Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

If \(\mathrm{Cl}_{2}\) is passed through hot \(\mathrm{NaOH}\), oxidation number of \(\mathrm{Cl}\) changes from (a) \(-1\) to 0 (b) 0 to \(-1\) (c) 0 to \(+5\) (d) 0 to \(+1\)

Step-by-Step Solution

Answer

The oxidation number of Cl changes from 0 to +5 in hot NaOH.

Step 1: Understand the Reaction

When \(\mathrm{Cl}_{2}\) is passed through hot \(\mathrm{NaOH}\), it undergoes a disproportionation reaction where part of the chlorine gets reduced and another part gets oxidized. The overall reaction is \[\mathrm{3Cl_{2} + 6NaOH \rightarrow 5NaCl + NaClO_{3} + 3H_{2}O}\,\] where chlorine is both reduced and oxidized.

Step 2: Assign and Identify Oxidation States

In \(\mathrm{Cl}_{2}\), chlorine has an oxidation state of 0. During the reaction, chlorine in \(\mathrm{NaCl}\) is reduced to \(-1\), and chlorine in \(\mathrm{NaClO_{3}}\) is oxidized to \(+5\).

Key Concepts

Disproportionation Reaction

A disproportionation reaction is a unique type of redox reaction. In these reactions, a single element undergoes both oxidation and reduction. Quite fascinatingly, this involves the same element taking on dual roles in the chemical transformation. This is different from a typical redox reaction where there is a clear exchange of electrons between different elements.
In our exercise example, the element chlorine undergoes disproportionation. When chlorine gas \( \mathrm{Cl}_{2} \) interacts with sodium hydroxide \( \mathrm{NaOH} \), it results in chlorine being both oxidized and reduced. This means chlorine atoms gain electrons in some places while losing them in others.

• Reduction: \( \mathrm{Cl} \) in \( \mathrm{NaCl} \) changes its oxidation state from 0 to -1.
• Oxidation: \( \mathrm{Cl} \) in \( \mathrm{NaClO_3} \) changes its oxidation state from 0 to +5.

Besides understanding what happens to chlorine, appreciating why this occurs is central. The reaction moves towards a state of lower energy where some chlorine atoms are more stable in their reduced form while others stabilize under oxidation conditions. This balancing of oxidation and reduction on chlorine typifies a disproportionation reaction.

Oxidation States

The concept of oxidation states is essential in understanding redox reactions. An oxidation state is like a bookkeeping number associated with electrons in a compound's atoms, reflecting how many electrons are lost or gained. These states are determined based on a series of rules focused on electronegativity and elemental priority in compounds.
In the context of our example, chlorine undergoes changes in its oxidation state during the reaction with sodium hydroxide. Initially, in chlorine gas \( \mathrm{Cl}_2 \), both chlorine atoms are uncharged with an oxidation state of 0. This serves as the reference point in identifying changes as the reaction proceeds:

• Sodium chloride (\( \mathrm{NaCl} \)) sees chlorine's oxidation state change to -1. This indicates a gain of one electron, or reduction.
• Sodium chlorate (\( \mathrm{NaClO_3} \)) involves chlorine being oxidized to an oxidation state of +5, meaning a loss of five electrons.

Understanding these shifts helps track the movement of electrons, illustrating the dual role chlorine plays in itself being both oxidized and reduced.

Chlorine Chemistry

Chlorine is an incredibly versatile chemical element, playing roles in many reactions. Its reactivity comes from its ability to easily gain or lose electrons, which is why it often participates in redox reactions.
Chlorine exists mostly as diatomic molecules (\( \mathrm{Cl}_2 \)) and can take several oxidation states. This variability lets it interact with a vast range of other elements and compounds. In the reaction with sodium hydroxide, we see chlorine's ability to form both \( \mathrm{NaCl} \) and \( \mathrm{NaClO_3} \), demonstrating its adaptability.

• Sodium chloride (\( \mathrm{NaCl} \)) is commonly known as table salt, representing chlorine in a more reduced form.
• Sodium chlorate (\( \mathrm{NaClO_3} \)) features chlorine in a highly oxidized form often used in bleaching and as an herbicide.

This adaptable chemistry brings chlorine to numerous industrial and biological applications, such as in water purification and disinfection. Chlorine's varied chemical behavior underpins its importance in creating a wide array of compounds.