven non-polar, there are no permanent dipole-dipole interactions between them. Now, look for the hydrogen bonds. Hydrogen bonds only exist when there are hydrogen atoms bonded with one of three: Fluorine, Oxygen or Nitrogen. The examples above have no hydrogen bonds. Now it all comes to the Dispersion Forces. The Molecule with the highest molecular weight has stronger intermolecular dispersion forces. This is C2I6 (hexaiodoethane).

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