Problem 80 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Potassium manganate \(\left(\mathrm{K}_{2} \mathrm{MnO}_{4}\right)\) is formed when (a) chlorine is passed into aqueous \(\mathrm{KMnO}_{4}\) solution (b) manganese dioxide is fused with potassium hydroxide in air (c) formaldehyde reacts with potassium permanganate in presence of a strong alkali (d) potassium permanganate reacts with conc. sulphuric acid

Step-by-Step Solution

Answer

Reaction (b) forms potassium manganate.

Step 1: Understanding the Exercise

We need to determine which reaction forms potassium manganate (\(\mathrm{K}_2 \mathrm{MnO}_4\)). Four different reactions are given, and we will analyze each to see which one results in \(\mathrm{K}_2 \mathrm{MnO}_4\) as a product.

Step 1: Analyze Reaction (a)

Reaction (a) involves chlorine being passed into an aqueous \(\mathrm{KMnO}_4\) solution. This reaction typically converts the purple permanganate ion (\(\mathrm{MnO}_4^-\)) to the green dichloromanganate ion (\(\mathrm{MnO}_4^{2-}\)), which is not a standard reaction for forming \(\mathrm{K}_2 \mathrm{MnO}_4\).

Step 2: Analyze Reaction (b)

In reaction (b), manganese dioxide (\(\mathrm{MnO}_2\)) is fused with potassium hydroxide (\(\mathrm{KOH}\)) in the presence of air. This reaction does produce potassium manganate (\(\mathrm{K}_2 \mathrm{MnO}_4\)) alongside potassium oxide and water due to the oxidation state changes in manganese.

Step 3: Analyze Reaction (c)

Reaction (c) involves formaldehyde reacting with potassium permanganate in the presence of a strong alkali. Typically, this reaction leads to the formation of formate and reduces the permanganate but does not result in potassium manganate.

Step 4: Analyze Reaction (d)

In reaction (d), potassium permanganate reacts with concentrated sulfuric acid. This reaction usually produces manganese heptoxide and potassium bisulfate, rather than potassium manganate.

Step 6: Conclusion

After analyzing all options, reaction (b) is the correct one that leads to the formation of potassium manganate (\(\mathrm{K}_2 \mathrm{MnO}_4\)). Manganese dioxide fused with potassium hydroxide in air forms this compound.

Key Concepts

Redox Reactions

Redox reactions are a fundamental concept in chemistry where oxidation and reduction occur simultaneously. These reactions play a significant role in the formation of many compounds, including potassium manganate. In these reactions, one substance is oxidized, meaning it loses electrons, while another is reduced, gaining those electrons.
To understand redox reactions, remember the acronym OIL RIG—Oxidation is Loss (of electrons), Reduction is Gain (of electrons). In the case of potassium manganate formation, the manganese dioxide (\(\mathrm{MnO}_2\)) is oxidized. It means it increases its oxidation state, effectively gaining oxygen in this context and facilitating the formation of \(\mathrm{K}_2 \mathrm{MnO}_4\).
Recognizing redox processes can help predict reaction outcomes, balancing equations, and even understanding energy transfer in chemical systems.

• Oxidation: Loss of electrons
• Reduction: Gain of electrons
• Potassium manganate is formed via redox involving oxidation of manganese

Manganese Compounds

Manganese, a transition metal, is known for forming several oxidation states, which makes its compounds versatile and valuable in chemistry. The key manganese compounds involved in this exercise include manganese dioxide (\(\mathrm{MnO}_2\)), potassium permanganate (\(\mathrm{KMnO}_4\)), and potassium manganate (\(\mathrm{K}_2 \mathrm{MnO}_4\)).
Potassium permanganate is a powerful oxidizing agent, often recognized by its characteristic deep purple color. It's widely used in redox reactions, including titrations and oxidations in organic chemistry. Potassium manganate, on the other hand, is green, reflecting its different oxidation state and chemical properties.
Manganese dioxide is naturally occurring as a mineral and is used in the process described here to form potassium manganate by fusion with potassium hydroxide in air. This shows the diversity in manganese chemistry, handling different electron exchange processes and oxidation states.

• Potassium permanganate: Deep purple, strong oxidizer
• Potassium manganate: Green, differs in oxidation state
• Manganese dioxide: Used in manganate formation

Inorganic Chemistry Concepts

Inorganic chemistry explores the properties and behavior of inorganic compounds, which include metals, minerals, and organometallic compounds. The formation of potassium manganate is an interesting case study of several key inorganic chemistry concepts.
First, the synthesis reactions often involve ionic compounds and require knowledge of how ions interact in solutions and solids. The fusion of manganese dioxide and potassium hydroxide in air demonstrates this interaction.
Second, the oxidation states of elements like manganese are crucial. In this context, understanding oxidation state changes helps predict products and balance chemical equations.
Lastly, the role of the medium, such as air or a strong alkali, is critical as it influences reaction rates and pathways, acting as a reactant or catalyst.

• Understanding ionic interactions is key
• Manganese's various oxidation states play a pivotal role
• The reaction medium affects the chemical process