Problem 121 - A Complete Resource Book in Chemistry for JEE Main

Question

The aqueous solution of an inorganic compound \((\mathrm{X})\), gives a white precipitate with \(\mathrm{NH}_{4} \mathrm{OH}\), which does not dissolve in excess of \(\mathrm{NH}_{4} \mathrm{OH}\). This aqueous solution also gives white precipitate with \(\mathrm{AgNO}_{3}\) and the precipitate is soluble in dilute \(\mathrm{HNO}_{3} .\) Here ( \(\mathrm{X}\) ) is (a) \(\mathrm{AlCl}_{3}\) (b) \(\mathrm{AlBr}_{3}\) (c) AIN (d) \(\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}\)

Step-by-Step Solution

Answer

The compound \( \mathrm{X} \) is \( \mathrm{AlCl}_3 \).

Step 1: Analyze the Reaction with NH4OH

When compound \( \mathrm{X} \) is mixed with \( \mathrm{NH}_4 \mathrm{OH} \), it forms a white precipitate. This suggests the presence of cations that form insoluble hydroxides. For example, aluminum in the form of \( \mathrm{AlCl}_3 \), can form aluminum hydroxide, \( \mathrm{Al(OH)_3} \), which is white and does not dissolve in excess \( \mathrm{NH}_4 \mathrm{OH} \). Other potential candidates are explored later.

Step 2: Analyze the Reaction with AgNO3

The solution also gives a white precipitate with \( \mathrm{AgNO}_3 \), indicating the presence of chloride ions, as silver chloride, \( \mathrm{AgCl} \), is a white precipitate. However, \( \mathrm{AgCl} \), is known to dissolve in dilute \( \mathrm{HNO}_3 \), which matches the behavior described in the problem.

Step 3: Exclude Incorrect Options

- \( \mathrm{AlBr}_3 \) would give \( \mathrm{AgBr} \) with \( \mathrm{AgNO}_3 \), a yellowish precipitate, less soluble than \( \mathrm{AgCl} \) in \( \mathrm{HNO}_3 \).- \( \mathrm{AlN} \) is not typically reactive in this context for the provided reactions.- \( \mathrm{Al_2(SO_4)_3} \) would not precipitate with \( \mathrm{NH_4OH} \) as it would form a soluble complex.

Step 4: Confirm the Correct Answer

Since the reactions match perfectly with \( \mathrm{AlCl}_3 \)—forming insoluble \( \mathrm{Al(OH)_3} \) in \( \mathrm{NH_4OH} \) and soluble \( \mathrm{AgCl} \) in \( \mathrm{HNO}_3 \)—this compound is consistent with all given data. Thus, \( \mathrm{X} \) must be \( \mathrm{AlCl}_3 \).

Key Concepts

Aqueous Solutions

Aqueous solutions are a fundamental part of inorganic chemistry, serving as the medium for a plethora of reactions. These solutions occur when a substance, typically a solid, dissolves in water, forming a homogeneous mixture. In the context of aqueous solutions, water acts as a solvent, and the dissolved substance is the solute.

A key feature of aqueous solutions is their ability to conduct electricity, as they often contain ions. Ions are charged particles, and their presence allows the solution to act as an electrolyte. This characteristic is crucial in many chemical reactions, where the transfer of electrons is necessary for reaction progression.

• A solute like Aluminum Chloride ( AlCl_3 ) can release Cl^- ions into the solution.
• The solution's behavior—including reactions with other chemicals—is dictated by these ions.

Chemical Reactions

Chemical reactions in aqueous solutions are common in both natural processes and industrial applications. These reactions involve the transformation of reactants into products with different chemical properties. The study of these reactions helps chemists understand how substances interact on a molecular level.

In an aqueous solution, chemical reactions can occur when the solute interacts with other substances. For example, when AlCl_3 is added to an aqueous solution, the Aluminum and Chloride ions separate to form a new ionic environment ready for further reactions:

• In this environment, ions can collide with and react with ions from other substances like NH_4OH or AgNO_3 .
• These interactions can lead to the formation of precipitates, gases, or aqueous products.

Understanding the specific conditions and products of these reactions is crucial for predicting the behavior of solutions in various circumstances.

Precipitation Reaction

Precipitation reactions occur when two soluble salts in an aqueous solution react to form an insoluble solid, known as a precipitate. This process is key in determining the presence of certain ions in a solution and is often utilized in qualitative analysis.

During a precipitation reaction, reactants dissolve in the solution, and their ions interact to form a new compound:

• A classic example is seen when Aluminum ions from AlCl_3 react with hydroxide ions from NH_4OH to form Aluminum hydroxide ( Al(OH)_3 ), a white solid precipitate.
• Similarly, the reaction of Chloride ions with silver ions from AgNO_3 results in the formation of Silver Chloride ( AgCl ), another white precipitate.

These reactions are easily observable since the solid precipitate is distinct from the aqueous environment.

Aluminum Chloride

Aluminum Chloride ( AlCl_3 ) is a commonly studied compound in chemistry due to its interesting reactions and properties. In its solid-state, it forms colorless crystals, but it is highly soluble in water, dissociating into Aluminum and Chloride ions.

• In the presence of a base like NH_4OH , AlCl_3 reacts to form Aluminum hydroxide, highlighting its amphoteric nature. This means it can react with both acids and bases under specific conditions.
• With AgNO_3 , it produces a white precipitate of AgCl , demonstrating the presence of free chloride ions in the solution.

AlCl_3's chemistry is intricate yet fascinating, making it a classic example of how compounds behave in aqueous solutions and during chemical reactions.