Question

Rank the following acids in order of increasing acid strength: HClO, $HCl{O}_2$, HBrO, HIO

Short answer

$HIO<HBrO<HClO<HCl{O}_2$

Step-by-step solution

Acids

Compounds that can give a proton in solution are called acids. The strength of an acid is defined by the ease with which the proton is removed from it.

Compounds that release a proton easily are stronger acids.

The acid in a solution attains an equilibrium at which the rate of dissociation and association are equal. The equilibrium constant of this equilibrium is called the acid dissociation constant and is used to compare the strength of acids.

Acidity of oxyacids

Acids containing oxygen, hydrogen, and any other atom in which the acidic proton is attached to an oxygen atom are called oxyacids. Therefore, oxyacids will contain an X-O-H bond, where X is an element acting as the central atom.

The acidity of oxyacids depend on the electronegativity of the central atom and the oxidation state of the central atom. The oxidation state of the central atom can be determined by the number of oxygen atoms in the acid. Generally, compounds that contain a more electronegative central atom in a higher oxidation state are more acidic.

Comparing acidity of given oxyacids

The given oxyacids contain oxygen, hydrogen and a halogen atom. Elements of Group 17 are called halogens, and they are highly electronegative.

The electronegativity of halogens decreases down the group due to an increase in the size of the atom and a decrease in effective nuclear charge. Therefore, the order of electronegativity of halogens is given by

$\mathrm{I}<\mathrm{Br}<\mathrm{Cl}<\mathrm{F}$

The acidity of oxyacids is directly proportional to the electronegativity of the central atom. Therefore, the acidity of oxyacids can be shown as

$\mathrm{HIO}<\mathrm{HBrO}<\mathrm{HClO}$

For acids having the same central atom, the acidity is directly proportional to the oxidation state of the central atom. The oxidation state of chlorine in HClO is +1 and $HCl{O}_2$ is +3. Therefore, the order of acidity can be shown as

$HClO<HCl{O}_2$

From the above observations, the order of acidity can be shown as

$HIO<HBrO<HClO<HCl{O}_2$