Acid-Base Properties of Ions and Salts Pick the stronger base from each pair. \(\begin{array}{l}{\text { a. } \mathrm{ClO}_{4}^{-} \text { or } \mathrm{ClO}_{2}^{-}} \\ {\text { b. } \mathrm{Cl}^{-} \text { or } \mathrm{H}_{2} \mathrm{O}} \\ {\text { c. } \mathrm{CN}^{-} \text { or } \mathrm{ClO}^{-}}\end{array}\)

When studying the acid-base properties of ions, a fundamental concept to grasp is the idea of conjugate acid-base pairs. These pairs include a proton (H⁺) donor, known as the acid, and a proton acceptor, the base. When an acid donates a proton, it transforms into its conjugate base, while the base that accepts a proton becomes its conjugate acid.

For example, when hydrogen chloride (HCl) donates a proton, it becomes chloride ion (Cl⁻), its conjugate base. Conversely, when water (H₂O) accepts a proton, it becomes hydronium (H₃O⁺), its conjugate acid. The relationship is easily expressed using the formula:
Acid ⇌ Conjugate Base + H⁺.
Understanding this relationship helps in predicting the strength of bases and acids. If the conjugate acid is strong, meaning it has a high tendency to donate a proton, its counterpart—the conjugate base—will typically be weak, as it has a low affinity for protons.

The strength of acids and bases significantly impacts the behavior of ions in chemical reactions. Strong acids fully dissociate in water, releasing all their protons. Conversely, weak acids only partially dissociate, holding onto some of their protons even in an aqueous solution. For bases, the strength is determined by their proton affinity; strong bases readily acquire protons, while weak bases have a lower tendency to do so.

As seen in the exercise, comparing the strength of two conjugate bases hinges on their corresponding acids. For instance, since hydrochloric acid (HCl) is a strong acid, its conjugate base, chloride ion (Cl⁻), is a weak base. This concept is key to solving the problems in the original exercise, where the strength of a base is inferred by considering the strength of its conjugate acid.

The stability of conjugate acids is another vital factor in determining the strength of a base. The stability of a conjugate acid is influenced by various factors, including the electronegativity of the elements involved and the molecule's ability to delocalize the positive charge. Generally, the more stable the conjugate acid, the less likely it is to release a proton, and hence, its conjugate base is considered stronger.

In the textbook exercise, understanding that perchloric acid (HClO₄) is a very strong acid helps to deduce that its conjugate base, perchlorate ion (ClO₄⁻), is quite weak due to the compound's stability and lack of tendency to attract protons. This principle of stability versus reactivity is a useful heuristic for predicting the behavior of acids and bases in chemical reactions.