Section V. HYDROLYSIS OF SALTS
9-16. DISCUSSION
If you were to place a salt in water and then perform a pH determination, you
would be surprised to find that some salts would have an acidic pH, some would be
basic, and others would be neutral. Most salts are formed by the reaction of acids with
bases. The anion (-) of the acid and the cation (+) of the base are the component parts
of a salt molecule. The term hydrolysis refers to a reaction with water. A salt when
placed in water will ionize and depending on the type of salt, will undergo hydrolysis
(remember water itself ionizes to a small degree). There are three types of salts we will
consider: salts that are composed of the ions of a weak acid and a strong base, salts of
a strong acid and a weak base, and salts of a weak acid and a weak base. Anions that
form a strong acid (NO3-, Cl-, C104-, Br- and I-) and cations that form strong bases
(Group IA and IIA) do not undergo hydrolysis but simply dissociate into ions: (i.e., NaCl
does not hydrolyze in water). This information will help in understanding the action of
buffers.
9-17. HYDROLYSIS OF THE SALT OF A WEAK ACID AND A STRONG BASE
This salt will hydrolyze to form a basic solution. The salt sodium acetate will
ionize into sodium ions and acetate ions. The sodium ion does not undergo hydrolysis
but the acetate ion will react with free hydrogen ions forming acetic acid.
+
+
-
Na + C2H3O2- + HOH <====> HC2H3O2 + Na +OH
+
-
+
This reaction causes a shift in the [H ]/[OH ] by removing [H ]. This salt is
classified as a basic salt because the acetate ion, by definition, is a weak
Bronsted-Lowry base (proton acceptor).
9-18. HYDROLYSIS OF THE SALT OF A STRONG ACID AND A WEAK BASE
The ions of this type of salt will hydrolyze to form an acidic solution. The salt
ammonium chloride reacts as follows:
+
+
-
NH4 + Cl- + HOH <====> NH3 + H3O + Cl
In this case, the ammonium cation is a Bronsted-Lowry acid (proton donor). This
salt is classified as an acidic salt.
9-19. HYDROLYSIS OF THE SALT OF A WEAK ACID AND A WEAK BASE
The ions of this type of salt will hydrolyze to form a solution that may be acidic,
basic, or neutral, depending on the strength (ionization constant) of the weak acid and
weak base that formed the salt. If the ionization constant of the weak acid is greater
MD0837
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