weak acid
H+ +ion
A. Equilibrium Considerations
The importance of the equilibria involved in the ionization of water and of weak electrolytes in water cannot be underestimated. Whenever water is
present, whether in the ocean, in a raindrop, or in blood, hydrogen and hydroxide ions are present in amounts that satisfy the equation
Kw = [H+][OH-] = 1.0 X 10-14
Whenever an ion present in water is related through ionization to a weak acid, that equilibrium, too, must be satisfied. For example, whenever acetate ion is present in water, the equilibrium
| HC2H3O2 weak acid |
H+ + |
C2H3O2- ion |
is present. The concentrations of hydrogen ion, acetate ion, and acetic acid molecules must satisfy the acid dissociation constant for acetic acid:
| Ka = | [H+][C2H3O2-] [HC2H3O2] |
= 1.8 X 10-5 |
In a solution of sodium acetate, the hydrogen ion concentration must satisfy two equilibrium constants: the ion product of water (Kw) and the acid dissociation constant of acetic acid (Ka). The hydrogen ion concentration of pure water is decreased by the amount of hydrogen ions that react with acetate ions to satisfy the acid dissociation constant of acetic acid. This decrease in the concentration of hydrogen ions means that the solution is no longer neutral (pH > 7) but now contains an excess of hydroxide ions and is basic (pH 7).
Similarly, if the cation of the salt is ammonium ion, NH4+, its equilibrium reaction in solution
NH4+ + H2O
produces hydronium ions and the solution is acidic (pH < 7).
B. Brønsted-Lowry Considerations
When a salt dissolves in water, it dissociates into ions. If the anion is a strong Brønsted-Lowry base, such as the acetate ion, or the cation is a weak Brønsted-Lowry acid, such as the ammonium ion, there is an acid-base reaction with water. The reaction is called hydrolysis.
The reaction of the anion base would produce hydroxide ions as the acetate ion does in the following equation:
C2H3O2- + H2O
If the cation of the salt is a weak Brønsted-Lowry acid, like ammonium ion, the reaction with water produces hydronium ions and the solution is acidic:
NH4+ + H2O
|
Example: Write equations for the hydrolysis of the appropriate ion of the following compounds: a. sodium carbonate Predict whether a solution of each is acidic, basic, or neutral. Justify you decision by considering the equilibri involved. Solution a. Sodium in is neither a weak acid nor a strong base. Carbonate ion is a
strong base (the anion of a weak acid). The equation for its hydrolysis
is b. A solution of ammonium cloride contains ammonium and chloride ions. Ammonium ion is a weak acid; chloride ion is a weak base. Weak acids will react with water to produce hydronium ion and an acidic solution: A solution of potassium nitrate contains potassium and ntrate ions. Potassium in not a weak acid; nitrate ion is a weak base. Neither ion will hydrolyze; the solution is neutral. The only equilibrium present is that of water with its ions: |
HCO3- + OH-