http://www.ck12.org Chapter 21. Acids and Bases
Reactions Involving a Weak Acid or Weak Base
Reactions where at least one of the components is weak do not generally result in a neutral solution. The reaction
between weak nitrous acid and strong potassium hydroxide is shown below.
HNO 2 (aq)+KOH(aq)→KNO 2 (aq)+H 2 O(l)In order to write the net ionic equation, the weak acid must be written as a molecule, since it does not ionize to a
great extent in water. The base and the salt are fully dissociated.
HNO 2 (aq)+K+(aq)+OH−(aq)→K+(aq)+NO− 2 (aq)+H 2 O(l)
The only spectator ion is the potassium ion, resulting in the net ionic equation:
HNO 2 (aq)+OH−(aq)→NO− 2 (aq)+H 2 O(l)The strong hydroxide ion essentially “forces” the weak nitrous acid to become ionized. The hydrogen ion from
the acid combines with the hydroxide ion to form water, leaving the nitrite ion as the other product. The resulting
solution is not neutral (pH = 7) but, instead, is slightly basic. We will investigate why this is the case in the next
lesson.
Reactions can also involve a weak base and strong acid, resulting in a solution that is slightly acidic. The molecular
and net ionic equations for the reaction of hydrochloric acid and ammonia are shown below.
HCl(aq)+NH 3 (aq)→NH 4 Cl(aq)
H+(aq)+NH 3 (aq)→NH+ 4 (aq)(Cl−is spectator ion)Reactions between acids and bases that are both weak may result in solutions that are neutral, acidic, or basic.
Titrations
When hydrochloric acid is reacted with sodium hydroxide, an acid/base mole ratio of 1:1 is required for full
neutralization.
HCl(aq)+NaOH(aq)→NaCl(aq)+H 2 O(l)If instead the hydrochloric acid were reacted with barium hydroxide, the mole ratio would be 2:1.
2HCl(aq)+Ba(OH) 2 (aq)→BaCl 2 (aq)+2H 2 O(l)Two moles of HCl are required to completely neutralize one mole of Ba(OH) 2. In a balanced neutralization equation,
the moles of H+ions supplied by the acid will be equal to the moles of OH−ions supplied by the base. The
equivalence pointis the point in a neutralization reaction where the number of moles of hydrogen ions is equal to
the number of moles of hydroxide ions.
In the laboratory, you may need to determine the concentration of an acid or a base whose concentration is not
known. This can be accomplished by performing a controlled neutralization reaction. Atitrationis an experiment
in which a solution, whose concentration is known, is gradually added to a measured volume of another solution in
order to determine its concentration. Many titrations are acid-base neutralization reactions, although other types of
titrations can also be performed.
In order to perform an acid-base titration, the chemist must have a way to visually detect that the neutralization
reaction has reached the equivalence point. Anindicatoris a substance that has a distinctly different color when