Acid Strength and Molecular Structure

Hydrochloric acid is a strong acid, whereas hydrofluoric acid is a weak acid. But what determines their strength?

The strength of binary acids, with only two elements, is determined by bond energy and bond polarity.

An acid with a higher bond energy has a stronger bond and, therefore, will be a weaker acid. Comparing acids in a group, the bond in a weak acid, like hydrofluoric acid, is harder to break, so the acid is less likely to donate protons.

In contrast, an acid with lower bond energy has a weaker bond and, therefore, will be a stronger acid. For example, the bond in a strong acid, like hydrochloric acid, breaks more easily and donates protons more readily than hydrofluoric acid.

A bond, like that in hydrochloric acid, is polar when one atom is more electronegative than the other atom.

A compound can act as an acid when the atom attached to hydrogen has a higher electronegativity than hydrogen. The atom will have a partial negative charge, allowing the hydrogen to have a partial positive charge so it can be released as a proton.

An acid with higher bond polarity has a weaker bond and, therefore, will be a stronger acid.

Comparing compounds across a period, hydrochloric acid i