Acid-Base Properties of Ions and Salts Determine whether each cation is acidic or pH-neutral. For the cat- ions that are acidic, write an equation that shows how the cation acts as an acid. \(\begin{array}{l}{\text { a. } \mathrm{NH}_{4}^{+}} \\ {\text { b. } \mathrm{Na}^{+}} \\ {\text { c. } \mathrm{Co}^{3+}} \\ {\text { d. } \mathrm{CH}_{3} \mathrm{NH}_{3}^{+}}\end{array}\)

Acids have a characteristic ability to donate protons (hydrogen ions, H+) to other substances, a process fundamental to acid-base chemistry. This act of proton donation is defined by the Bronsted-Lowry acid theory. In essence, when an acid donates a proton to water, it forms a hydronium ion ((H_3O^+)), thus lowering the pH of the solution. As seen with the ammonium ion ((NH_4^+)), when it donates a proton to water, it transforms into ammonia ((NH_3)) and a hydronium ion, which makes the solution acidic.

The ability of an ion to donate a proton also dictates its strength as an acid. Strong acids can completely dissociate and donate all of their available protons to water, while weak acids only partly dissociate, leading to an equilibrium between the acid and its conjugate base in solution. Students can often struggle with this concept, but a practical approach is to remember that substances with a ready-to-donate proton, typically indicated by the presence of a hydrogen atom in their structure, will exhibit acidic properties in water.

To determine if a cation is acidic, we must consider its origin and its ability to interact with water. A cation, which is a positively charged ion, can often be the conjugate acid of a base. For instance, the cation resulting from the removal of a hydroxide ion (OH^-) from a base will exhibit acidic properties because it seeks to reacquire a proton to return to its original base form.

This behavior is illustrated by cations like (CH_3NH_3^+), the conjugate acid of the weak base methylamine. Since weak bases have weak conjugate acids, (CH_3NH_3^+) will act as an acid in water by donating a proton. In contrast, cations derived from strong bases, such as sodium ion (Na^+), do not demonstrate this tendency and are thus considered pH-neutral. Their conjugate acids are so weak that they virtually have no tendency to donate a proton back to water.

Conjugate acids and bases are paired substances that transform into each other by the exchange of a proton. Identified by the Bronsted-Lowry theory, a conjugate acid-base pair consists of a base that gains a proton to form its conjugate acid, or an acid that loses a proton to form its conjugate base. For example, water acts as a base by accepting a proton from (NH_4^+), with the resulting substances being ammonia, the conjugate base, and the hydronium ion, the conjugate acid.

Understanding this relationship can greatly simplify the analysis of many acid-base reaction problems. When students encounter difficulties with this complex concept, visualizing the transfer of the proton as a 'hand-off' between acid and base can help to solidify their grasp on how these reactions occur.

pH-neutral cations are those that either cannot donate a proton or do not produce a significant amount of hydronium ions when dissolved in water. Cations that are the conjugate acids of strong bases like sodium ion (Na^+) fall into this category. These cations are considered the counterparts to atoms or molecules that, as bases, readily accept protons and completely disassociate in water, such as the hydroxide ion from sodium hydroxide (NaOH).

Because these strong bases are so proficient in accepting protons, their resulting conjugate acids have little to no inclination to reverse this process. The Na^+ ion is a clear example of a cation that is pH-neutral because its conjugate base, NaOH, is a strong base. In general, if a student is trying to ascertain the impact of an ion on pH, they should consider the strength of the corresponding base from which the ion is derived.