SaltSoln

CHM 1046
General Chemistry II
Dr. Michael Blaber

Acid-Base Equilibria

Acid-Base Properties of Salt Solutions

Ions can exhibit acidic or basic properties

NH₄⁺ can donate a proton (i.e. act as an acid)
F^- can accept a proton (i.e. act as a base)

Can salt solutions have acidic or basic properties?

Salts are strong electrolytes - they are completely ionized in solution.
Any acid or base properties must therefore be due to the properties of their cations or anions

Many ions are able to react with H₂O(l) to produce H⁺(aq) or OH^-(aq) ions. This is termed Hydrolysis.

When weak acids dissociate, they produce anions that are the conjugate base

HA(aq) ó A^-(aq) + H⁺(aq)

The A^- ion conjugate base can react with H₂O(l) to produce a hydroxide ion, and is therefore a base

A^-(aq) + H₂O(l) ó AH(aq) + OH^-(aq)

The anions of strong acids, by definition, are such weak bases that they do not react with H₂O(l) to produce OH^-(aq) ions and thus, don't affect the pH of aqueous solutions

Amphoteric ions can behave as either an acid or a base

HSO₃^- ions can accept a proton, and behave as a base
HSO₃^- ions also have a proton they can still donate, thus, they may act as an acid
Their behavior in water will be determined by the relative magnitudes of K_a and K_b.

Cations

All cations, with the exception of group 1 metals, and the heavy group 2 metals (i.e. Ca²⁺, Sr²⁺, Ba²⁺) act as weak acids in aqueous solution
Predicting the pH of salt solutions:

The ions in a salt represent the spectator ions left over after the neutralization reaction

NaOH(aq) + HCl(aq) ó Na⁺(aq) + Cl^-(aq) + H₂O(l)

In the above reaction, the cation (Na⁺) is the conjugate acid of the base NaOH, and the anion (Cl^-) is the conjgate base of the acid HCl.
NaOH is a strong base, and therefore, the conjugate acid (Na⁺) is so weak as to not have any acidic properties at all
HCl is a strong acid, and therefore, the conjugate base (Cl^-) is so weak as to not have any basic properties at all

Thus, the salt NaCl is composed of ions that do not have any acid or base properties, and will not affect the pH of an aqueous solution

and
One can predict whether a salt will have acidic or basic properties by considering the nature of its conjugate acid and base

A salt derived from a strong acid and a strong base: As conjugate acid to a strong base and conjugate base to a strong acid both salt ions will be so weak as to have no acid or base properties (see above for NaCl ions)

A salt derived from a strong acid and a weak base: The conjugate base to the strong acid (i.e. the anion of the salt) will have no basic properties. The conjugate acid to the weak base (i.e. the cation) will have significant acidic property. Thus, this type of salt in aqueous solution will be acidic in nature.

A salt derived from a weak acid and a strong base: The conjugate acid to the strong base (i.e. the cation of the salt) will have no acidic properties. The conjugate base to the weak acid (i.e. the anion) will have significant base property. Thus, this type of salt will be basic in nature.

A salt derived from a weak acid and a weak base: The conjugate acid to the weak base (i.e. the cation) will have significant acidic properties and the conjugate base to the weak acid (i.e. the anion) will have significant basic properties. Both cation and anion hydrolyze. The overall affect on pH is dependent upon which is the stronger.