Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

By passing \(\mathrm{H}_{2} \mathrm{~S}\) gas in acidified \(\mathrm{KMnO}_{4}\) we get (a) \(\mathrm{S}\) (b) \(\mathrm{MnO}_{2}\) (c) \(\mathrm{K}_{2} \mathrm{SO}_{3}\) (d) \(\mathrm{K}_{2} \mathrm{~S}\)

Step-by-Step Solution

Answer

The correct answer is (a) \(\mathrm{S}\).

Step 1: Understand the chemical reaction

In this reaction, hydrogen sulfide (\(\mathrm{H}_{2} \mathrm{~S}\)) is passed through an acidified potassium permanganate (\(\mathrm{KMnO}_{4}\)) solution. The reaction occurs in an acidic medium (often sulfuric acid). \(\mathrm{KMnO}_{4}\) acts as an oxidizing agent.

Step 2: Analyze reactants and oxidizing agent

\(\mathrm{H}_{2} \mathrm{~S}\) is a reducing agent and will be oxidized by \(\mathrm{KMnO}_{4}\) in the acidic solution. The \(\mathrm{KMnO}_{4}\) will be reduced in the process. \(\mathrm{H}_{2} \mathrm{~S}\) is known to typically convert to elemental sulfur (\(\mathrm{S}\)) when oxidized.

Step 3: Determine the reduction product

In acidic solutions, \(\mathrm{KMnO}_{4}\) is known to be reduced to \(\mathrm{Mn}^{2+}\) ions as it loses oxygen. This means \(\mathrm{KMnO}_{4}\) will not become \(\mathrm{MnO}_{2}\) unless in a neutral or basic environment.

Step 4: Identify the reaction products

The final products of the reaction will be elemental sulfur (\(\mathrm{S}\)) from the oxidation of \(\mathrm{H}_{2} \mathrm{~S}\), while \(\mathrm{KMnO}_{4}\) is reduced to \(\mathrm{Mn}^{2+}\). Other options (b), (c), and (d) are not typically associated with the products when \(\mathrm{H}_{2} \mathrm{~S}\) is bubbled through acidified \(\mathrm{KMnO}_{4}\).

Step 5: Conclude with correct answer

Given the understanding of \(\mathrm{H}_{2} \mathrm{~S}\) and \(\mathrm{KMnO}_{4}\) interactions, the correct answer to the question is (a) \(\mathrm{S}\) as elemental sulfur is a direct product.

Key Concepts

Oxidizing Agents

In chemical reactions, an oxidizing agent is a substance that facilitates the oxidation of another substance by accepting electrons from it. Oxidizing agents themselves get reduced in the process.
In the exercise, potassium permanganate (\(\mathrm{KMnO}_{4}\)) is the oxidizing agent. It plays a crucial role by accepting electrons from hydrogen sulfide (\(\mathrm{H}_{2} \mathrm{~S}\)), transforming itself from a higher oxidation state to a lower one.

• The oxidation state of manganese in \(\mathrm{KMnO}_{4}\) is +7.
• During the reaction, \(\mathrm{KMnO}_{4}\) is reduced to \(\mathrm{Mn}^{2+}\), a lower oxidation state, thereby serving as an effective oxidizing agent.
• This conversion typically occurs in an acidic medium, which stabilizes the \(\mathrm{Mn}^{2+}\) ions.

This ability to accept electrons makes \(\mathrm{KMnO}_{4}\) a very strong oxidizing agent, frequently used in various chemical analyses and syntheses.

Reducing Agents

Reducing agents, like hydrogen sulfide (\(\mathrm{H}_{2} \mathrm{~S}\)), are substances that donate electrons in a chemical reaction, thereby becoming oxidized themselves. This donation results in the reduction of another compound.
For the reaction in our exercise:

• \(\mathrm{H}_{2} \mathrm{~S}\) donates electrons to \(\mathrm{KMnO}_{4}\), which leads to the oxidation of \(\mathrm{H}_{2} \mathrm{~S}\) itself.
• This results in \(\mathrm{H}_{2} \mathrm{~S}\) turning into elemental sulfur (\(\mathrm{S}\)), as it loses hydrogen and forms a stable, oxidized product.
• The process highlights the dual nature of reduction and oxidation occurring simultaneously.

Any reducing agent, including \(\mathrm{H}_{2} \mathrm{~S}\), is characterized by its ability to give up electrons to the oxidizing agent, thereby aiding in its own transformation. Overall, the interplay between oxidizing and reducing agents drives many chemical processes.

Chemical Reactions

At the heart of this exercise is the concept of chemical reactions. A chemical reaction is a process where substances (reactants) are transformed into different substances (products).
Chemical reactions involve the rearrangement of atoms and the making or breaking of chemical bonds:

• In our particular example, the reactants are \(\mathrm{H}_{2} \mathrm{~S}\) and acidified \(\mathrm{KMnO}_{4}\).
• Upon interaction, \(\mathrm{H}_{2} \mathrm{~S}\) is oxidized to sulfur, and \(\mathrm{KMnO}_{4}\) is reduced to \(\mathrm{Mn}^{2+}\) ions.
• The reaction takes place in an acidic environment, which assists in the effective reduction of \(\mathrm{KMnO}_{4}\).

Thus, the essence of chemical reactions involves either transfer of electrons, as in redox reactions, or the formation/elimination of bonds. This fundamental understanding helps in predicting and determining the resulting products, like how elemental sulfur is predicted in this scenario.