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 \(\text{Cl}\) changes from 0 to \(+5\) when \(\text{Cl}_2\) is passed through hot \(\text{NaOH}\), so the answer is (c).

Step 1: Identify the Reaction

When chlorine gas \(\mathrm{Cl}_{2}\) is passed through hot \(\mathrm{NaOH}\), a disproportionation reaction occurs. The reaction is: \[\mathrm{3Cl}_{2} + 6\mathrm{NaOH} \rightarrow \mathrm{5NaCl} + \mathrm{NaClO}_{3} + 3\mathrm{H}_{2}\mathrm{O}\] In this reaction, chlorine is both oxidized and reduced.

Step 2: Determine initial Oxidation State of Chlorine

In \(\mathrm{Cl}_{2}\), the oxidation state of chlorine is 0 because it is a diatomic molecule of the same element and is electrically neutral.

Step 3: Determine Reduced Form of Chlorine

Chlorine is reduced to chloride ions \(\mathrm{Cl}^{-}\) in \(\mathrm{NaCl}\). Here, the oxidation state of chlorine is \(-1\).

Step 4: Determine Oxidized Form of Chlorine

Chlorine is oxidized to chlorate ions in \(\mathrm{NaClO}_{3}\). In this compound, the oxidation state of chlorine is \(+5\).

Step 5: Conclude the Change in Oxidation State

The initial oxidation state of chlorine is 0. It changes to \(-1\) when reduced, and changes to \(+5\) when oxidized. Hence, chlorine undergoes a change in oxidation state from 0 to \(+5\).

Key Concepts

Understanding Oxidation States

Oxidation states, also referred to as oxidation numbers, are a concept used in chemistry to describe the degree of oxidation of an atom in a compound. They help track electron transfer in reactions. An atom's oxidation state is generally determined using the rules based on electronegativity and standard oxidation numbers. For chlorine in its diatomic form, each chlorine atom has an oxidation state of 0 because it shares electrons equally between the atoms. Note that in compounds with other elements, chlorine usually takes an oxidation state that reflects its tendency to gain one electron to form a chloride ion (\(\text{Cl}^{-}\)), which has an oxidation state of \(-1\).

• The oxidation state can increase (indicating loss of electrons or oxidation),
• or it can decrease (indicating gain of electrons or reduction).

For complex ions like chlorate \(\text{ClO}_3^{-}\), chlorine's oxidation state can be determined by considering the known oxidation numbers of oxygen (usually \(-2\)). Thus, we calculate chlorine’s oxidation state by ensuring the sum of oxidation states equals the charge of the compound.

The Dynamics of Chlorine Reactions

Chlorine reactions cover a fascinating area of chemistry, especially when chlorine interacts with alkalis like sodium hydroxide (\(\text{NaOH}\)). The reaction between chlorine gas (\(\text{Cl}_2\)) and hot \(\text{NaOH}\) results in a disproportionation reaction. In this type of reaction, the same element undergoes simultaneous oxidation and reduction.

• Chlorine is reduced to \(\text{NaCl}\), where it is in the \(-1\) oxidation state.
• It is also oxidized to \(\text{NaClO}_3\), where the oxidation state rises to \(+5\).

This dual process highlights the unique behavior of chlorine, as it can achieve multiple oxidation states within a single reaction. Understanding its reactions can reveal much about redox behavior and electron transfer. The change in oxidation state illustrates the diverse chemistry that chlorine can participate in.

Introduction to Redox Reactions

Redox reactions are fundamental to understanding chemical changes, as they involve the transfer of electrons between substances. The term "redox" is a combination of "reduction" and "oxidation," the two processes that occur simultaneously in these reactions.

• Oxidation refers to the loss of electrons, often resulting in an increase in oxidation state.
• Reduction describes the gain of electrons, leading to a decrease in oxidation state.

In the given reaction where chlorine gas reacts with \(\text{NaOH}\), chlorine acts as both the oxidizing and reducing agent due to its change in oxidation states. The simultaneous occurrence of oxidation and reduction of chlorine shows how elements can behave in interdependent ways depending on the circumstances. By understanding redox reactions, students can gain insight into reaction mechanics, electron flow, and the principles of balancing chemical equations. Thus, mastering redox concepts is crucial for anyone exploring chemistry's vast landscape.