15.10: Ions as Acids and Bases

Salts with Acidic Ions

Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt’s constituent ions. For example, dissolving the ammonium chloride in water results in its dissociation, as described by the equation:

The ammonium ion is the conjugate acid of the base ammonia, NH₃; its acid ionization (or acid hydrolysis) reaction is represented by

Since ammonia is a weak base, K_b is measurable and K_a > 0 (ammonium ion is a weak acid).

The chloride ion is the conjugate base of hydrochloric acid, and so its base ionization (or base hydrolysis) reaction is represented by

Since HCl is a strong acid, K_a is immeasurably large and K_b ≈ 0 (chloride ions don’t undergo appreciable hydrolysis). Thus, dissolving ammonium chloride in water yields a solution of weak acid cations (NH₄⁺) and inert anions (Cl⁻), resulting in an acidic solution.

Salts with Basic Ions

As another example, consider dissolving sodium acetate in water:

The sodium ion does not undergo appreciable acid or base ionization and has no effect on the solution pH. This may seem obvious from the ion's formula, which indicates no hydrogen or oxygen atoms, but some dissolved metal ions function as weak acids, as addressed later in this section. The acetate ion, CH₃CO₂⁻, is the conjugate base of acetic acid, CH₃CO₂H, and so its base ionization (or base hydrolysis) reaction is represented by

Because acetic acid is a weak acid, its K_a is measurable and K_b > 0 (acetate ion is a weak base). Dissolving sodium acetate in water yields a solution of inert cations (Na⁺) and weak base anions (CH₃CO₂⁻), resulting in a basic solution.

Salts with Acidic and Basic Ions

Some salts are composed of both acidic and basic ions, and so the pH of their solutions will depend on the relative strengths of these two species. For such types of salts, a comparison of the K_a and K_b values allows the prediction of the solution’s acid-base status.

The Ionization of Hydrated Metal Ions

Unlike the group 1 and 2 metal ions of the preceding examples (Na⁺, Ca²⁺, etc.), some metal ions function as acids in aqueous solutions. These ions are not just loosely solvated by water molecules when dissolved; instead they are covalently bonded to a fixed number of water molecules to yield a complex ion (see the chapter on coordination chemistry). As an example, the dissolution of aluminum nitrate in water is typically represented as

However, the aluminum(III) ion actually reacts with six water molecules to form a stable complex ion, and so the more explicit representation of the dissolution process is

The Al(H₂O)₆³⁺ ions involve bonds between a central Al atom and the O atoms of the six water molecules. Consequently, the bonded water molecules' O–H bonds are more polar than in nonbonded water molecules, making the bonded molecules more prone to the donation of a hydrogen ion:

The conjugate base produced by this process contains five other bonded water molecules capable of acting as acids, and so the sequential or step-wise transfer of protons is possible as depicted in few equations below:

Aside from the alkali metals (group 1) and some alkaline earth metals (group 2), most other metal ions will undergo acid ionization to some extent when dissolved in water. The acid strength of these complex ions typically increases with increasing charge and decreasing size of the metal ions. The first-step acid ionization equations for a few other acidic metal ions are shown below:

This text is adapted from Openstax, Chemistry 2e, Section 14.4: Hydrolysis of Salts.

Acids, bases, and ionic compounds form anions and cations when they dissolve in water.

Anions that are the conjugate base of strong acids, like chloride formed by the dissociation of hydrochloric acid, are too weak to accept a proton from water. Therefore, chloride ions are pH neutral; that is, they are neither acidic nor basic.

In contrast, anions formed by weak acids, like acetate, the conjugate base of acetic acid, acts as a weak base as it can accept a proton from water.

Cations that are the conjugate acid of strong bases, like sodium ions formed by sodium hydroxide, cannot accept protons and, therefore, are also pH neutral.

In contrast, cations produced by weak bases, like ammonium, the conjugate acid of the ammonia, acts as a weak acid as it can donate protons to water.

When they ionize, salts can produce acidic and basic solutions if they contain a conjugate acid or a conjugate base of a weak acid or base.

Ammonium bromide produces ammonium and bromide ions in water. Bromide ions are pH neutral, whereas ammonium ions act as a weak acid as they can donate protons.

When sodium acetate is dissolved in water, the sodium ions do not react with water; however, the acetate ion can accept a proton—forming a basic solution.

Salts that contain pH neutral cations and anions form neutral solutions. For example, sodium chloride dissociates into sodium ions and chloride ions when dissolved in water. As both of these ions can neither accept nor donate protons, they form a neutral solution.

Small and highly charged metal ions, like iron(III) and aluminium(III), can also act as weak acids when they become hydrated.

When aluminum(III) is hydrated, it acts as a weak acid and transfers protons from its waters of hydration to the free water molecules, leading to the production of hydronium ions.

The smaller the metal ion and the higher the charges present on it, the greater is its tendency to act as an acid. For example, the K_a for Fe(III) is 6.3 × 10⁻³, whereas the K_a for Ni(II) is 2.5 × 10⁻¹¹.