Reactions in Aqueous Solution
Interactive Student Tutorial

 4.5 Acids, Bases, and Neutralization Reactions

Acids and bases were introduced in the Interactive Student Tutorial section of Chapter 2.9. In your study of chemistry, you will encounter several systems used to classify the acid and base properties of substances. One system that is used widely was proposed in 1887 by the Swedish chemist Svante Arrhenius (1859–1927). The Arrhenius system classifies acids as substances that dissociate in water to produce hydrogen ions (H+), and bases as substances that dissociate in water to yield hydroxide ions (OH). The general formula HA is commonly used to represent an acid, and MOH to represent a metal hydroxide.

The hydrogen ion H+(aq) does not exist as such in aqueous solutions. Hydrogen ions combine with water molecules to give a more stable species, the hydronium ion H3O+, as demonstrated in the following equation:

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

Acids dissociate to different extents in aqueous solution. Acids that dissociate to a large extent are strong electrolytes and strong acids. In contrast, acids that dissociate only to a small extent are weak acids and weak electrolytes.

Acids can contain different numbers of acidic hydrogens, and can yield different numbers of H3O+ ions in solution.

 Introduction to Aqueous Acids

In a similar manner, bases can be strong or weak depending on the extent to which they dissociate and produce OH ions in solution. Most metal hydroxides are strong electrolytes and strong bases. Ammonia, NH3, is a weak electrolyte and weak base.

 Introduction to Aqueous Bases

Table 4.2 summarizes the names, formulas, and classifications of some common acids and bases.

When an acid and a base are mixed in stoichiometric proportions, their acidic and basic properties disappear as the result of a neutralization reaction.

Because the salts that form in neutralization reaction are generally strong electrolytes, we can write the neutralization reaction as an ionic equation.

When the spectator ions are removed, the net ionic equation is revealed.

This net ionic equation is the same for the neutralization reaction of any strong acid and strong base.

For the reaction of a weak acid with a strong base, a similar neutralization occurs. Consider the neutralization of HF with KOH.

HF(aq) + KOH(aq) KF(aq) + H2O(l) molecular equation
HF(aq) + (aq) + OH-(aq) (aq) + F-(aq) + H2O(l) ionic equation
HF(aq) + OH-(aq) F-(aq) + H2O(l) net ionic equation

The weak acid HF is written as a molecular formula because its dissociation is incomplete.