Question

Which of the following hypohalous acids is most acidic?
a.) HClO
b.) HBrO
c.) HIO
d.) All are same acidic

Answer 1

Hint: Acidity of any compound is measured according to its tendency to release a proton. More stable the conjugate base, more stable is the acid.

Complete step by step solution:
A hypohalous acid is an oxyacid consisting of a hydroxyl group single-bonded to any halogen. Examples include hypofluorous acid (HFO), hypochlorous acid (HClO), hypobromous acid (HBrO), and hypoiodous acid (HIO). The conjugate base is called a hypohalite. It is formed by reacting the corresponding diatomic halogen molecule (\[{{\text{F}}_{\text{2}}}\], \[\text{C}{{\text{l}}_{\text{2}}}\], \[\text{B}{{\text{r}}_{\text{2}}}\], \[{{\text{I}}_{\text{2}}}\]) with water according to the given reaction:

\[{{\text{X}}_{\text{2}}}\text{+}{{\text{H}}_{\text{2}}}\text{O}\rightleftharpoons \text{HXO+HX}\]

Now let us try to arrange the given compounds on the basis of its acidity. As we can see, in all the given oxy-acids, the same atom O is attached to H in all acids.
Let us consider the following points –

As we go down the group, electronegativity decreases. Electronegativity of given halogens is in the order: Cl > Br > I.
More, the electronegativity, more will be its tendency to pull an electron pair towards itself.
Therefore, stability of conjugate bases of the given compounds are in the order:
\[\text{Cl}{{\text{O}}^{\text{-}}}, \text{Br}{{\text{O}}^{\text{-}}} ,\text{I}{{\text{O}}^{\text{-}}}\]
Greater the stability of the conjugate base, stronger is its acid.
From here, we can conclude that the order of acidity in hypohalous acids is: HClO > HBrO > HIO.
Therefore, the answer is – option (a) – HClO is the most acidic hypohalous acid.

Note: This question can also be solved by comparing subsequent electronegativities. Greater the electronegativity and smaller the size of the halogen, greater will be attraction of electron pairs towards it. Therefore, such compounds will release protons easily.