III. Inorganic acids

The naming of inorganic acids does not follow the rules of the ionic compounds or covalent compounds. For example, HNO₃ is called nitric acid, not hydrogen nitrate nor hydrogen nitrogen trioxide. How can one recognize an acid by looking at its chemical formula? You will learn about the properties of acids in detail in the second semester of general chemistry. Here we will simply present the rules for naming acids. An acid is a proton donor. Therefore, for the purpose of nomenclature, an acid can be viewed as a molecule with one or more protons (H⁺) bonded to an anion. Note that the molecule must not carry a charge. For example, HSO₃^- is not an acid molecule; it is an anion because it carries a - 1 charge. Even though it shows acidic properties, it is named like a polyatomic anion. Also, the molecule must not contain metal atoms. For example, NaHSO₃, should not be named as an acid. Instead, it should be named as an ionic compound because it consists of a Na⁺ cation and an HSO₃^- anion. Thus, it is named sodium bisulfite or sodium hydrogen sulfite.

Many acids consist of protons bonded to an oxoanion (e.g. HNO₃ is H⁺ bonded to NO₃^- and H₂SO₄ is two H⁺s bonded to a SO₄^2- ). These acids are called oxoacids. To name an oxoacid, one should change the - ate or - ite suffixes of the oxoanions to - ic or - ous respectively and add the word acid at the end. For example,

HNO₃ is H⁺ bonded to NO₃^- (nitrate), thus it is called nitric acid.

HNO₂ is H⁺ bonded to NO₂^- (nitrite), thus it is called nitrous acid.

Besides the oxoacids, there are other acids in which the anions end with the suffix -ide. The names of these acids begin with hydro- and end with -ic. For example, aqueous HCl is called hydrochloric acid because the anion, Cl^- , is named chloride.

The names of the inorganic acids are closely related to the names of the anions in the acid. The correlations of the names of the anions and the names of the acids is summarized in Table V below with examples:

Note: The gaseous HCl, HBr, H₂S etc. do not bear the names of acids. They are named as covalent compounds. A compound that dissolves in water to form an acid is called an acid anhydride (acid without water). Only the aqueous solutions of acid anhydrides are named as acids. Therefore, HCl(g) is called hydrogen chloride while HCl(aq) is called hydrochloric acid; HCN(g) is called hydrogen cyanide while HCN(aq) is called hydrocyanic acid. The distinction in naming the anhydrides and the acids is not critical for oxoacids, because all their anhydrides are different molecules. For example, the anhydride of H₂SO₄ is SO₃, not gaseous H₂SO₄. Thus H₂SO₄ is always called sulfuric acid, not hydrogen sulfate.

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