Problem 112 - A Complete Resource Book in Chemistry for JEE Main

Question

Find \([\mathrm{A}],[\mathrm{B}]\) and \([\mathrm{C}]\) respectively in the reaction given below: \([\mathrm{A}]+\mathrm{Na}\left[\mathrm{Cr}(\mathrm{OH})_{4}\right] \stackrel{\mathrm{H}_{2} \mathrm{O}_{2}, \text { boil }}{\longrightarrow}[\mathrm{B}] \frac{\mathrm{H}_{2} \mathrm{SO}_{4}}{\text { Yellow }}-[\mathrm{C}]\) compound (a) \(\mathrm{NaOH}, \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}, \mathrm{Na}_{2} \mathrm{CrO}_{4}\) (b) \(\mathrm{NaOH}, \mathrm{Na}_{2} \mathrm{CrO}_{4}, \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (c) \(\mathrm{H}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{CrO}_{4}, \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (d) \(\mathrm{NaOH}, \mathrm{K}_{2} \mathrm{CrO}_{4}, \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\)

Step-by-Step Solution

Answer

Option (b): \(\mathrm{NaOH}, \mathrm{Na}_{2} \mathrm{CrO}_{4}, \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\).

Step 1: Understand the Reaction Components

The reaction involves transforming a chromium complex (\[\mathrm{Na}\left[\mathrm{Cr}(\mathrm{OH})_{4}\right]\]) using \(\mathrm{H}_{2} \mathrm{O}_{2}\) under boiling conditions. We are asked to find intermediates and the final product.

Step 2: Identify [A]

Since \(\mathrm{Na}\left[\mathrm{Cr}(\mathrm{OH})_{4}\right]\) is a basic complex, \([\mathrm{A}]\) must be \(\mathrm{NaOH}\). This is because \(\mathrm{Na}\left[\mathrm{Cr}(\mathrm{OH})_{4}\right]\) is formed from \(\mathrm{NaOH}\) reacting with chromium hydroxide.

Step 3: Determine [B] After Reaction with H2O2

When \(\mathrm{Na}\left[\mathrm{Cr}(\mathrm{OH})_{4}\right]\) reacts with \(\mathrm{H}_{2} \mathrm{O}_{2}\) in boiling conditions, it gets oxidized to form the yellow chromate ion \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\). This ion corresponds to \([\mathrm{B}]\).

Step 4: Determine [C] with H2SO4 Addition

Adding \(\mathrm{H}_{2} \mathrm{SO}_{4}\) to \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\) leads to the formation of dichromate ions \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\), due to the medium turning acidic. This corresponds to \([\mathrm{C}]\).

Step 5: Match the Compounds with the Choices

Based on the steps above, \([\mathrm{A}] = \mathrm{NaOH}\), \([\mathrm{B}] = \mathrm{Na}_{2} \mathrm{CrO}_{4}\), and \([\mathrm{C}] = \mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\). These match option (b).

Key Concepts

Oxidation Reactions

Oxidation reactions are chemical processes where a substance loses electrons. In the context of inorganic chemistry, these reactions often involve metals and can significantly change the chemical properties of the involved elements or compounds.

• Electron Transfer: During an oxidation reaction, electrons are transferred from one substance to another. This usually results in one species undergoing oxidation, while another undergoes reduction.
• Example in Reaction: In this exercise, when \( \mathrm{Na[Cr(OH)_4]} \) reacts with \( \mathrm{H_2O_2} \), chromium is oxidized. The oxidation state of chromium changes, and it forms the yellow chromate ion \( \mathrm{Na_2CrO_4} \).
• Significance: Oxidation is crucial in many chemical reactions and is often used in industrial processes to produce materials or refine metals.

Understanding these reactions help in knowing how different substances interact and transform.

Transition Metal Complexes

Transition metal complexes consist of a central transition metal atom or ion bonded to surrounding molecules or anions, known as ligands. These complexes are essential in studying the chemistry of metals such as chromium.

• Coordination Compounds: These complexes have a number of ligands attached to the central metal ion. In the exercise, \( \mathrm{Na[Cr(OH)_4]} \) is a transition metal complex where chromium is the central metal ion.
• Ligand Properties: Ligands can be neutral molecules like water or charged ions like hydroxide. They affect the properties and reactivity of the metal center.
• Role in Reactions: Such complexes can undergo changes in coordination when reacting with oxidizing agents, as shown by the transformation into \( \mathrm{Na_2CrO_4} \).

Transition metal complexes are vital in diverse applications, including catalysis and in the coloring of compounds.

Acid-Base Chemistry

Acid-base chemistry involves the transfer and acceptance of protons (\( \mathrm{H}^+ \)). This concept is fundamental in many chemical reactions and is demonstrated when an acidic substance interacts with a basic one.

• Reaction Environment: The addition of \( \mathrm{H_2SO_4} \) to \( \mathrm{Na_2CrO_4} \) changes the reaction environment from basic to acidic. This occurs because \( \mathrm{H_2SO_4} \) donates protons, increasing the concentration of hydrogen ions in the solution.
• Effect on Compounds: The acidification in this reaction results in the conversion of yellow chromate ions to orange dichromate ions \( \mathrm{Na_2Cr_2O_7} \). This change is due to the shifting equilibrium in response to the increase in hydrogen ion concentration.
• Real-life Importance: Understanding acid-base interactions is essential in many fields, such as environmental science for controlling pH levels and in laboratories for reaction optimization.

Mastery of these principles allows for control over chemical processes and reaction products.