Boiling Point Trends

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Sep 29, 2020
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So I know that the boiling point is highest in ionic compounds, followed by those with hydrogen bonds, then dipole-dipole forces, then London forces. My question is, where do compounds with covalent bonds fit in here? Is their boiling point lower than ionic compounds but higher than those with hydrogen bonds?

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When you are looking at boiling point, you are looking at intramolecular intermolecular interactions of like molecules with each other. Some of the interactions as you identified correctly are dipole-dipole forces, hydrogen bond, ionic interactions with molecules involved in an ionic bond. A covalent bond on the other hand is an intermolecular intramolecular bond. This bond only affects bp by affecting the intramolecular intermolecular bonds.

For e.g. in a water molecule, the bond between hydrogen and oxygen within one molecule of water is a covalent bond, an intermolecular, intramolecular bond. Because of the high dipole moment, this covalent bond allows for a hydrogen bond, which is an intramolecular intermolecular bond. The hydrogen bond, in the end, affects the boiling point. I hope that helps.

Edited my answer because I flubbed between intra and inter.
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This is a common misconception about how substances boil.

Covalent bonds and Ionic bonds are intramolecular bonds, which are bonds that occur within a molecule.

Bonds like H-bonds, dipole-dipole, London Dispersion, and ion-dipole are all intermolecular bonds, which are bonds that occur between two different molecules of the same type. You can think of intermolecular bonds as similar to the internet. The internet connects two computers together and intermolecular bonds connect two molecules together.

When a chemical boils intermolecular bonds break not intramolecular bonds.

If you take something like water that has two covalent bonds (intramolecular bonds) when it boils those bonds do not break. A physical change takes place not a chemical change so the reaction looks something like this:

H2O (l) —> H2O (g)

What did break are the hydrogen bonds (which is a special type of dipole-dipole intermolecular bond) holding the H2O together in the liquid phase. Now not all of the hydrogen bonds broke. H2O (g) also has hydrogen bonds but much fewer on average than H2O (l).

By asking this question I think what you could be thinking of are London Disperson Forces, which is an attractive force that all covalent molecules have (Ionic bonds are not molecules - molecules do not exist for ionic bonds they are crystals). The greater the molecular weight of a compound the greater the london dispersion force and the greater the london dispersion force the stronger the intermolecular bonds and the greater the intermolecular bond the harder it is to push that chemical to a higher phase ( s -> l -> g ).
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H-bonds, dipoles-dipole, and london dispersion forces only apply to covalent molecules because these all require covalent bonds in their nature and therefor ionic compounds are excluded from these types of intermolecular forces