Introduction:

Oxoacids are the acid in which ionisable hydrogen atoms are joined to the central atom via oxygen atoms, example - Cl-OH, N, P, As and Sb form a number of oxoacids and as well do S, Se and Te in Group 16, and Cl, Br and I in group 17. Let us, thus, first familiarize ourselves by the nomenclature of oxoacids.

To differentiate between the oxidation states of the central atom in oxoacids, suffixes, 'ous' and 'ic' are utilized. The acids in which the central atom is in the lower oxidation state are known as ous acids, while those having central atom in the higher oxidation state are known as ic acids. The oxoacids containing halogens in their highest oxidation states are known as peracids. In  hypo-ous  and  hypo-ic  acids  the  oxidation  state  of  the central atom is lower than that in ous and ic acids, correspondingly.

The prefixes ortho, meta and pyro are employed to differentiate acids differing in the content of water. The most highly hydroxylated acid of the element in a specific oxidation state is known as the ortho acid. The acid that consists of one water molecule less than the ortho acid is known as meta acid. The pyro acid corresponds to the loss of one water molecule between the two molecules of the ortho acid.

Oxoacids of Nitrogen:

Nitrogen forms a number of oxoacids. Most of them are recognized only in the aqueous solutions or as their salts. We will now discuss here the two better identified oxoacids, that is, nitrous acid and nitric acid.

Nitrous Acid, HNO₂:

This is an unstable, weak acid that is known only in the aqueous solution. This can be obtained by acidifying an aqueous solution of a nitrite or via passing an equimolar mixture of NO and NO₂ into water: 

Ba(NO₃)₂ + H₂SO₄ → 2HNO₂ + BaSO₄ ↓

NO + NO₂ + H₂O → 2HNO₂

On attempting to concentrate, the acid decomposes:

3HNO₂ → HNO₃ + 2NO + H₂O

Nitrous acid and nitrites are very good oxidising agents and convert iodides to iodine, ferrous salts to ferric, stannous to stannic and sulphites to the sulphates, example:

2KI + 2HNO₂ + 2HCl → 2H₂O + 2NO + 2KO + I₂

By strong oxidising agents, such as KMnO₄, nitrous acid and nitrites function as the reducing agents and get oxidized to NO₃ ions:

2KMnO₄ + 5KNO₂ + 6HCl → 2MnCl₂ + 5KNO₃ + 3H₂O + 2KCl

The structures of nitrous acid and nitrite ion are illustrated in the figure below:

Fig: Structures of nitrous acid and nitrite ion

Nitrite ion is a good coordinating agent. Both the nitrogen and oxygen contain lone pairs able of making coordinate bond by metal ions. Nitrite ion can coordinate either via N or via O. Therefore, isomerism takes place between M←NO₂ and M←ONO structures. The analogous organic derivatives are as well identified, the nitrites, R←ONO and the nitro compounds, R-NO₂ here R is any alkyl or aryl group. These ligands which can coordinate in two different manners are known as ambidentate.

Nitric Acid, HNO₃:

Nitric acid is one of the main acids of modern chemical industry. It has been familiar as the corrosive solvent for metals since 13^th century. HNO₃ is now almost exclusively prepared by Ostwald process. In this method NH₃ is catalytically oxidized to give NO:

4NH + 5O₂ + (Pt-Rh catalyst) → 4NO + 6H₂O, ΔH = -904 kJ mol^-1

In the above reaction, around 96-98% of NH₃ is transformed into NO. As, the reaction is exothermic, reaction temperature can be maintained devoid of external heating provided a heat exchanger is employed. The mixture of gases is cooled and diluted by air. NO joins with O₂ to provide NO₂ which is absorbed in water to give HNO₃ and NO, which is then recycled. The given equations represent different steps in this procedure:

2NO + O₂ → 2NO₂

3NO₂ + H₂O → 2HNO₃ + NO

Nitric acid can be concentrated to 68% through distillation, when a constant boiling mixture is prepared. More concentrated acid can be made via distilling the mixture with concentrated sulphuric acid.

Pure nitric acid is a colourless liquid (boiling point 359 K); it decomposes readily in light providing a yellow colour due to the formation of nitrogen dioxide. It is a strong acid and is almost fully dissociated into ions in solution. It reacts with metals and by metal oxides, hydroxides and carbonates making salts known as nitrates.

The reaction of HNO₃ with metals is of, particular interest due to the great variety of products obtained, example: H2, N2, NH4NO3, N2O, NO and NO2 in addition to the nitrate or oxide of the metal. The nature of products made based on the nature of the metal and reaction conditions, such as concentration of acid and temperature.

By having very dilute acid, magnesium and manganese give hydrogen:

Mg + 2HNO₃ → Mg(NO₃)₂ + H₂ ↑

Other metals such as Zn, Sn, Fe, and so on that as well lie above the hydrogen in electrochemical series discharge hydrogen from dilute nitric acid. However, as nitric acid is a strong oxidising agent and hydrogen a reducing agent, secondary reactions take place resultant in the reduction of nitric acid to NH₃, N₂O or N₂. Therefore, Zn reacts by dilute HNO₃ in cold giving N₂O or N₂ according to the given equations:

Zn + 2HNO₃ → Zn(NO₃)₂ + 2H] x 4

2HNO₃ + 8H → N₂O + 5H₂O

4Zn + 10HNO₃ → 4Zn(NO₃)₂ + N₂O + 5H₂O

Zn + 2HNO₃ → Zn(NO₃)₂ + 2H] x 5

2HNO₃ + 10H → N₂ + 6H₂O

5Zn + 12HNO₃ → 5Zn(NO₃)₂ + N₂ + 6H₂O

Extremely dilute HNO₃ reacts with Zn to give NH₃, which is obviously neutralized to form NH₄NO₃:

Zn + 2HNO₃ → Zn(NO₃)₂ + 2H] x 4

HNO₃ + 8H → NH₃ + 3H₂O 

HNO₃ + NH₃ → NH₄NO₃

4Zn + 10HNO₃ → 4Zn(NO₃)₂ + NH₄NO₃ + 3H₂O

Likewise, iron and tin as well provide NH₄NO₃ with dilute HNO₃ in cold.

As we are familiar that metals like Cu, Bi, Hg, Ag and so on, lying beneath hydrogen in the electrochemical series don't discharge hydrogen from acids. With such metals, the action of nitric acid comprises the oxidation of metals into  the  metallic  oxides  that  dissolve  in  the  acid  to  form  nitrates accompanied through evolution of NO or NO₂ based on whether the acid is dilute or concentrated. For example - Cu, Ag, Hg and Bi react with dilute acid discharging NO:

3Cu + 2HNO₃ → 3CuO + 2NO + H₂O

CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O] x 3

3Cu + 8HNO₃ → 3Cu(NO₃)₂ + 2NO + 4H₂O

By concentrated acid NO₂ is given off:

Cu + 2HNO₃ → CuO + 2NO₂ + H₂O

CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O

Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + H₂O

Concentrated nitric acid works essentially as an oxidising agent and metals such as Al, Fe, Cr, and so on, are rendered passive due to the formation of a layer of insoluble oxide on the metal surface.

Noble metals such as Au, Pt, Rh and Ir are not attacked via nitric acid. Though, a 1:3 mixture of concentrated HNO₃ and concentrated HC1 termed as aqua regia, dissolves Au and Pt as it includes free chlorine:

HNO₃ + 3HCl → 2H₂O + 2Cl + NOCl

Au + 3Cl + HCl → HAuCl₄

Pt + 4Cl + 2HCl → H₂PtCl₆

Concentrated HNO₃ readily oxidizes the solid non-metals and metalloids to their corresponding oxoacids or hydrated oxides. Therefore, P, As, Sb, S, I and Sn are oxidized to phosphoric acid (H₃PO₄), arsenic acid (H₃AsO₄), antimony pentoxide (Sb₂O₅), sulphuric acid (H₂SO₄), iodic acid, (HIO₃) and metastannic acid (H₂SnO₃), correspondingly, example:

2HNO₃ → 2NO₂ + H₂O + O] x 10

P₄ + 10O → P₄O₁₀

P₄O₁₀ + 6H₂O → 4H₃PO₄

P₄ + 20HNO₃ → 4H₃PO₄ + 20NO₂ + 4H₂O

Dilute nitric acid as well acts as an oxidising agent. Therefore, reducing agents, like, H₂S, HI and FeSO₄ are oxidized to S, I₂ and Fe₂(SO₄)₃; correspondingly: 

2HNO₃ → 2NO + H₂O + 3O

H₂S + O → S + H₂O] x 3

2HNO₃ + 3H₂S → 2NO + 4H₂O + 3S

Nitric acid as well oxidizes numerous organic compounds. This converts a mixture of cyclohexanol and cyclohexanone to adipic acid which is the primary material for nylon polymers. Nitric acid also oxidized p-xylene to terephthalic acid which is employed for the preparation of Terylene.

Concentrated nitric acid, in the presence of concentrated sulphuric acid, reacts by certain aromatic compounds making nitro compounds, example: benzene is transformed to nitrobenzene.

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

This method termed as nitration is of great industrial significance due to its usefulness of nitro compounds. Nitronium ion, NO₂⁺, which is made up in the presence of the concentrated H₂SO₄, is assumed to be the active nitrating agent.

HNO₃ + H₂SO₄ ↔ NO₂ + HSO₄ + H₂O

The molecular structure of nitric acid is as illustrated in the figure given below. In nitric acid the nitrogen and the three oxygen atoms arc coplanar. Terminal N-O bonds are equivalent; the other N-O bond is much longer and corresponds to the single bond.

Fig: Molecular and resonance structure of HNO₃

The nitrate ion is planar by equivalent N-O bonds. Its structure can be symbolized as a resonance hybrid as illustrated in the figure given below.

Fig: Resonance Structures of Nitrate ion

Oxoacids of Phosphorus:

Phosphorus, arsenic and antimony as well prepare a number of oxoacids. Oxoacids of arsenic and antimony are not well characterized. Their salts, though, are acknowledged. Phosphorus forms two series of oxoacids, the phosphoric and the phosphorous acids. The oxidation state of phosphorus is +5 in the phosphoric acids while it is +3 in the phosphorous acids. We might observe that hypo phosphorous and phosphorous acids have direct P-H bond(s) too. Though, this P-H bond is not ionisable, it doesn't give H⁺, and therefore, it doesn't confer acidity. These acids are, thus, monobasic and dibasic, correspondingly.

A great number of condensed phosphoric acids or their salts are known which encompass rings or chains of PO₄ tetrahedra linked altogether via P- O-P linkages, example - di or pyrophosphoric acid (H₄P₂O₇) and triphosphoric acid (H₅P₃O₁₀).

Sodium salt of triphosphoric acid, Na₅P₃O₁₀, makes stable chelate complexes by alkaline earth metal cations. It is, thus, used in water softening. What is recognized as metaphosphoric acid and given the empirical formula HPO₃ is however a mixture of cyclo-polyphosphoric acids having a -P-O-P-O- linkages. Two significant cyclo-polyphosphoric acids are cyclotriphosphoric acid (H₃P₃O₉) and cyclo-tetraphpsphoric acid (H₄P₄O₁₂).

Salts of cyclo-polyphosphoric acids having 3-8 phosphorus atoms are identified. Sodium cyclo-hexaphosphate, Na₆P₆O₁₈·6H₂O, termed as calgon is a helpful compound. It forms soluble complexes by alkaline earth metal cations. Therefore, it is employed in water softening.

The phosphate link, P-O-P, is very significant in biological systems, as it is assumed to be the prime store of energy. The energy of the bond (29 kJ mol^-1) is discharged to the system whenever required, by enzyme catalyzed hydrolysis of the phosphate link in adenosine triphosphate (ATP), the high energy molecule.

Tutorsglobe: A way to secure high grade in your curriculum (Online Tutoring)

Expand your confidence, grow study skills and improve your grades.

Since 2009, Tutorsglobe has proactively helped millions of students to get better grades in school, college or university and score well in competitive tests with live, one-on-one online tutoring.

Using an advanced developed tutoring system providing little or no wait time, the students are connected on-demand with a tutor at www.tutorsglobe.com. Students work one-on-one, in real-time with a tutor, communicating and studying using a virtual whiteboard technology.  Scientific and mathematical notation, symbols, geometric figures, graphing and freehand drawing can be rendered quickly and easily in the advanced whiteboard.

Free to know our price and packages for online Chemistry tutoring. Chat with us or submit request at [email protected]