Acids are a fundamental class of chemical compounds, recognized by their ability to donate a hydrogen ion (\(\text{H}^+\)) when dissolved in an aqueous solution. Systematic chemical nomenclature provides a clear, uniform method for identifying these substances based on their molecular composition. The specific rules used depend primarily on whether the acid contains oxygen, which dictates the prefixes and suffixes applied to the non-hydrogen component. This structured approach helps differentiate between chemically similar yet distinct substances.
Naming Binary Acids (Acids Without Oxygen)
The simplest acids consist of hydrogen and only one other nonmetallic element, such as a halogen, and are known as binary acids. These compounds follow a straightforward naming convention that always begins with the prefix hydro-. This prefix specifically denotes the absence of oxygen in the acid’s structure.
The name is completed by taking the root name of the second element and attaching the suffix -ic acid. For example, the compound \(\text{HCl}\) is named hydrochloric acid. Similarly, \(\text{HF}\) is named hydrofluoric acid, and \(\text{HBr}\) becomes hydrobromic acid.
Even acids containing sulfur, such as \(\text{H}_2\text{S}\), adhere to this pattern, though the root is extended to include the full element name for better pronunciation. The resulting name is hydrosulfuric acid, rather than simply “hydrosulfic acid”.
Naming Oxyacids Based on Polyatomic Ions
Acids that contain oxygen (oxyacids) are derived from polyatomic ions that already contain oxygen. The naming convention depends entirely on the suffix of the polyatomic anion to which the hydrogen is attached. The hydro- prefix used for binary acids is never used for oxyacids.
If the polyatomic ion ends with the suffix -ate, the acid’s name is formed by changing this suffix to -ic acid. For instance, the sulfate ion (\(\text{SO}_4^{2-}\)) forms sulfuric acid (\(\text{H}_2\text{SO}_4\)), and the nitrate ion (\(\text{NO}_3^-\)) forms nitric acid (\(\text{HNO}_3\)).
Conversely, if the polyatomic ion ends with the suffix -ite, the acid’s name is constructed by changing this suffix to -ous acid. Therefore, the sulfite ion (\(\text{SO}_3^{2-}\)), which has one less oxygen atom than sulfate, forms sulfurous acid (\(\text{H}_2\text{SO}_3\)). Similarly, the nitrite ion (\(\text{NO}_2^-\)) yields nitrous acid (\(\text{HNO}_2\)).
Handling Complex Oxyacids (Prefixes and Suffixes)
Certain elements, particularly halogens like chlorine, can form a series of four different oxyacids, each distinguished by a varying number of oxygen atoms. To differentiate these compounds, the standard -ic and -ous suffixes are combined with the prefixes per- and hypo-. These prefixes are retained directly from the naming of the corresponding polyatomic ion.
The prefix per- is used to denote the acid with one more oxygen atom than the standard -ic acid form. For example, perchloric acid (\(\text{HClO}_4\)) is derived from the perchlorate ion (\(\text{ClO}_4^-\)), which has one more oxygen than chloric acid (\(\text{HClO}_3\)).
The prefix hypo- is used to designate the oxyacid with the fewest oxygen atoms, specifically one less than the -ous acid form. The series progresses from the standard chloric acid (\(\text{HClO}_3\)) to chlorous acid (\(\text{HClO}_2\)), and then to hypochlorous acid (\(\text{HClO}\)). Hypochlorous acid is derived from the hypochlorite ion (\(\text{ClO}^-\)). Its hypo- prefix combined with the -ous suffix indicates the lowest oxygen count and lowest oxidation state in the complete four-acid family.