21.4. Acid-Base Neutralization http://www.ck12.org
FIGURE 21.9
Over-the-counter antacids are bases that
are used to neutralize excess acid in the
stomach.Strong Acid-Strong Base Reactions
When equal amounts of a strong acid such as hydrochloric acid are mixed with a strong base such as sodium
hydroxide, the result is a neutral solution. In other words, the products of the reaction do not have the characteristics
of either an acid or a base. Here is the balanced molecular equation.
HCl(aq)+NaOH(aq)→NaCl(aq)+H 2 O(l)Aneutralization reactionis a reaction in which an acid and a base react in an aqueous solution to produce a
salt and water. Note that the neutralization reaction above is an example of a double-replacement reaction. The
aqueous sodium chloride that is produced in the reaction is called a salt. Asaltis an ionic compound composed of a
cation from a base and an anion from an acid. However, because essentially any anion or cation can be paired with
hydrogen or hydroxide ions to make an acid or base, the term salt can be used to describe any ionic compound, and
the two terms are often used interchangeably.
Recall that chemical reactions occurring in aqueous solution can often be written as a net ionic equation. The full
ionic equation for the neutralization of hydrochloric acid with sodium hydroxide is written as follows:
H+(aq)+Cl−(aq)+Na+(aq)+OH−(aq)→Na+(aq)+Cl−(aq)+H 2 O(l)
Since the acid and base are both strong, they are fully ionized, so they can be written as ions. Because the NaCl
product is also water-soluble, it stays dissociated. The sodium and chloride ions are spectator ions, leaving the
following net ionic reaction:
H+(aq)+OH−(aq)→H 2 O(l)All neutralization reactions between a strong acid and a strong base simplify to this net ionic equation.
What if the acid is a diprotic acid, such as sulfuric acid? The balanced molecular equation now involves a 1:2 ratio
between acid and base.
H 2 SO 4 (aq)+2NaOH(aq)→Na 2 SO 4 (aq)+2H 2 O(l)In order for the reaction to be a full neutralization, twice as many moles of NaOH must react with the H 2 SO 4. The
sodium sulfate salt is soluble, and so the net ionic reaction is again the same. Different mole ratios can occur when
using polyprotic acids or bases that include multiple hydroxide ions per formula unit, such as Ca(OH) 2.