Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{FeCl}_{3}\) is acidic towards litmus. On treatment with excess of \(\mathrm{NH}_{4} \mathrm{SCN}\) it gives red coloured compound (A) and on treatment with excess of \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) in the presence of conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\), it evolves deep red vapours of on passing the vapours of (B) into \(\mathrm{NaOH}\), then adding a solution of acetic acid and lead acetate it gives yellow precipitate of compound of chromium (C). The compound (A) is (a) \(\mathrm{NH}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{4}\right]\) (b) \(\mathrm{Fe}(\mathrm{SCN})_{2}\) (c) \(\mathrm{FeCl}_{3}\) (d) \(\mathrm{Fe}(\mathrm{SCN})_{3}\)

Step-by-Step Solution

Answer

The compound (A) is \( \mathrm{Fe(SCN)}_{3} \).

Step 1: Analyze the Reaction with NH4SCN

When excess \( \mathrm{NH}_{4} \mathrm{SCN} \) is added to \( \mathrm{FeCl}_{3} \), it forms a red-colored complex. The most likely compound here is \( \mathrm{Fe(SCN)}_{3} \), where thiocyanate ions form a complex with iron (III), known for its deep red color.

Step 2: Identify the Deep Red Vapors

On treating \( \mathrm{FeCl}_{3} \) with \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) in the presence of concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \), chromyl chloride vapors (\( \mathrm{CrO}_{2} \mathrm{Cl}_{2} \)) are evolved. This is a characteristic deep red vapor that is released in this reaction.

Step 3: Determine the Yellow Precipitate

The vapor \( \mathrm{CrO}_{2} \mathrm{Cl}_{2} \) reacts with \( \mathrm{NaOH} \) to form \( \mathrm{NaCrO}_{4} \), which can further react to produce chromium hydroxide in the presence of lead acetate and acetic acid, giving a yellow precipitate of \( \mathrm{PbCrO}_{4} \).

Step 4: Conclude the Identity of Compound (A)

Based on the first step, the red-colored compound (A) formed when \( \mathrm{NH}_{4} \mathrm{SCN} \) reacts with \( \mathrm{FeCl}_{3} \) is \( \mathrm{Fe(SCN)}_{3} \), because of its distinct red color, which matches the description given.

Key Concepts

Complex Formation

Coordination compounds are often formed by metal ions approaching and binding with ligands, which are ions or molecules that can donate a pair of electrons. When this interaction occurs, it gives rise to a complex ion or a coordination compound. This chemical phenomenon is key in many natural and synthetic processes, where the ligands can modulate the properties of the metal ion they are attached to. In our case, adding excess ammonium thiocyanate (\( \mathrm{NH}_{4} \mathrm{SCN} \)) to \( \mathrm{FeCl}_{3} \) leads to the formation of a complex between iron (III) and thiocyanate ions. This complex is the red-colored \( \mathrm{Fe(SCN)}_{3} \), where three thiocyanate ligands are coordinated to the central metal ion, iron. The color change is a visual indication of the complex formation, which is a handy tool in chemistry to indicate reactions occur. Complex formation not only affects the color but also influences a metal ion's reactivity and solubility, which can be pivotal for reactions and separations in chemistry.

Thiocyanate Complex

Thiocyanate ions (\( \mathrm{SCN}^{-} \)) are interesting ligands in chemistry, known for their versatility and ability to form stable complexes with transition metals. When \( \mathrm{FeCl}_{3} \) reacts with an excess of \( \mathrm{NH}_{4} \mathrm{SCN} \), it forms \( \mathrm{Fe(SCN)}_{3} \), a thiocyanate complex known for its deep red hue. This happens because thiocyanate ions can bind through their nitrogen or sulfur atoms, creating multiple coordination modes that stabilize the complex. Thiocyanate complexes such as \( \mathrm{Fe(SCN)}_{3} \) are used extensively in chemical analysis due to their distinct color changes, which serve as qualitative indicators. This characteristic deep red color is indicative of the thiocyanate ions forming a bond with the iron (III) ion, signifying a change in the coordination environment around the metal ion.

Chromyl Chloride Vapors

Chromyl chloride (\( \mathrm{CrO}_{2} \mathrm{Cl}_{2} \)) is a fascinating compound notable for its deep red vapor. In a reaction of \( \mathrm{FeCl}_{3} \) with \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) in the presence of concentrated \( \mathrm{H}_{2} \mathrm{SO}_{4} \), these chromyl chloride vapors are produced. The generation of these vapors is characteristic of the interaction between chromates and chloride ions under acidic conditions. Chromyl chloride's unique properties make it a significant tool in qualitative analysis to confirm the presence of chloride ions in a sample. Upon contact with alkalis like \( \mathrm{NaOH} \), the vapors undergo hydrolysis, producing chromate ions. This intriguing transformation is key in analytical chemistry, providing a pathway to identify and confirm various components in a complex mixture.

Chemical Reactions

Chemical reactions are the processes where substances are transformed into new products. They are the heart of chemistry and describe how molecules interact, break bonds, and form new ones. In this specific exercise, the interactions between \( \mathrm{FeCl}_{3} \), \( \mathrm{NH}_{4} \mathrm{SCN} \), and \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \) showcase a sequence of colorful transformations. When \( \mathrm{FeCl}_{3} \) reacts with \( \mathrm{NH}_{4} \mathrm{SCN} \), a vivid red thiocyanate complex forms. This is a fine example of a redox reaction where the iron's coordination environment changes. In the reaction with \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} \), deep red chromyl chloride vapors emerge, reflecting the change in oxidation states and new compound formations. The chromyl chloride vapors reacting with \( \mathrm{NaOH} \) to precipitate \( \mathrm{PbCrO}_{4} \) is an example of a sequential reaction. Each step in these fascinating exchanges showcases principles of stoichiometry, conservation of mass, and energy transfer, all fundamental to understanding the complex world of chemical reactions.