Problem 82 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{MnO}_{2}\) is fused with 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) \(\mathrm{Mn}_{i} \mathrm{O}_{4}\), black

Step-by-Step Solution

Answer

The product is \( \mathrm{K}_{2} \mathrm{MnO}_{4} \), green.

Step 1: Identify the Reaction

When \( \mathrm{MnO}_{2} \) is fused with \( \mathrm{KOH} \) in the presence of oxygen, it undergoes an oxidation process. The manganate ion \( \mathrm{MnO}_{4}^{2-} \) is usually produced in this process.

Step 2: Determine the Compound Formed

The oxidation of \( \mathrm{MnO}_{2} \) in basic medium by \( \mathrm{KOH} \) results in the formation of potassium manganate, which has the chemical formula \( \mathrm{K}_{2} \mathrm{MnO}_{4} \).

Step 3: Identify the Colour of the Compound

Potassium manganate \( \mathrm{K}_{2} \mathrm{MnO}_{4} \) is well-known for its green color. This distinguishes it from potassium permanganate \( \mathrm{KMnO}_{4} \), which is purple.

Step 4: Choose the Correct Option

Based on the information above, \( \mathrm{K}_{2} \mathrm{MnO}_{4} \) is green, which matches the description given in option (a).

Key Concepts

Oxidation Process

The oxidation process is a fundamental chemical reaction that involves the loss of electrons from a substance. Typically, it means one or more elements within the compound are being transformed to a higher oxidation state by gaining oxygen or losing hydrogen. In the case of manganese dioxide (\(\mathrm{MnO}_{2}\)), when it is fused with potassium hydroxide (\(\mathrm{KOH}\)) and heated in an oxygen-rich environment, an oxidation process occurs.
\(\mathrm{MnO}_{2}\) is oxidized, meaning it loses electrons to form a higher state compound like potassium manganate \(\mathrm{K}_{2} \mathrm{MnO}_{4}\).
This reaction illustrates the essence of oxidation in chemical reactions:

• Loss of electrons (oxidation) as opposed to gaining electrons (reduction).
• Transformation of manganese from \(\text{Mn}^{4+}\) in \(\mathrm{MnO}_{2}\) to \(\text{Mn}^{6+}\) in \(\mathrm{K}_{2} \mathrm{MnO}_{4}\).
• Involvement of oxygen as a principal reactant facilitating oxidation.

Understanding oxidation is crucial in inorganic chemistry, as it underpins various chemical phenomena and processes.

Potassium Manganate

Potassium manganate is an important compound formed during some inorganic chemical reactions. It bears the chemical formula \( \mathrm{K}_{2} \mathrm{MnO}_{4} \). This compound is significant not only for its structural properties but also for its characteristic green color, setting it apart from its chemical cousin, potassium permanganate, which is purple.
In chemistry, the distinction in color between these two compounds is essential:

• Potassium manganate appears green due to the manganese in the oxidation state +6 in the structure \(\mathrm{MnO}_{4}^{2-}\).
• In contrast, potassium permanganate \(\mathrm{KMnO}_{4}\) is purple because manganese has an oxidation state of +7 forming \(\mathrm{MnO}_{4}^{-}\).

Potassium manganate is usually formed through the high-temperature reaction of \(\mathrm{MnO}_{2}\) with \(\mathrm{KOH}\) in an oxidizing environment. Its formation and use in reactions illustrate the dynamic nature of oxidation states in transition metal chemistry.

Chemical Reactions

Chemical reactions are processes where substances interact, leading to the formation of new products. These transformations involve breaking and forming chemical bonds, often accompanied by energy changes.
In the case of the reaction between manganese dioxide (\(\mathrm{MnO}_{2}\)) and potassium hydroxide (\(\mathrm{KOH}\)), a specific type of chemical reaction occurs known as a solid-state fusion reaction. Here:

• Reactants, \(\mathrm{MnO}_{2}\) and \(\mathrm{KOH}\), are fused together with thermal energy, typically involving oxygen.
• The process leads to the conversion of \(\mathrm{MnO}_{2}\) into potassium manganate \(\mathrm{K}_{2} \mathrm{MnO}_{4}\).

Such reactions highlight the importance of basic conditions (provided by \(\mathrm{KOH}\)) and the presence of an oxidizing agent (oxygen) in chemical processes. They showcase the chemical principle where reactants change under specific conditions to convert into products, demonstrating the interplay of chemical forces and energy.