Acids donate a proton, or hydrogen ion (H+), when dissolved in an aqueous solution. Oxyacids (or oxoacids) are a class of acids whose molecular structure specifically includes oxygen atoms. An oxyacid is composed of hydrogen, oxygen, and at least one other element, usually a nonmetal atom.
Defining the Core Structure
The defining characteristic of an oxyacid is the X-O-H arrangement, where X represents the central nonmetal atom (e.g., sulfur, nitrogen, or phosphorus). This configuration distinguishes oxyacids from binary acids, such as hydrochloric acid (HCl), which contain only hydrogen and one other element. The acidic hydrogen atom is always bonded directly to an oxygen atom, forming a hydroxyl group (-OH). When the oxyacid dissolves in water, the O-H bond breaks, releasing the H+ ion.
The central atom X does not directly release the acidic proton. Instead, X influences acidity through its electronegativity and the number of non-hydrogen oxygen atoms attached to it. These additional oxygen atoms pull electron density away from the O-H bond, weakening it and making the hydrogen easier to release. The overall properties of the oxyacid, including its strength, are dictated by the nature of this central X atom.
Rules of Nomenclature
The naming system for oxyacids is systematic and directly correlates with the oxidation state of the central nonmetal atom X. This nomenclature avoids the prefix “hydro-” used in binary acids and instead relies on suffixes and prefixes derived from the name of the corresponding polyatomic ion. The foundation of the naming rule is based on the polyatomic anion that remains after the acid releases its H+ ions.
If the polyatomic ion ends in “-ate,” the corresponding acid replaces the suffix with “-ic acid” (e.g., the sulfate ion forms sulfuric acid (H2SO4)). If the polyatomic ion ends in “-ite,” the acid name replaces the suffix with “-ous acid” (e.g., the sulfite ion forms sulfurous acid (H2SO3)).
When an element forms more than two oxyacids, prefixes are used to indicate the relative oxidation state or number of oxygen atoms. Chlorine is a common example, forming four different oxyacids. The prefix “per-” is used for the highest oxidation state, pairing with the “-ic acid” suffix, such as perchloric acid (HClO4). The prefix “hypo-” is used for the lowest oxidation state, pairing with the “-ous acid” suffix, resulting in hypochlorous acid (HClO). The intermediate compounds are chloric acid (HClO3) and chlorous acid (HClO2).
Practical Examples and Uses
Oxyacids are among the most heavily produced and widely used chemicals globally, serving as fundamental materials across numerous industries. Sulfuric acid (H2SO4) is often cited as the most important industrial chemical because its production volume is an indicator of a nation’s industrial strength. It is extensively used in the manufacture of phosphate-based fertilizers, as well as in petroleum refining and the processing of metals. The compound’s dehydrating and oxidizing properties make it valuable for a vast array of chemical syntheses.
Nitric acid (HNO3) is another economically significant oxyacid, primarily used in the production of ammonium nitrate for agricultural fertilizers. Its strong oxidizing nature also makes it an ingredient in the manufacture of explosives, such as trinitrotoluene (TNT), and in various processes for etching metals. Due to its reactivity, nitric acid is also employed in the synthesis of organic dyes and pharmaceutical compounds.
Phosphoric acid (H3PO4), which is weaker than sulfuric or nitric acid, plays an important role in commerce. It is a major component in the production of detergents and is used in the creation of phosphate salts for various applications. Phosphoric acid is also widely used in the food and beverage industry as an acidulant, providing the sharp, tangy flavor found in many soft drinks.