Ions as Acids and Bases

Chloride ions are the conjugate base of the strong acid hydrochloric acid. Since strong acids dissociate completely, their conjugate bases are too weak to accept protons from water molecules. Therefore, chloride ions neither donate nor accept protons, making them pH neutral and unable to affect solution acidity or basicity.

Acetate ions are the conjugate base of the weak acid acetic acid. Because acetic acid is weak, acetate ions retain the ability to accept protons from water molecules. This proton acceptance produces hydroxide ions, creating a basic solution. Sodium ions do not react with water, so they have no effect on pH.

Ammonium bromide produces ammonium and bromide ions in water. Bromide ions are pH neutral because they are the conjugate base of the strong acid hydrobromic acid. Ammonium ions act as weak acids by donating protons to water. Since only ammonium ions affect pH, the solution becomes acidic.

When aluminum(III) ions dissolve in water, they form hydrated complex ions with six bonded water molecules. The positive charge on the metal ion polarizes these bonded water molecules, making their O-H bonds more polar and prone to donating hydrogen ions. This proton transfer produces hydronium ions, making the solution acidic.

The acid strength of hydrated metal ions increases with higher charge and smaller size. For example, Fe(III) with Ka = 6.3 × 10−3 is a stronger acid than Ni(II) with Ka = 2.5 × 10−11. Smaller, more highly charged ions create stronger polarization of bonded water molecules, increasing their tendency to donate protons and making them stronger acids.

Sodium chloride dissociates into sodium and chloride ions. Sodium ions are the conjugate acid of the strong base sodium hydroxide and cannot accept protons. Chloride ions are the conjugate base of the strong acid hydrochloric acid and cannot donate protons. Since neither ion reacts with water, the solution remains neutral.

Conjugate bases of strong acids, like chloride, are too weak to accept protons from water and remain pH neutral. Conjugate bases of weak acids, like acetate, retain significant ability to accept protons from water, acting as weak bases. This difference determines whether salt solutions containing these ions will be acidic, basic, or neutral.