Problem 83 - 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 \(\mathrm{K}_{2} \mathrm{MnO}_{4}\), and its color is green.

Step 1: Understanding the Reaction

When \(\mathrm{MnO}_2\) is fused with \(\mathrm{KOH}\), an oxidation reaction takes place. It helps to first predict the likely products of this reaction based on chemical properties and reactivity. \(\mathrm{Mn}^{4+}\) in \(\mathrm{MnO}_2\) can be further oxidized to \(\mathrm{Mn}^{6+}\), forming \(\mathrm{K}_{2} \mathrm{MnO}_4\).

Step 2: Analyzing the Oxidation State

To determine the formulation and color of the compound formed, calculate the oxidation state of manganese in different potential compounds. In the compound \(\mathrm{K}_{2} \mathrm{MnO}_4\), the oxidation state of \(\mathrm{Mn}\) is +6, which usually leads to a green colored compound.

Step 3: Identifying the Product and Color

The correct product of the reaction between \(\mathrm{MnO}_2\) and \(\mathrm{KOH}\) is \(\mathrm{K}_{2} \mathrm{MnO}_4\), due to the oxidation of \(\mathrm{MnO}_2\) in the alkaline medium. This compound is known to have a green color, which matches option (a).

Key Concepts

Oxidation States

The concept of oxidation states is a foundational topic in chemistry, serving as a bookkeeping method to keep track of electrons during chemical reactions. Oxidation states, sometimes referred to as oxidation numbers, indicate the degree of oxidation of an atom in a chemical compound. They are assigned based on a set of rules, such as:

• Elements in their elemental form have an oxidation state of 0.
• The sum of oxidation states in a neutral molecule must be zero.
• For ions, the sum of oxidation states is equal to the charge of the ion.
• Common oxidation states for elements often reflect their group on the periodic table.

For example, in the exercise, manganese in \(\mathrm{MnO}_2\), has an oxidation state of +4. During the reaction with \(\mathrm{KOH}\), manganese is further oxidized to an oxidation state of +6, resulting in the formation of \(\mathrm{K}_2 \mathrm{MnO}_4\). This particular oxidation change is crucial for determining the properties of the compound, including its color.

Inorganic Chemistry

Inorganic chemistry is a branch of chemistry focused on substances that are not carbon-based. While most organic compounds include carbon atoms, inorganic chemistry tackles the remaining segments of the periodic table.Some important points about inorganic chemistry include:

• Some common inorganic compounds are salts, metals, and nonmetals.
• Inorganic compounds often involve metals and usually form ionic bonds.
• Reactions in inorganic chemistry are often explained through concepts such as oxidation-reduction (redox), coordination chemistry, and acid-base behavior.

In the given exercise, the reaction between \(\mathrm{MnO}_2\) and \(\mathrm{KOH}\) serves as a typical example of an inorganic reaction taking place in an alkaline medium. This type of reaction showcases the principles of redox chemistry, with manganese undergoing oxidation. Inorganic chemistry provides the framework to predict such reactions and assess their outcomes, both in terms of the products formed and their corresponding properties.

Color of Compounds

The color of a compound is an intriguing property and is determined by the specific wavelengths of light that the compound absorbs and reflects. This color can often be used to predict the chemical behavior of a compound, particularly in inorganic chemistry.Here are a few key points:

• The color of compounds often comes from the presence of transition metals in the structure.
• Transition metals can absorb certain wavelengths due to electron transitions between d-orbitals, emitting the complementary color.
• The oxidation state of the metal greatly influences the color exhibited by a compound.

For instance, manganese compounds display a variety of colors depending on their oxidation states. In the case of \(\mathrm{K}_2 \mathrm{MnO}_4\), the oxidation state of manganese is +6, resulting in a green color. Understanding this relationship between oxidation state and color is vital for identifying unknown compounds and predicting chemical behavior.