Question

You have three covalent compounds with three very different boiling points. All of the compounds have similar molar mass and relative shape. Explain how these three compounds could have very different boiling points ?

Answer 1

If the compounds have similar shape and molar mass but different boiling points, they must have different intermolecular forces.

Consider the compounds propane, dimethyl ether and ethanol.

Propane, `"CH"_3"CH"_2"CH"_3`: molar mass = 44 g/mol; boiling point = -42 °C

Dimethyl ether, `"CH"_3"OCH"_3`: molar mass = 46 g/mol; boiling point = -24 °C

Ethanol, `"CH"_3"CH"_2"OH"`: molar mass = 46 g/mol; boiling point = 78 °C

These compounds all consist of a chain of three "heavy" atoms with hydrogen atoms attached, and they have similar molar masses, but widely different boiling points.

Propane is a nonpolar compound. It has only weak London dispersion forces, so the molecules can easily escape into the gas phase.

It has a low boiling point and is a gas at room temperature.

Dimethyl ether has polar `"C-O"` bonds, so its intermolecular forces are the stronger dipole-dipole attractions.

It has a higher boiling point, but it is still a gas at room temperature.

Ethanol has strong hydrogen bonds. The intermolecular forces are so strong that ethanol is a liquid at room temperature.