8.6 Molecular Structure of Acids and Bases

Molecular Structures of Acids and Bases

Acid strength is largely justified by two components of molecular structure: the ease with which the hydrogen is removed from a molecule and the stability of the conjugate base. Let’s walk through these factors with a very clear example: ${HI}$ and ${HCl}$.

Now, both of these are strong acids, and are therefore assumed to have 100% dissociation. In truth, neither acid is 100% exactly—they’re just very close. As a result, some strong acids can be even stronger than others. ${HCl}$ and ${HI}$ are both binary acids (2 elements) that consist of halogens bonded to a hydrogen. The main difference, however, is the bond length. {Cl^–} has a smaller ionic radius than {I^–}, and the ${H-Cl}$ bond length is therefore shorter than the ${H-I}$ bond length. Since ${H-I}$ has a longer, therefore weaker, bond, the hydrogen is easier to remove, making ${HI}$ the stronger acid.

Additionally, since {I^–} is larger, the negative charge is spread out more—and therefore more stabilized. Since {I^–}, the conjugate base of ${HI}$, is more stable than {Cl^–}, the conjugate base of ${HCl}$, it has a stronger tendency to remain in that stable state. Therefore, ${HI}$ is a stronger acid than ${HCl}$.