Problem 35 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

When \(\mathrm{MnO}_{2}\) is fused with \(\mathrm{KOH}\), a coloured compound is formed, the product and its colour is: (a) \(\mathrm{K}_{2} \mathrm{MnO}_{4}\), purple green (b) \(\mathrm{KMnO}_{4}\), purple (c) \(\mathrm{Mn}_{2} \mathrm{O}_{3}\), brown (d) \(\mathrm{Mn}_{3} \mathrm{O}_{4}\), black

Step-by-Step Solution

Answer

(a) \(\text{K}_{2} \text{MnO}_{4}\), green

Step 1: Identify Reactants and Reaction

When manganese dioxide, \(\text{MnO}_{2}\), is fused with potassium hydroxide (KOH), in the presence of an oxidizing agent, such as oxygen or air, a chemical reaction occurs. The reaction typically results in the formation of a manganate compound.

Step 2: Analyze Possible Products

The reaction of \(\text{MnO}_{2}\) with \(\text{KOH}\) in an oxidizing environment forms potassium manganate (\(\text{K}_{2} ext{MnO}_{4}\)). This is a well-known product from this reaction under basic conditions.

Step 3: Determine the Colour of the Product

Potassium manganate, \(\text{K}_{2} ext{MnO}_{4}\), is characterized by its distinctive green color. This helps us identify the product formed under these reaction conditions.

Step 4: Select the Correct Answer Based on Analysis

Given the analysis, the correct choice is option (a), \(\text{K}_{2} ext{MnO}_{4}\), which is a green compound. This matches the product and its color obtained from the fusion of \(\text{MnO}_{2}\) and \(\text{KOH}\).

Key Concepts

Manganese Compounds

Manganese is an essential element, found in many compounds known for their vibrant colors and interesting chemical properties. This transition metal is capable of forming various oxidation states, ranging from +2 to +7. As a result, manganese compounds can exhibit different colors and chemical behaviors based on their oxidation state.

Common manganese compounds include:

• **Manganese dioxide (\(\text{MnO}_{2}\))**: A dark, black or brown compound, often used as a depolarizer in dry cell batteries.
• **Potassium permanganate (\(\text{KMnO}_{4}\))**: Known for its deep purple color, it is widely used as a strong oxidizing agent in various applications.
• **Potassium manganate (\(\text{K}_{2}\text{MnO}_{4}\))**: This compound is green and forms when manganese dioxide reacts with potassium hydroxide in the presence of oxygen.

These compounds are used in diverse applications, such as in medicine, water treatment, and as catalysts in chemical reactions. Understanding manganese compounds and their properties is essential for exploring their potential uses in various fields.

Oxidizing Agents

Oxidizing agents are substances that can accept electrons from other molecules during a chemical reaction. By doing so, they cause the oxidation of those molecules.

Typically, oxidizing agents are elements or compounds that have high electronegativity or possess high oxidation states. The presence of an oxidizing agent is crucial in many reactions, especially those involving manganese compounds.

Some common characteristics of oxidizing agents include:

• They gain electrons and are reduced in the process.
• They often help generate energy or facilitate chemical transformations.
• Examples include oxygen, chlorine, and permanganate ions.

In the chemical reaction involving \(\text{MnO}_{2}\) and \(\text{KOH}\), an oxidizing environment encourages the transformation of manganese into a higher oxidation state, particularly forming \(\text{K}_{2}\text{MnO}_{4}\). This highlights the essential role of oxidizing agents in creating specific compounds with desired properties.

Potassium Manganate

Potassium manganate (\(\text{K}_{2}\text{MnO}_{4}\)) is a fascinating inorganic compound notable for its vibrant green color. It forms when manganese dioxide is heated with potassium hydroxide in an oxidizing atmosphere.

This compound carries manganese in the +6 oxidation state, a relatively rare form compared to the more common +7 state in potassium permanganate.

Potassium manganate has several interesting characteristics:

• It acts as an intermediate in the production of potassium permanganate during chemical synthesis.
• Being soluble in water, it creates alkaline solutions due to the presence of \(\text{OH}^{-}\) ions.
• It decomposes in neutral or acidic conditions, transitioning to the purple \(\text{KMnO}_{4}\).

Understanding potassium manganate is essential for chemists who aim to utilize its oxidative properties or transition it into other manganese-based compounds, demonstrating the versatility and importance of this green compound.