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Group 17: Oxoacids Of Halogens

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Group 17: Oxoacids Of Halogens - Lesson Summary

Halogens usually form four series of oxoacids, namely, hypohalous acids, halous acids, halic acids and perhalic acids. X represents the halogen, which may be fluorine, chlorine, bromine or iodine.

Oxidation states of the 4 series of oxoacids:

Halogen oxoacid
       series
General
formulae Oxidation
states
                Halogen oxoacid   F   Cl   Br   I   Hypohalous acids   HOX   +1   HOF   HOCl   HOBr   HOI   Halous acids   HOXO   +3   -   HOClO   -   -   Halic acids   HOXO 2   +5   -   HOClO 2   HOBrO 2   HOIO 2   Perhalic acids   HOXO 3   +7   -   HOClO 3   HOBrO 3   HOIO 3

Structures of the oxoacids of halogens:

In all of these oxoacids, the central halogen atom is sp3 hybridised.

One X-OH bond is essentially present in every oxoacid. In most of these oxoacids, "X=O" bonds are present. The double bond present in an oxoacid between the central halogen atom and oxygen is d pi - p pi in nature.

Hypohalous acids include hypofluorous acid, hypochlorous acid, hypobromous acid and hypoiodous acid. The halogen has the oxidation state of +1 in hypohalous acids.

The respective hypohalite ions have a linear shape owing to the presence of three lone pairs of electrons on the central halogen atom.

Hypohalous acids, being less stable, tend to form halic acids.

          3HOX          →          HOXO2      +       3HX
 Hypohalous acid               Halic acid         Hydrogen halide

Only oxoacid of fluorine is hypofluorous acid. No higher oxoacid of fluorine is known due to the absence of d orbitals in the fluorine atom.

Halous acids, includes only chlorous acid. The chlorine in chlorous acid has the oxidation state of+3. The chlorite ion has a V - shape owing to the presence of two lone pairs on the central chlorine atom.

Halic acids, includes chloric acid, bromic acid and iodic acid. The halogen has the oxidation state of +5 in these oxoacids. The respective halate ions are pyramidal in shape owing to the presence of a lone pair of electrons on the central halogen atom.

Perhalic acids, includes perchloric acid, perbromic acid and periodic acid. The oxidation state of the halogen in these oxoacids is+7. The shape of the perhalate ion is tetrahedral.

In any series of oxoacids of halogens, the first member possesses high acidic strength. This is due to the high electro-negativity and small size of the respective halogens.

Ex: Among perhalic acids, perchloric acid has high acidic strength. As chlorine is more electro-negative than bromine or iodine, the shared electron pair lies relatively nearer to chlorine in a Cl-O bond than bromine in a Br-O bond or iodine in an I-O bond. As a consequence, the O-H bond becomes much weaker in case of perchloric acid, thereby facilitating the release of a proton readily.

      HOClO3              >            HOBrO3          >          HOIO3
Perchloric acid                   perbromic acid            Periodic acid

The acidic strength of an oxoacid increases with an increase in the oxidation number of the halogen.

EX: Among the four oxoacids of chlorine, the acidic strength increases markedly from hypochlorous acid to perchloric acid.

Perchloric acid is the strongest acid among all the oxoacids of halogens.

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