Problem 28 - 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: option (c).

Step 1: Identify the Reaction

When chlorine gas (\(\text{Cl}_2\)) is passed through hot sodium hydroxide (\(\text{NaOH}\)), the chemical reaction that occurs isoftentimes written as: \[\text{3Cl}_2 + 6\text{NaOH} \rightarrow 5\text{NaCl} + \text{NaClO}_3 + 3\text{H}_2\text{O}.\] In this reaction, \(\text{Cl}_2\) is involved in a disproportionation reaction.

Step 2: Determine Oxidation States of Chlorine

Initially, in \(\text{Cl}_2\), chlorine is in the 0 oxidation state since it is in its elemental form. During the reaction, chlorine is both reduced and oxidized. Some chlorine turns into \(\text{NaCl}\), where chlorine has an oxidation state of -1, and some become \(\text{NaClO}_3\), where chlorine has an oxidation state of +5.

Step 3: Identify the Change in Oxidation Number

In this reaction, the oxidation number of chlorine (\(\text{Cl}\)) changes from 0 in \(\text{Cl}_2\) to -1 in \(\text{NaCl}\) and +5 in \(\text{NaClO}_3\). These two changes correspond to two possible outcomes: oxidation and reduction, which indicates that multiple answers may meet the question requirements.

Key Concepts

Understanding Oxidation States

Oxidation states, also known as oxidation numbers, represent the degree of oxidation of an atom in a substance. It essentially tells us how many electrons an atom has gained or lost during a reaction. In its elemental form, like Cl₂, chlorine has an oxidation state of 0 because it is neither gaining nor losing electrons. But when it undergoes a chemical reaction, this can change significantly.
In our specific reaction where chlorine gas reacts with hot sodium hydroxide, chlorine experiences a change in its oxidation state from 0 to two different values. Some of the chlorine atoms change to an oxidation state of -1 as they form NaCl. This indicates that chlorine gains an electron. On the other hand, some chlorine atoms change to an oxidation state of +5 as they form NaClO₃, indicating a loss of electrons.
This simultaneous increase and decrease in oxidation state in a single reaction is a hallmark of a disproportionation reaction, where the same element is both oxidized and reduced.

Chlorine Reactions Explored

Chlorine is a highly reactive element, making it involved in various chemical reactions, including disproportionation reactions, like the one described with sodium hydroxide. When chlorine reacts, it can form different compounds depending on the conditions present during the reaction.

• Under normal conditions, chlorine might react with metals to form chloride salts, such as sodium chloride ( NaCl), where chlorine has an oxidation state of -1.
• In the presence of strong bases like NaOH, especially when heated, chlorine can undergo more complex reactions, leading to the formation of substances like NaClO₃, where chlorine achieves a higher oxidation state of +5.

These adaptations in reactions underline chlorine's ability to flexibly adjust its oxidation state to form various products depending on the reaction environment.

Sodium Hydroxide Reactions

Sodium hydroxide (NaOH) is a strong base that can engage in robust reactions, especially with reactive nonmetals like chlorine. In the case of its interaction with chlorine gas at high temperatures, NaOH facilitates a reaction that leads to the formation of different chlorine compounds, demonstrating its ability to alter reaction pathways.

• NaOH reacts with chlorine to form sodium chloride ( NaCl) and sodium chlorate ( NaClO₃), demonstrating its role in both reducing and oxidizing chlorine.
• The reaction is characterized by the release of water (H₂O), further pushing the reaction forward.

Sodium hydroxide's strong base nature allows it to break bonds in chlorine molecules, leading to the observed disproportionation reaction and facilitating various changes in oxidation states, essential for understanding its chemical behavior in such reactions.