Oxoacids, also known as ternary acids, are acids that have oxygen atoms. They contain oxygen and hydrogen, which are bonded to one other element. They have at least one -OH group bonded to the central atom. The central atom has one doubly bonded oxygen atom. Some common examples of oxoacids are sulphurous acid (H2SO3), nitrous acid (HNO2), chloric acid (HClO3) and pyrophosphoric acid (H4P2O7).

Oxoacids of Sulphur

Sulphur forms various oxoacids, but some are unstable. They’re usually found in aqueous solutions or the form of their salts. They typically have one S=O bond and one S-OH bond, showing tetrahedral structure. S atom shows a tetrahedral structure with respect to oxygen.

Some important oxoacids of sulphur

All oxoacids of sulphur are dibasic (basicity = 2)

Sulphuric Acid (H2SO4)

Another name for it is Oil of Vitriol. It is very corrosive.

Manufacture

It is manufactured by Contact Process, which involves four steps:

• Burning sulphur/sulphide ores to make SO2

            S + O2 → SO2

• Converting SO2 to SO3 by adding oxygen in the presence of vanadium pentoxide

2SO2 + O2 → 2SO3

• Formation of Oleum

SO3 + H2SO4 → H2S2O7

• Dilution of Oleum to give concentrated sulphuric acid

H2S2O7 + H20 → H2SO4

Physical properties of sulphuric acid

• It is colourless and dense.

1. Its boiling point is 611K. This is due to hydrogen bonding.
2. Its freezing point is 283K.

Chemical properties of sulphuric acid

1. It’s a powerful dehydrating agent.

1. It dehydrates sucrose to sugar charcoal.
2. It dehydrates formic acid to give carbon monoxide.
3. It dehydrates oxalic acid to give carbon monoxide and carbon dioxide.

2. It is a strong dibasic and diprotic acid.
3. It has low volatility.
4. It is an excellent oxidising agent and can oxidise both metals and non-metals.

Oxoacids of Chlorine

Chlorine forms four types of oxoacids. They have essentially one Cl-OH bond. The central atom shows sp3 hybridisation.

Some important oxoacids of chlorine

Acidic strength of oxoacids of chlorine follows the order HClO < HClO2  < HClO3 < HClO4

Hypochlorous acid

HOCl can be manufactured in a pure gaseous state.

Chlorous acid

It is prepared by the reaction of ClO2 with a base. The next step involves precipitation of the ClO2-  salt with barium chloride. The barium salt is dried and then reacted with H2SO4.

Chloric acid

Chloric acid anion (ClO3-)  is a strong oxidising agent, showing disproportionation. It is pyramidal in shape in solid salts and solution. Many salts are of chloric acid known to exist. Those with organic cations are explosive.

Perchloric acid

The perchlorate anion (ClO4-) is formed by electrolytic oxidation of chlorate using an aqueous solution. 100% pure perchloric acid can be obtained by dehydration with H2SO4.

Oxoacids of Nitrogen

They have nitrogen as a central atom in them.

Some important oxoacids of nitrogen

Nitric Acid

Out of all the oxoacids of nitrogen, nitric acid is the most important.

Nitric acid is a highly corrosive acid and has a planar structure. The molar mass of nitric acid is  63.01 g mol-1.

Manufacture

On a large scale, it’s mainly prepared by Ostwald’s process. It is based on catalytic oxidation of NH3 by oxygen and involves the following steps:

• Formation of NO in the presence of a catalyst

4NH3 + 5O2 → 4NO + 6H20

• Formation of NO2

2NO + O2 → 2NO2

• Dissolving in water to form Nitric acid

3NO2 + H2O → 2HNO3 + NO

Physical properties of nitric acid

1. It is a colourless liquid.
2. Its boiling point is 355.6K.
3. It has a freezing point of 231.4K.

Chemical properties of nitric acid

1. On decomposition, nitric acid gives nitrogen dioxide.
2. It is one of the best oxidising agents.
3. Only concentrated nitric acid reacts with non-metals; dilute does not.

Oxidation of non-metals by nitric acid

1. C + HNO3 → CO2 + NO2
2. S + HNO3 → H2SO4 + NO2
3. P4 + HNO3 → H3PO4 + NO2

Oxidation of metals by nitric acid

• Very dilute nitric acid

Out of all metals, only magnesium and manganese react with very dilute nitric acid. They liberate hydrogen gas and metal nitrates.

Mg + 2HNO3 → Mg(NO3)2 + H2

Mn + 2HNO3 → Mn(NO3)2 + H2

• Cold dilute nitric acid

Metals like magnesium, zinc and iron react with cold dilute nitric acid to liberate ammonium nitrate and metal nitrate.

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

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

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

• Hot dilute nitric acid

When metals react with hot dilute nitric acid, nitrous oxide (N2O) is formed.

4Mg + 10HNO3 → 4Mg(NO3)2 + 5H2O + N2O

4Zn + 10HNO3 → 4Zn(NO3)2 + 5H2O + N2O

4Fe + 10HNO3 → 4Fe(NO3)2 + 5H2O + N2O

• Concentrated nitric acid

In reaction with concentrated nitric acid, metals give off NO2 gas.

Zn + 4HNO3 → 4Zn(NO3)2 + 2H2O + NO2

Mg + 4HNO3 → 4Mg(NO3)2 + 2H2O + NO2

Uses of nitric acid

1. Nitric acid is essential for the manufacturing of fertilisers.
2. Nitric acid is used for making various organic nitrogen compounds, like derivatives of aniline.
3. It is also used in woodwork.
4. It is commonly used for cleaning and dairy equipment.
5. It is helpful in pickling stainless steel as well as for cleaning silicon wafers in electronics.

Three parts of concentrated HCl and one part of concentrated Nitric acid forms Aquarezia, which can dissolve noble metals like gold and platinum.

Important Notes

1. Order of acidity of halogens – F > Cl > Br > I
2. Acidity increases with an increase in the number of oxygen atoms.
3. Acidity is directly proportional to the oxidation state of the central atom.
4. An oxyacid molecule contains the X−O−H, where X is the central atom.
5. When oxyacids are heated, many of them break down to give water and the anhydride of the acid.

Conclusion

Oxoacids contain at least one -OH group and one double-bonded oxygen atom. The number of oxygen atoms and electronegativity of the central atom determine the acidity of an oxoacid. Sulphur forms mainly seven oxoacids, 2 of which show peroxy-linkage. Sulphuric acid is a dehydrating and oxidising agent. Chlorine forms four oxoacids, and nitrogen forms mainly 5.

Nitric acid is an oxidising agent. It reacts with metals and non-metals in concentrated form.