Problem 208 - A Complete Resource Book in Chemistry for JEE Main

Question

What products are expected from the disproportionation reaction of hypochlorous acid? (a) \(\mathrm{HClO}_{3}\) and \(\mathrm{Cl}_{2} \mathrm{O}\) (b) \(\mathrm{HClO}_{2}\) and \(\mathrm{HClO}_{4}\) (c) \(\mathrm{HCl}\) and \(\mathrm{Cl}_{2} \mathrm{O}\) (d) \(\mathrm{HCl}\) and \(\mathrm{HClO}_{3}\)

Step-by-Step Solution

Answer

Choice (d): HCl and HClO_{3}.

Step 1: Understanding Disproportionation

A disproportionation reaction is a type of redox reaction where a single substance undergoes oxidation and reduction simultaneously, producing two different products with different oxidation states.

Step 2: Identifying Hypochlorous Acid

Hypochlorous acid (HClO) is the reactant in this problem. The chlorine in HClO is in the +1 oxidation state.

Step 3: Determining Possible Products

In a disproportionation reaction of HClO, the chlorine can both increase and decrease its oxidation state. Possible oxidation state changes are from +1 to -1 for Cl in HCl (reduction) and from +1 to +5 in HClO_{3} (oxidation).

Step 4: Writing the Disproportionation Reaction

The disproportionation reaction can be written as: 2 HClO → HCl + HClO_{3} Here, one HClO is reduced to HCl, and the other is oxidized to HClO_{3}.

Step 5: Selecting the Correct Option

Among the given options, (d) HCl and HClO_{3} is the correct answer, as this corresponds to both oxidation and reduction products.

Key Concepts

Hypochlorous Acid

Hypochlorous acid, often represented by its chemical formula \( \mathrm{HClO} \), is a weak acid formed when chlorine dissolves in water. It is characterized by the chlorine atom being bonded to an oxygen atom, which in turn is bonded to a hydrogen atom. This particular arrangement is important because it renders hypochlorous acid an effective disinfectant and bleaching agent.
One noteworthy attribute of \( \mathrm{HClO} \) is its ability to undergo disproportionation. In such reactions, it can both gain and lose electrons to form different compounds with chlorine in varied states. This property is central to understanding its behavior in redox reactions, as seen in this exercise.
When considering reactions involving \( \mathrm{HClO} \), its reactivity is largely dictated by the oxidation state of chlorine and the propensity to undergo both oxidation and reduction simultaneously.

Oxidation States

The concept of oxidation states is crucial for understanding the redox behavior of hypochlorous acid in chemical reactions. An oxidation state represents the degree of oxidation (electron loss) of an atom in a compound.
In hypochlorous acid, the chlorine atom has an oxidation state of +1. This indicates a slight positive charge due to the electronegative nature of the oxygen atom.
During disproportionation, \( \mathrm{HClO} \) serves as both an oxidizing and a reducing agent. This means that the chlorine atom shifts from its original +1 oxidation state both upwards and downwards in terms of its oxidation number.

• The reduction part of the reaction involves chlorine reducing from +1 to -1, forming \( \mathrm{HCl} \).
• The oxidation part sees chlorine increasing from +1 to +5, leading to the formation of \( \mathrm{HClO}_{3} \).

These shifts in oxidation states are what define the novel character of disproportionation reactions.

Redox Reaction

Redox reactions, short for reduction-oxidation reactions, are processes in which electrons are transferred between substances. Understanding redox reactions is essential to analyzing disproportionation.
In the context of hypochlorous acid, a redox reaction involves both a reduction and an oxidation process occurring at the same time with the same element.
In this specific exercise, the chlorine atom in \( \mathrm{HClO} \) undergoes a redox reaction, where it experiences two concurrent changes:

• Reduction: The chlorine atom gains an electron, decreasing its oxidation state from +1 to -1, forming hydrochloric acid, \( \mathrm{HCl} \).
• Oxidation: Simultaneously, the chlorine loses electrons, raising its oxidation state from +1 to +5, forming chloric acid, \( \mathrm{HClO}_{3} \).

These dual changes demonstrate the classic case of a disproportionation reaction, where one element transitions in both directions of electron transfer.

Reaction Products

The products formed from the disproportionation of hypochlorous acid are the key focus of this exercise. In a typical disproportionation reaction, the reactant is transformed into two distinct compounds.
In the case of hypochlorous acid \( \mathrm{HClO} \), the disproportionation process yields:

• Hydrochloric acid \( \mathrm{HCl} \): This product results from the reduction of the chlorine atom, where it gains electrons and decreases its oxidation state to -1.
• Chloric acid \( \mathrm{HClO}_{3} \): This product forms as the chlorine atom loses electrons and increases its oxidation state to +5, undergoing oxidation.

The correct identification of these products is crucial. It demonstrates an understanding of how chlorine atoms find equilibrium in a chemical environment through shifts in their electron configurations, providing a real-life application of redox principles in chemistry.