Problem 151 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{Cl}_{2}\) is passed through hot \(\mathrm{NaOH}\) solution, oxidation number of chlorine changes from (a) 0 to \(-1\) (b) 0 to \(+5\) (c) 0 to \(+7\) (d) \(-1\) to 0

Step-by-Step Solution

Answer

When \( \text{Cl}_2 \) is passed through hot \( \text{NaOH} \), its oxidation number changes from 0 to \(-1\) and 0 to \(+5\).

Step 1: Identify Initial Oxidation State

The oxidation state of chlorine in \( \text{Cl}_2 \) is 0 because it is in its elemental form.

Step 2: Reaction Description

When \( \text{Cl}_2 \) is passed through hot \( \text{NaOH} \) solution, a disproportionation reaction occurs, producing \( \text{NaCl} \), \( \text{NaClO}_3 \), and \( \text{H}_2\text{O} \).

Step 3: Determine Oxidation State in \( \text{NaCl} \)

In \( \text{NaCl} \), chlorine is present as \( \text{Cl}^- \), so its oxidation state is \(-1\).

Step 4: Determine Oxidation State in \( \text{NaClO}_3 \)

In \( \text{NaClO}_3 \), let's assume \( x \) as the oxidation state of chlorine. The formula can be written as: \( 1 + x + 3(-2) = 0 \). Solving gives \( x = +5 \).

Step 5: Identify Changes

Initially, chlorine is at 0 in \( \text{Cl}_2 \). It changes to \(-1\) in \( \text{NaCl} \) and \(+5\) in \( \text{NaClO}_3 \).

Step 6: Conclusion of Changes

The oxidation number of chlorine changes from 0 to \(-1\) for some atoms and from 0 to \(+5\) for others. Therefore, options (a) and (b) are both correct.

Key Concepts

Oxidation State

The oxidation state, also known as oxidation number, represents the degree of oxidation of an atom in a chemical compound. It's an essential concept in understanding redox reactions. When an element is in its elemental form, such as chlorine in \( \text{Cl}_2 \), the oxidation state is zero. This indicates that the element hasn't gained or lost any electrons. In compounds, the oxidation state differs as elements combine and either lose or gain electrons to stabilize. The key to determining the oxidation state in compounds is using rules such as:

• The oxidation state of a pure element is always zero.
• The oxidation state of a monoatomic ion is equal to its charge.
• The sum of oxidation states in a neutral compound is zero, while in a charged ion, it is equal to the ion's charge.

Understanding these rules helps unravel redox reactions where oxidation states change as electrons are transferred.

Chlorine in Chemical Reactions

Chlorine is a versatile chemical element used in many reactions. In its diatomic form, \( \text{Cl}_2 \), chlorine is a powerful oxidizing agent. It possesses the capacity to accept electrons due to its ability to form strong bonds with elements, resulting in various oxidation states ranging from -1 in compounds like \( \text{NaCl} \) to higher positive states in oxyanions like \( \text{ClO}_3^- \). In reactions, chlorine often undergoes redox processes where it can simultaneously exhibit both oxidation and reduction, known as disproportionation. This phenomenon allows chlorine to change its oxidation state multiple times within a single reaction, as seen in the reaction with hot \( \text{NaOH} \). This property of chlorine makes it unique, allowing it to participate actively in diverse chemical transformations.

Chemical Reactions in Hot NaOH

When chlorine gas is introduced to a hot \( \text{NaOH} \) solution, it undergoes a disproportionation reaction. This type of reaction involves the simultaneous oxidation and reduction of a single element. In this specific process, \( \text{Cl}_2 \) transforms into two chloride-containing products: sodium chloride \( \text{NaCl} \) and sodium chlorate \( \text{NaClO}_3 \).In this reaction:

• Chlorine is reduced to form \( \text{NaCl} \), where its oxidation state goes from 0 to \(-1\).
• Simultaneously, it's oxidized to form \( \text{NaClO}_3 \), where the oxidation state of chlorine shifts to \(+5\).

This dual change in oxidation state is characteristic of disproportionation reactions and highlights the interplay between chlorine's different valences within chemical reactions. Knowledge of these processes is crucial for predicting the products of chemical interactions and understanding their properties.