84 Hydrolysis of Salts
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Learning Objectives
By the end of this section, you will be able to:
- Predict whether a salt solution will be acidic, basic, or neutral
- Calculate the concentrations of the various species in a salt solution
- Describe the acid ionization of hydrated metal ions
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 ammonium chloride in water results in its dissociation, as described by the equation
The ammonium ion is the conjugate acid of the weak base ammonia, NH3, and so it will undergo acid ionization (or acid hydrolysis):
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, Kb is measurable and Ka > 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, Ka is immeasurably large and Kb ≈ 0 (chloride ions don’t undergo appreciable hydrolysis).
Thus, dissolving ammonium chloride in water yields a solution of weak acid cations (\({\text{NH}}_{4}{}^{+}\)) and inert anions (Cl−), resulting in an acidic solution.
Calculating the pH of an Acidic Salt Solution Aniline is an amine that is used to manufacture dyes. It is isolated as anilinium chloride, \(\left[{\text{C}}_{6}{\text{H}}_{5}{\text{NH}}_{3}{}^{\text{+}}\right]\text{Cl},\) a salt prepared by the reaction of the weak base aniline and hydrochloric acid. What is the pH of a 0.233 M solution of anilinium chloride
SolutionThe Ka for anilinium ion is derived from the Kb for its conjugate base, aniline (see Appendix H):
Using the provided information, an ICE table for this system is prepared:
Substituting these equilibrium concentration terms into the Ka expression gives
Assuming x << 0.233, the equation is simplified and solved for x:
The ICE table defines x as the hydronium ion molarity, and so the pH is computed as
Check Your LearningWhat is the hydronium ion concentration in a 0.100-M solution of ammonium nitrate, NH4NO3, a salt composed of the ions \({\text{NH}}_{4}{}^{\text{+}}\) and \({\text{NO}}_{3}{}^{\text{−}}.\) Which is the stronger acid \({\text{C}}_{6}{\text{H}}_{5}{\text{NH}}_{3}{}^{\text{+}}\) or \({\text{NH}}_