Question

Type the following formulas in the boxes below in order of increasing boiling point: RbF, C02, CH3OH, CH3Br.

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Answer 1

RbF is ionic, and it is necessary to overcome strong electrostatic forces of attraction to boil it, so it will have a high boiling point. The other three are all covalent molecular substances, so have only weak intermolecular interactions between their molecules, and so have a low boiling point.

The intermolecular interactions are:
CO2 - dispersion forces only
CH3OH - dispersion forces, dipole-dipole interactions and hydrogen bonds
CH3Br - dispersion forces and dipole-dipole interactions.

Since hydrogen bonds are the strongest of the intermolecular interactions, and dispersion forces are the weakest, carbon dioxide will have the lowest boiling point of this group, and methanol will have the highest.

So, the order is: CO2 < CH3Br < CH3OH < RbF

Answer 2

CO2 < CH3Br < CH3OH < RbF

The boiling point is related to the molar heat of vaporization, i.e. generally speaking the higher the molar heat of vaporization the higher the boiling point. Both molar heat of vaporization and boiling point are determined by the strength of intermolecular forces.