Oxoacids of halogens preparation and properties, Group 17 P Block Elements
Oxo Acids of Halogens :
→ Halogens have high electronegativity so they have tendency to form oxo acids. But among all halogens fluorine have highest electronegativity and small size so it form only one oxo acid.
→ It is hypofluorous acid (HOF or HFO), Chlorine, Bromine and lodine form four types of Oxy acids, they are HXO, HXO2, HXO3 and HXO4. Among all these oxo acids few are quite unstable. They exist only in their alt solutions. The important oxoacids are given in table 7.20.
Table 7.20 Some oxo acids of Halogens
|Halic (I) acid OR Hypohalous acid||HOF (Hypo fluoro us acid)||HOCl Hypochloro us acid||HOBr Hypobromo us acid||HOI Hypoiodous acid|
|Halic (III) acid OR Halous acid||–||HOClO or HClO2 Chlorous acid||–||–|
|Halic (V) acid OR Halic acid||–||HOClO2 or HClO3 Chloric acid||HOBrO2 or HBrO3 Bromic acid||HOIO2 or HIO3 Iodic acid|
|Halic (VII) acid OR Perhalic acid||–||HOClO3 or HClO4 Perchloric acid||HOBrOg or HBrO4 Perbromic acid||HOIO3 or HIO4 Periodic acid|
Structure of oxo acids of Chlorine
Acidic strength of oxoacids of halogens
→ Acidie strength of hypohalous acids: The acidic strength of hypohalous acids decreases with increase in atomic number because electronegativity decreased
HClO > HBrO > HIO (Acidic strength)
→ Higher the electronegativity higher will be the tendency to attract electons hences the tendency to release H ion from O – H bond incore and acidity incrase.
→ Acidic strength of perhalic acid : The acidic strength of perhalic acid decreases as electronegativity decre
HClO4 > HBrD4 > HIO4 (Acidic strength)
Because as electronegativity decreases the tendency to withdraw electron form O – H XO3 group decreases hence acidie strength decreases.
→ Acidie strength of oxo Acids. The acidic strength of oxoacids of the same halogen increases with increase in oxidation number.
→ The above order of acidie strength can be explained on the basic principle of Bronsted-lowry concept. This concept states that a strong acid will have a weak conjugate base while a weak acid will have a strong eonjugate bare The four conjugate base of oxo acids are
ClO–, ClO–2 ClO–3, ClO–4. The stability of these conjugate bases are as follows:
OCl– < ClO–2 < ClO–3 < ClO–4 (Stability)
→ The stability of above given conjugate base anions depend upon the delocalisation of charge between oxygen atoms. Larger the dekealisation of charge higher the stability of conjugate base anion, lesser will be tendency to attract proton.
→ Hence, the conjugate base will behave as weaker base. But according to Bronsted-Lowery concept stronger the conjucate base weaker will be acid and vice versa. Now the charge delocalisation between oxygen atoms can be shown in fig. 7.42 by the dotted lines.
→ The charge delocalisation or stabilisation is minimum in ClO– and maximum in ClO–4. Hence, order of charge stabilisation is as follow:
ClO– < ClO–2 < ClO–3 < ClO–4 (Charge stabilisation)
→ From above discussion, we can conclude that ClO– har minimum stability therefore it will have maximum tendency to attract the H+ ion i.c.. ClO– is strongest Bronsted base so HClO is weakest acid. On the other hand ClO–4 is weakest Brunsted base 0 HClO4 is the strongest acid.
HClO < CHClO2 <HClO3 < HClO4 (Acidic strength)
Oxidising power of oxoacids
→ Oxidising power of hypohalous acid: As the size of halogen increases the thermal stability of O – X bond increases but oxidising power of hypohalous acid or hypohalite ion decreases. The order of oxidising power is as follows:
OCl– ≈ OBr– > OI– (Oxidising power)
→ The above order of oxidising power is supported by their electrode potentials.
→ Oxidising power of perhalic acid. The ion of perhalie acid is perhalates. Perhalates are very strong oxidising agent. Their oxidising power is as follows:
BrO–4 > IO–4 > ClO–4 (Oxidising power)
→ The above order is on the basis of values of electrode potentials which are as follows:
→ BrO–4 is the strongest oxidising agent but it is weaker than F2
BrO–3 + F2 + 2OH– – BrO–4 + 2F– + H2O
→ Oxidising power of halous acids: As the stability of relative anions of acid increases the oxidising power decreases. Thus,
HClO4 < HClO3 < HClO2 HClO (Oxidising power)