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 13th 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:
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.
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