Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Thermal decomposition of a Mn compound (X) at \(513 \mathrm{~K}\) results in compound \(\mathrm{Y}, \mathrm{MnO}_{2}\) and a gaseous product. \(\mathrm{MnO}_{2}\) reacts with \(\mathrm{NaCl}\) and concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) to give a pungent gas \(\mathrm{Z} . \mathrm{X}, \mathrm{Y}\), and \(\mathrm{Z}\), respectively, are: (a) \(\mathrm{KMnO}_{4}, \mathrm{~K}_{2} \mathrm{MnO}_{4}\) and \(\mathrm{Cl}_{2}\) (b) \(\mathrm{K}_{2} \mathrm{MnO}_{4}, \mathrm{KMnO}_{4}\) and \(\mathrm{SO}_{2}\) (c) \(\mathrm{K}_{3} \mathrm{MnO}_{4}, \mathrm{~K}_{2} \mathrm{MnO}_{4}\) and \(\mathrm{Cl}_{2}\) (d) \(\mathrm{K}_{2} \mathrm{MnO}_{4}, \mathrm{KMnO}_{4}\) and \(\mathrm{Cl}_{2}\)

Step-by-Step Solution

Answer

The correct sequence is (a): KMnO₄, K₂MnO₄, and Cl₂.

Step 1: Identify Initial Compound X

The problem mentions that the compound X undergoes thermal decomposition, generating MnO₂, a common decomposition result of permanganate compounds. This suggests compound X could be a permanganate, likely KMnO₄.

Step 2: Decomposition Reaction

When KMnO₄ is heated, it decomposes to produce K₂MnO₄ (compound Y), MnO₂, and O₂ as the gas. The equation for this reaction is: \[ 2 \text{KMnO}_4 \rightarrow \text{K}_2\text{MnO}_4 + \text{MnO}_2 + \text{O}_2 \] This aligns with compound Y being K₂MnO₄.

Step 3: Reaction with NaCl and H2SO4

The problem suggests that MnO₂ reacts with NaCl and concentrated H₂SO₄ to generate a pungent gas. This reaction is known to yield chlorine gas (Cl₂), a result of MnO₂ acting as an oxidizing agent.

Step 4: Verification and Comparison

From the reactions analyzed, we obtained that X is KMnO₄, Y is K₂MnO₄, and Z is the pungent gas Cl₂. Per the provided options, the correct choice matching this conclusion is option (a), which lists X as KMnO₄, Y as K₂MnO₄, and Z as Cl₂.

Key Concepts

Permanganate Compounds

Permanganate compounds are fascinating substances known for their vibrant purple color in solutions and their significant chemical reactivity. These compounds typically contain the permanganate ion, MnO₄⁻.
One of the most common permanganate compounds is potassium permanganate (KMnO₄). It's widely used in laboratories as a strong oxidizing agent. When heated to a temperature like 513 K, KMnO₄ undergoes thermal decomposition. This means it breaks down into simpler substances due to the application of heat.
In the case of KMnO₄, heating results in the formation of potassium manganate (K₂MnO₄), manganese dioxide (MnO₂), and the release of oxygen gas.

• Potassium Permanganate (KMnO₄): A purple salt, often used in titration due to its strong oxidizing properties and distinct color change.
• Potassium Manganate (K₂MnO₄): Green in color, it is one of the decomposition products of KMnO₄.

This reaction is not only an example of thermal decomposition but also demonstrates the interconnected nature of different manganese compounds.

Oxidizing Agents

Oxidizing agents play a crucial role in numerous chemical reactions, including those involving permanganate compounds. An oxidizing agent is a substance that can accept electrons, thereby oxidizing another substance while itself being reduced.
Manganese dioxide (MnO₂) is considered a strong oxidizing agent. This characteristic is evident when it reacts with sodium chloride (NaCl) and concentrated sulfuric acid (H₂SO₄). In this interaction, MnO₂ oxidizes the chloride ions in NaCl to chlorine gas (Cl₂), while itself getting reduced.

• Manganese Dioxide (MnO₂): Can be found as a product of KMnO₄ decomposition, acting as an oxidizing agent in subsequent reactions.
• Chlorine Gas (Cl₂): A pungent gas released during the reaction, recognized for its distinctive smell and use as a disinfectant.

Understanding the behavior of oxidizing agents like MnO₂ allows chemists to predict and control chemical transformations effectively.

Chemical Reactions at High Temperatures

Chemical reactions can be significantly influenced by high temperatures, often providing the necessary energy to initiate reactions that would otherwise be slow or non-spontaneous under normal conditions. High-temperature reactions are common in both industrial processes and educational demonstrations.
In the context of permanganate compounds, heating KMnO₄ initiates its decomposition. This breakdown not only forms new compounds such as K₂MnO₄ and MnO₂ but also releases oxygen, showcasing the powerful effects of thermal energy.
High temperatures can lead to changes in chemical kinetics, affecting reaction rates and equilibrium. They can also enable reactions by overcoming activation energy barriers.

• Decomposition of KMnO₄: Requires substantial heat to convert the permanganate into magnesium dioxide and other products.
• Role of Heat: It acts as a catalyst, increasing the energy available for bond breaking and formation in these reactions.

By understanding these principles, students and scientists can harness high temperatures to efficiently drive desired chemical changes.