Question

How do I rank the following compounds from lowest to highest boiling point: calcium carbonate, methane, methanol (CH₄O), dimethyl ether (CH₃OCH₃)?

Answer 1

The order of boiling points is: `"CH"_4 < "CH"_3"OCH"_3 < "CH"_4"O" < "CaCO"_3`

Explanation:

The order of strengths of intermolecular forces is: `"ion-ion > H-bonding > dipole-dipole > London dispersion"`.

Compounds with stronger intermolecular forces have higher boiling points.

The strongest intermolecular force in each of the compounds is:

`"CaCO"_3` — ion-ion attractions.
`"CH"_4` — London dispersion forces
`"CH"_3"OH"` — hydrogen bonding
`"CH"_3"OCH"_3` — dipole-dipole attractions

`"CaCO"_3` is an ionic compound. It has the highest boiling points

Next comes methanol, `"CH"_4"O"` or `"CH"_3"OH"`.

Methanol has strong hydrogen bonds. It will have the next highest boiling point.

Dimethyl ether, `"CH"_3"OCH"_3`, is a polar molecule.

The `"C-O"` bond dipoles reinforce each other, so the molecule has a dipole moment.

Dipole-dipole forces are not as strong as hydrogen bonds, so dimethyl ether has a lower boiling point than methanol does.

Finally, the `"C-H"` bonds in methane are nonpolar, so the molecule is also nonpolar.

It has only weak London dispersion forces,

`"CH"_4`, has the lowest boiling point.

The order of boiling points is:

`"CH"_4 < "CH"_3"OCH"_3 < "CH"_4"O" < "CaCO"_3`

Here's a good video on ordering compounds according to their intermolecular forces and boiling points.