Oxyacid
An oxyacid, also known as an oxoacid, is a compound that contains oxygen, at least one other element, and hydrogen atoms bound to oxygen, from which it can lose hydrons to form a conjugate base; this distinguishes it from hydracids like HCl, which lack oxygen in the acidic group.[1] These acids typically follow the general formula \ce{H_mXO_n}, where \ce{X} is a central atom—usually a nonmetal or early transition metal—and m and n indicate the number of hydrogen and oxygen atoms, respectively, with the acidic hydrogens attached to oxygen atoms.[2] Common examples of oxyacids include sulfuric acid (\ce{H2SO4}), nitric acid (\ce{HNO3}), phosphoric acid (\ce{H3PO4}), and perchloric acid (\ce{HClO4}), many of which are derived from the reaction of nonmetallic oxides with water and play crucial roles in industrial processes, biological systems, and laboratory chemistry.[1][2] Oxyacids are often polyprotic, meaning they can donate multiple protons, with successive dissociation constants decreasing due to the increasing stability of the conjugate bases.[2] Naming of oxyacids follows systematic conventions based on the central atom and the number of oxygen atoms relative to the highest oxidation state; for instance, acids with fewer oxygen atoms end in -ous acid (e.g., \ce{H2SO3} as sulfurous acid), while those with more end in -ic acid (e.g., \ce{H2SO4} as sulfuric acid), with prefixes like hypo- for the lowest and per- for the highest oxygen content.[3] Their acidity trends are influenced by the electronegativity of the central atom—higher electronegativity increases acidity (e.g., \ce{HOCl} > \ce{HOBr} > \ce{HOI})—and the number of oxygen atoms attached to it, as additional oxygens stabilize the conjugate base through inductive effects (e.g., \ce{HClO4} > \ce{HClO3} > \ce{HClO2} > \ce{HClO}).[2]Definition and Nomenclature
Definition
Oxyacids, also known as oxoacids or oxygen acids, are acids containing oxygen in the acidic group, specifically compounds with at least one hydrogen atom bound to oxygen, which is further connected to a central atom—typically a nonmetal, metalloid, or early transition metal.[1] These acids produce their conjugate base, an oxoanion, upon dissociation by losing one or more hydron ions (H⁺).[1] The general structural formula for oxyacids is often expressed as H_m XO_n, where X represents the central atom, and m and n are positive integers denoting the number of hydrogen and oxygen atoms, respectively.[4] In these structures, the acidic hydrogens are directly attached to oxygen atoms, enabling ionization in aqueous solutions.[4] The term "oxyacid" is primarily used for inorganic acids and differs from binary acids (hydracids), such as hydrochloric acid (HCl), which consist solely of hydrogen and a single nonmetal element without oxygen in the acidic group. Although the structural definition could apply to some organic acids like carboxylic acids (which have oxygen in the acidic functional group), such compounds are conventionally classified as organic acids rather than oxyacids.[1][4] The term "oxyacid" originated in the early 19th century (first recorded 1830–1840), during a period of advancing chemical understanding that included the formulation of acid-base theory by Svante Arrhenius in the 1880s.[5] Common examples include sulfuric acid (H₂SO₄) and nitric acid (HNO₃).[4]Nomenclature
The nomenclature of oxyacids follows the recommendations of the International Union of Pure and Applied Chemistry (IUPAC), which provide both systematic and retained traditional names to reflect the oxidation state of the central atom and the number of oxygen atoms present.[6] In the traditional system, preferred for common use, the name is derived from the root of the central atom, with suffixes indicating the oxidation state: the "-ous" suffix denotes a lower oxidation state (fewer oxygen atoms), while the "-ic" suffix denotes a higher oxidation state (more oxygen atoms).[6] Prefixes modify these when multiple oxidation states exist: "hypo-" indicates the lowest state, and "per-" the highest.[6] Systematic names, less commonly used, employ additive nomenclature based on coordination entities, such as "tetraoxidosulfate(2−) with 2H" for sulfuric acid.[6] Common naming patterns illustrate these rules across element families. For sulfur oxyacids, the lower oxidation state compound H_2SO_3 is named sulfurous acid (+4 oxidation state), while the higher state H_2SO_4 is sulfuric acid (+6 oxidation state).[6] Similarly, chlorine oxyacids progress with increasing oxygen and oxidation state: HClO as hypochlorous acid (+1), HClO_2 as chlorous acid (+3), HClO_3 as chloric acid (+5), and HClO_4 as perchloric acid (+7).[6] These patterns ensure names convey the relative oxygen content and reactivity trends associated with oxidation states.[6] The corresponding oxyanions are named by replacing the acid suffixes with "-ate" or "-ite": for example, the anion from sulfuric acid, SO_4^{2-}, is sulfate, while from sulfurous acid, SO_3^{2-}, it is sulfite.[6] Prefixes carry over similarly, yielding hypochlorite (ClO^-) from hypochlorous acid and perchlorate (ClO_4^-) from perchloric acid.[6] This anion nomenclature extends to salts and other derivatives, maintaining consistency with the parent acid.[6] Certain oxyacids retain traditional or trivial names despite available systematic alternatives, as approved by IUPAC for historical and practical reasons. For instance, HNO_3 is universally called nitric acid, a retained name, rather than the additive form "trioxonitrate(1−) with H."[6] Other retained examples include phosphoric acid (H_3PO_4) and carbonic acid (H_2CO_3), which prioritize familiarity in scientific and industrial contexts.[6] These exceptions are listed in IUPAC tables to guide consistent usage.[6]Properties
Physical Properties
Oxyacids exhibit a range of physical states at room temperature, primarily as liquids or solids, depending on their molecular structure and intermolecular forces. Common examples include nitric acid (HNO₃), which appears as a fuming, pale yellow to reddish-brown liquid with a suffocating odor, and sulfuric acid (H₂SO₄), a colorless, viscous, oily liquid.[7][8] Pure phosphoric acid (H₃PO₄) is a transparent crystalline solid, though it is typically handled as a concentrated aqueous solution that remains liquid at room temperature. Perchloric acid (HClO₄) is also a clear, colorless liquid in its concentrated form.[9][10] Most oxyacids are highly soluble in water, owing to extensive hydrogen bonding between their hydroxyl groups and water molecules, often resulting in miscibility. For instance, sulfuric acid is completely miscible with water, releasing significant heat upon dilution, while nitric acid is similarly fully miscible. Many oxyacids form azeotropic mixtures with water, which complicates their purification by distillation; sulfuric acid forms a maximum-boiling azeotrope at approximately 98.3 wt% H₂SO₄, and nitric acid at 68 wt% HNO₃.[8][7][11][12] The melting and boiling points of oxyacids show trends influenced by molecular weight, the number of hydrogen bonds, and overall polarity, with higher values generally observed for those capable of stronger intermolecular interactions. Nitric acid has a relatively low boiling point of 83 °C and melting point of -42 °C, whereas sulfuric acid boils at 337 °C with a melting point of 10 °C, reflecting its greater viscosity and hydrogen-bonding capacity. Phosphoric acid melts at 42 °C, and perchloric acid at -18 °C, with the latter boiling at 203 °C. These properties establish the scale of thermal stability for handling and processing oxyacids.[7][8][9][10] Densities and viscosities among oxyacids vary significantly, often higher than those of simple binary acids due to their polar nature and molecular size. Sulfuric acid, for example, has a density of 1.84 g/cm³ at 20 °C and a viscosity of 21 mPa·s at 25 °C, contributing to its syrupy texture. In contrast, nitric acid has a lower density of 1.51 g/cm³ at 20 °C and viscosity of 0.75 mPa·s at 25 °C, making it more fluid. The following table summarizes key physical properties for representative oxyacids:| Oxyacid | State at 25 °C | Melting Point (°C) | Boiling Point (°C) | Density (g/cm³ at 20–25 °C) | Viscosity (mPa·s at 25 °C) |
|---|---|---|---|---|---|
| HNO₃ | Liquid | -42 | 83 | 1.51 | 0.75 |
| H₂SO₄ | Liquid | 10 | 337 | 1.84 | 21 |
| H₃PO₄ (85% aq.) | Liquid | ~21 | ~158 | 1.68 | ~40 |
| HClO₄ (70%) | Liquid | -18 | 203 | 1.67 | ~3.5 |