Oxoacids of Halogen: Definition, Properties

Oxoacids of halogen are compounds that contain at least one oxygen, hydrogen and no less than one other halogen (Group 17 elements include fluorine, chlorine, bromine, iodine, and astatine which are known as halogens). They form oxoacids in four different types with general formula (HOX) known as Hypohalous, (HOXO) called halous acid, halic acid (HOXO₂) and perhalic acid with formula (HOXO₃) where X is any halogen excluding fluorine as it only forms one oxoacid i.e., HOF. They are considered to be stable only in aqueous solutions or in their salt form.

Key Terms: Oxoacids, Halogens, Electronegativity, Thermal Strength, Acidic Character, Oxidation state, Double bond, Periodic, Hypohalous

Also Read: Group 17 elements

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Properties of Oxoacids of Halogens

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Oxoacids of halogens are found in four different oxidation stages:

• Hypohalous (HOX) with +1 oxidation stage
• Halous acid (HOXO) with +3 oxidation stage
• Halic acid (HOXO₂) with +5 oxidation stage
• Perhalic acid (HOXO₃) with +7 oxidation stage

Also Read: P- block elements

Oxidation Power

Oxidation power of the oxoacids of halogens is inversely proportional to the oxidation number. This means that, with the increase in the oxidation number of the oxoacids, the oxidation power of the compound decreases. For example, 

HClO > HClO₂ > HClO₃ > HClO₄

Acidic strength

• The acidic character of the oxoacids of halogen with the same oxidation number decreases as we move down the group 17 elements in the periodic table. For example,

HOClO₃ > HOBrO₃ > HOIO₃

• Acidic strength increases with an increase in the oxidation number of the oxoacid of halogen. For example, among the four oxoacids of chlorine, the acidic strength increases markedly from hypochlorous acid to perchloric acid.

HClO < HClO₂ < HClO₃ < HClO₄

Thermal strength

Thermal strength of the compound is the ability to resist the breakdown under thermal stress. The thermal stability of both acids and salts increases with an increase in the oxidation number of the halogens. For example, 

HClO < HClO₂ < HClO₃ < HClO₄

Also Read: Oxides of Nitrogen

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Oxoacid of fluorine

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Fluorine is the first element of Group 17. It is very small in size and high electronegative which results in the formation of the single oxoacid i.e., hypofluorous acid (HOF). It is only oxoacid with the negative oxidation state because the main atom in HOF gains electrons from oxygen. The bond lengths of the O–F and O–H bond are 144.2 and 96.4 pm respectively. The bond angle between H–O–F is 97.2° which is slightly narrower as it is examined in the gas phase.

Structure: hypofluorous acid

Also, it is considered as dangerous to keep it in room temperature because of the formation of HF and O₂

2 HOF → 2 HF + O₂

Also Read: Oxoacids of Sulphur

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Oxoacids of Chlorine

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Chlorine forms four oxoacids at each oxidation state. Hence, the four oxoacids of chlorine are

• Hypochlorous acid (HClO)
• Chlorous acid (HOClO or HCIO₂)
• Chloric acid (HOClO₂ or HClO₃)
• Perchloric acid (HOClO₃ or HClO₄)

All these oxoacids differ in their structure, acidic strength, thermal strength, oxidation number and other properties. The bond between Cl and OH (i.e., Cl–OH bond) is present in every oxoacid of chlorine whereas a double bond of Cl and O (i.e., Cl=O bond) is found in most of them. Moreover, the central atom is sp³ hybridised in the chlorine oxoacids. 

Oxoacids of chlorine are considered very useful in chemistry labs as they are ingredients in the preparation of chlorine, ammonium chloride and glucose. Apart from this, they are used to extract glue from bones and purify boneblack. They have a number of applications in medicine and laboratory.

Also Read:

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Things to Remember

• Acidic strength increases with an increase in oxidation number.
• Oxoacids with the same oxidation state decrease for element in order:

HCl > HBr > HI

• Fluorine is the only halogen that forms a single oxoacid i.e., HOF.
• HBrO₂ does not exist in nature.
• Oxoacids of halogens are stable in aqueous solution and in their salt form.
• Oxoacids of chlorine are water-soluble.
• The central atom in the oxoacid of halogen is sp³ hybridised. 
• Every oxoacid has X- OH bond where X is any element.

Also Read: Oxoacids of Phosphorus