Question

Is an ionic bond stronger than a comparable covalent bond? If not, then why do ionic compounds typically have higher melting points than covalent compounds?

Answer 1

Nope, ionic bonds are generally weak (and intramolecular) bonds. The electrons are poorly shared (in fact, we say they are mostly transferred instead of shared), which is the opposite of what an ideal chemical bond is supposed to be (even electron sharing).

The less covalent a bond is, the weaker the bond is, for the same point of comparison.

BOND-BREAKING (INTRAMOLECULAR)

Here's an example.

`"NaCl"` is ionic. We know it's going to dissociate completely in water, meaning that it can break apart into its ions, `"Na"^(+)` and `"Cl"^(-)`, easily.

That's because `"Cl"` atom is significantly more electronegative than `"Na"` atom, which means `"Cl"` skews the share of electrons significantly towards it.

Compare that to `"Cl"_2`, which is perfectly nonpolar (since the `"Cl"` atoms are identical, their electronegativity difference is precisely `0`). It doesn't make sense to write this reaction to dissociate `"Cl"_2(g)` in water, because it doesn't dissociate in water:

`"Cl"_2(g) stackrel("H"_2"O"(l))(->) 2"Cl"(g)`

`"Cl"_2(g)` just doesn't spontaneously break into two `"Cl"` gas atoms when placing it into water, since the electrons are exactly evenly shared, making an ideal covalent bond. Even if it dissolved in water to a significant extent, it would not be ionized.

So, the covalent bond in one molecule of `"Cl"_2` is stronger than the `"Na"^(+)-"Cl"^(-)` ionic interaction within one `"NaCl"` formula unit.

PHASE CHANGES (INTERMOLECULAR)

Melting point trends are not about bonds. They are about intermolecular forces, i.e. the interactions between compounds, NOT within compounds.

When you melt something, you try to separate the solid compounds themselves from each other (not into ions), increasing their motion until they move more freely than before, and become a liquid.

This involves breaking intermolecular interactions, such as dipole-dipole, hydrogen-"bonding" interactions, or ion-pairing interactions.

For instance, if you try to melt `"NaCl"`, you break apart its lattice structure to give many `"NaCl"` formula units that are moving more freely, which is molten `"NaCl"(l)`.

On the other hand, if you try to dissociate it in water instead of melting it, it exists as `"Na"^(+)` and `"Cl"^(-)`. They're not the same process.

Since ion-pairing interactions are the strongest intermolecular force, it is hardest to melt ionic compounds, and thus, they have high melting points relative to, say, water, which has hydrogen-"bonding" as its strongest intermolecular force, and thus a low melting point.