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

The products are HCl and HClO3, which corresponds to option (d).

Step 1: Understand What a Disproportionation Reaction Is

A disproportionation reaction is a redox reaction where a single substance acts both as an oxidizing agent and a reducing agent, resulting in the formation of two different products with different oxidation states.

Step 2: Determine the Oxidation State of Chlorine in Hypochlorous Acid

In hypochlorous acid (HClO), the oxidation state of chlorine is +1. This is because hydrogen typically has an oxidation state of +1, and oxygen is usually -2. So, to balance the charge in HClO, chlorine must be +1.

Step 3: Define Possible Oxidation States for Chlorine

Chlorine has several possible oxidation states, including -1 (as in HCl), 0 (as in Cl2), +1 (as in HClO), +3 (as in HClO2), +5 (as in HClO3), and +7 (as in HClO4). In a disproportionation reaction, the substance will form two different products, each with different oxidation states.

Step 4: Identify the Products of Disproportionation

The disproportionation of HClO involves the increase and decrease in the oxidation state of chlorine. The two possible highest and lowest oxidation states that can result from disproportionation of HClO (+1) in a typical aqueous solution are -1 (forming HCl) and +5 (forming HClO3), respectively.

Step 5: Match Possible Products with Answer Choices

Among the given choices, the pair of products that have the oxidation states -1 and +5 are hydrochloric acid (HCl, where Cl is -1) and chloric acid (HClO3, where Cl is +5). This matches with option (d).

Key Concepts

Redox Reactions

Redox reactions are a fascinating aspect of chemistry that involve both reduction and oxidation processes. During these reactions, the oxidation state of one substance is reduced, while another's oxidation state is increased.
A substance that loses electrons is oxidized, and the one that gains electrons is reduced. A simple way to remember this is with the acronym "OIL RIG" - Oxidation Is Loss, Reduction Is Gain of electrons. In a disproportionation reaction, which is a sub-type of redox reactions, a single element simultaneously undergoes oxidation and reduction. This means it serves as both a reducing agent and an oxidizing agent in the reaction. Such reactions result in a single reactant forming two products with distinct oxidation states.
This is what we see in the reaction involving hypochlorous acid, where chlorine in HClO plays a dual role. Understanding redox reactions helps in predicting the behavior and products of chemical reactions involving changes in oxidation states.

Oxidation States of Chlorine

Chlorine is a versatile element with various possible oxidation states ranging from -1 to +7. The oxidation state essentially tells us how many electrons an atom gains or loses when it forms compounds.
Some common oxidation states of chlorine include:

• -1, as seen in hydrochloric acid (HCl), a state where chlorine acts as an electron recipient.
• 0, the state found in diatomic chlorine gas ( Cl2), where chlorine is neither gaining nor losing electrons.
• +1 in hypochlorous acid (HClO), where chlorine loses an electron to form this compound.
• Transitions to higher oxidation states like +3 in chlorous acid (HClO2), +5 in chloric acid (HClO3), and +7 in perchloric acid (HClO4).

In disproportionation reactions, the chlorine atom in hypochlorous acid changes its oxidation state to form different products. This highlights the unique ability of chlorine to simultaneously increase and decrease its oxidation state in a single reaction.

Hypochlorous Acid

Hypochlorous acid (HClO) is a weak acid with a key role in many chemical reactions. It is most notable for its ability to act as both an oxidizing and reducing agent due to its +1 oxidation state. This allows it to participate in disproportionation reactions.
The chemical structure of hypochlorous acid consists of one hydrogen atom, one chlorine atom, and one oxygen atom. This configuration gives chlorine a +1 oxidation state, balancing the -2 oxidation state of oxygen and +1 of hydrogen.
When hypochlorous acid undergoes a disproportionation reaction, it transforms into two different substances with distinct chemical properties.
In such reactions, hypochlorous acid often forms hydrochloric acid ( HCl), where chlorine is reduced to a -1 oxidation state, and chloric acid ( HClO3), where chlorine is oxidized to a +5 state, demonstrating the dual nature of chlorine within the compound. This ability to transform into different products makes hypochlorous acid an intriguing subject of study in redox chemistry.