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) \(\mathrm{Mn}_{3} \mathrm{O}_{4}\), black

Step-by-Step Solution

Answer

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

Step 1: Analyze the Reaction

When manganese dioxide (\(\mathrm{MnO}_{2}\)) is fused with potassium hydroxide (KOH) in the presence of oxygen or oxidizing conditions, it forms manganate ion (\(\mathrm{MnO}_4^{2-}\)). This compound is ionic and typically forms a salt, \(\mathrm{K}_{2}\mathrm{MnO}_4\), with potassium ions present from KOH.

Step 2: Color Identification of the Product

The manganate ion (\(\mathrm{MnO}_4^{2-}\)) in \(\mathrm{K}_{2}\mathrm{MnO}_4\) gives the compound a distinctive green color. This is a known characteristic of the manganate ion in this form and is crucial in differentiating it from similar compounds.

Step 3: Select the Correct Option

Review the answer choices to identify which mentions \(\mathrm{K}_{2}\mathrm{MnO}_4\). The correct choice describes the formation of \(\mathrm{K}_{2}\mathrm{MnO}_4\) as a green compound, matching our findings in the previous steps. Thus, option (a) \(\mathrm{K}_{2}\mathrm{MnO}_{4}\), green, matches precisely.

Key Concepts

Manganese Compounds

Manganese is a versatile element that can combine with different elements to create various compounds with distinct properties. Manganese dioxide (MnO_{2}) is a common manganese compound. It often undergoes reactions to form other manganese compounds.
In particular, when MnO_{2} reacts with potassium hydroxide (KOH) under oxidizing conditions, it forms the manganate ion (MnO_4^{2-}). This ion is crucial because it bears a charge of -2 and combines with potassium ions from KOH to form the salt K_{2}MnO_4. The formation of K_{2}MnO_4d is an ionic reaction, which essentially means that ions from specific compounds recombine to create a new structure.
Understanding these reactions is essential in inorganic chemistry because it's how new compounds are identified and used in practical applications. Manganese itself can exist in multiple oxidation states, allowing it to form many different compounds with a range of properties and colors, which can be fascinating to explore!

Color Identification in Chemistry

Color identification plays an integral role in chemistry, especially in recognizing different compounds and their states. When studying manganese compounds, color can be an immediate clue about which compound is present and its oxidation state.
The green color of K_{2}MnO_4 is characteristic and distinguishes it from related compounds. This is thanks to the specific electronic transitions that occur in the manganate ion (MnO_4^{2-}).

• Electronic transitions refer to the movement of electrons between energy levels within an atom or molecule. When these electrons move, they can absorb certain wavelengths of light, which results in the observed color.
• In the case of manganate (MnO_4^{2-}), its electronic structure leads to a green color, a stark contrast to permanganate (KMnO_4) which appears purple.

Color serves as a quick reference in chemical analysis, which is why it's important for recognizing and understanding different compounds in chemistry.

Oxidation States

Oxidation states are a fundamental part of understanding how elements like manganese form various compounds. Oxidation states indicate the degree of oxidation of an atom; in essence, it's a concept that represents the number of electrons an atom gains or loses.
Manganese is known for having several oxidation states, ranging from -3 to +7, making it incredibly versatile in forming different compounds. For example:

• The oxidation state of manganese in MnO_{2} is +4.
• When forming MnO_4^{2-} through reaction with KOH, manganese transitions to an oxidation state of +6.
• This change in oxidation state is crucial as it results in the formation of the green K_{2}MnO_4 compound.

Understanding oxidation states is essential because they guide predictions about chemical reactions. They indicate not only which compounds will form but also signal potential color changes and other properties. For students, mastering oxidation states helps make sense of complex reactions and trends across the periodic table.