Problem 145 - A Complete Resource Book in Chemistry for JEE Main

Question

The following acids have been arranged in order or decreasing acid strength: ClOH(I), BrOH(II), IOH(III). Identify the correct order. (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (b) \(\mathrm{III}>\mathrm{I}>\mathrm{II}\) (c) \(\mathrm{II}>\mathrm{III}>\mathrm{I}\) (d) \(\mathrm{III}>\mathrm{II}>\mathrm{I}\)

Step-by-Step Solution

Answer

Option (a): I > II > III.

Step 1: Understand the problem

We need to identify the correct order of acid strength for the given oxyacids: ClOH, BrOH, and IOH.

Step 2: Recall the trend in halogen oxoacids

In halogen-containing oxyacids, the strength typically increases with increasing electronegativity of the central halogen atom. Therefore, we need to arrange the acids according to the electronegativity of the halogens: Cl, Br, and I.

Step 3: Determine electronegativity order

The electronegativity order for the halogens is: Chlorine (Cl) > Bromine (Br) > Iodine (I). This means ClOH is the strongest acid, followed by BrOH and then IOH.

Step 4: Arrange the oxyacids by acid strength

Using the electronegativity order, the correct order of decreasing acid strength for the given acids is ClOH > BrOH > IOH.

Step 5: Select the correct option

The correct order of decreasing acid strength is ClOH (I) > BrOH (II) > IOH (III), which corresponds to option (a).

Key Concepts

Electronegativity of Halogens

Electronegativity describes an atom's ability to attract and hold onto electrons. This property is a crucial factor in determining the behavior of atoms in chemical reactions, especially in the context of acids and bases.
When discussing halogens like chlorine (Cl), bromine (Br), and iodine (I), understanding electronegativity is key to predicting acid strength in their oxoacids. Among halogens, chlorine is the most electronegative, followed by bromine, and then iodine. This pattern implies that oxoacids with a more electronegative central atom will generally have a stronger acid.

• Chlorine (Cl): Highest electronegativity
• Bromine (Br): Intermediate electronegativity
• Iodine (I): Lowest electronegativity

Chlorine's high electronegativity allows it to pull electron density away from the acidic hydrogen, stabilizing the conjugate base once a proton is lost. This stabilization makes ClOH (hypochlorous acid) a stronger acid than both BrOH (hypobromous acid) and IOH (hypoiodous acid). Therefore, electronegativity effectively guides the determination of acid strength in these compounds.

Halogen Oxoacids

Oxoacids are acids that contain an oxygen atom bonded to a central element, such as a halogen. These acids notoriously vary in strength depending on the nature of the central atom, specifically the halogen's properties. With halogen oxoacids, the halogen atom is directly bonded to oxygen which is highly electronegative.
The most common halogen oxoacids fall into the series HXO where X is the halogen. The variations in their acid strength largely hinge on the electronegativity and size of the halogen atom.

• ClOH (Hypochlorous acid): Contains chlorine
• BrOH (Hypobromous acid): Contains bromine
• IOH (Hypoiodous acid): Contains iodine

Since oxygen is strongly electronegative, it significantly influences the distribution of charge across the molecule. If the central halogen is highly electronegative, like chlorine, this causes even greater pull on the electrons, facilitating the release of a proton (H+) from the molecule, thus rendering the acid stronger.

Order of Acidity

Acidity order determines the relative strength of acids and in our context, helps in predicting reactions involving halogen oxoacids, such as ClOH, BrOH, and IOH.
The order of acidity is determined by how readily an acid releases a proton. In halogen oxoacids, this readiness is largely dependent on the electronegativity of the halogen present.
For oxoacids, our rules of thumb include:

• The stronger the pull on electrons by the central atom, the stronger the acid.
• A more electronegative central atom makes a more potent acid.

Following these guidelines, we determine that ClOH, with chlorine as its central atom, is the strongest acid, followed by BrOH, and finally IOH. This is in line with the order: ClOH > BrOH > IOH, aligning with typical trends in oxoacid strength relative to the central atom's electronegativity.