4.2 Classifying chemical reactions  (Page 4/34)

Consider as an example the dissolution of lye (sodium hydroxide) in water:

This equation confirms that sodium hydroxide is a base. When dissolved in water, NaOH dissociates to yield Na ⁺ and OH ⁻ ions. This is also true for any other ionic compound containing hydroxide ions. Since the dissociation process is essentially complete when ionic compounds dissolve in water under typical conditions, NaOH and other ionic hydroxides are all classified as strong bases.

Unlike ionic hydroxides, some compounds produce hydroxide ions when dissolved by chemically reacting with water molecules. In all cases, these compounds react only partially and so are classified as weak bases . These types of compounds are also abundant in nature and important commodities in various technologies. For example, global production of the weak base ammonia is typically well over 100 metric tons annually, being widely used as an agricultural fertilizer, a raw material for chemical synthesis of other compounds, and an active ingredient in household cleaners ( [link] ). When dissolved in water, ammonia reacts partially to yield hydroxide ions, as shown here:

This is, by definition, an acid-base reaction, in this case involving the transfer of H ⁺ ions from water molecules to ammonia molecules. Under typical conditions, only about 1% of the dissolved ammonia is present as ${\text{NH}}_4{}^{+}$ ions.

The chemical reactions described in which acids and bases dissolved in water produce hydronium and hydroxide ions, respectively, are, by definition, acid-base reactions. In these reactions, water serves as both a solvent and a reactant. A neutralization reaction    is a specific type of acid-base reaction in which the reactants are an acid and a base, the products are often a salt    and water, and neither reactant is the water itself:

To illustrate a neutralization reaction, consider what happens when a typical antacid such as milk of magnesia (an aqueous suspension of solid Mg(OH) ₂ ) is ingested to ease symptoms associated with excess stomach acid (HCl):

Note that in addition to water, this reaction produces a salt, magnesium chloride.