Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

A solution, when diluted with water and boiled, gives a white precipitate. On addition of excess \(\mathrm{NH}_{4} \mathrm{Cl} /\) \(\mathrm{NH}_{4} \mathrm{OH}\), the volume of precipitate decreases leaving behind a white gelatinous precipitate. Identify the precipitate which dissolves in \(\mathrm{NH}_{4} \mathrm{OH} / \mathrm{NH}_{4} \mathrm{Cl}\) (a) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (b) \(\mathrm{Al}(\mathrm{OH})_{3}\) (c) \(\mathrm{Mg}(\mathrm{OH})_{2}\) (d) \(\mathrm{Ca}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

The precipitate that dissolves is \( \mathrm{Zn}( ext{OH})_{2} \).

Step 1: Understanding the Precipitation Reaction

When the solution is diluted and boiled, it forms a white precipitate, which suggests the formation of a metal hydroxide. The metal cations that commonly form white hydroxide precipitates are zinc, aluminum, magnesium, and calcium.

Step 2: Reaction with Ammonium Chloride and Ammonium Hydroxide

The addition of excess \( \mathrm{NH}_{4} \mathrm{Cl} / \mathrm{NH}_{4} \mathrm{OH} \) causes the volume of the initial precipitate to decrease, forming a white gelatinous precipitate. This suggests that some of the precipitate is dissolving due to complexation with \( \mathrm{NH}_{4} \).

Step 3: Complex Formation Analysis

Zinc hydroxide \( \mathrm{Zn}( ext{OH})_{2} \) and aluminum hydroxide \( \mathrm{Al}( ext{OH})_{3} \) dissolve in \( \mathrm{NH}_{4} \mathrm{OH} \) due to the formation of ammine complexes, reducing the amount of precipitate. This aligns with the observation that the precipitate's volume decreases. Magnesium and calcium do not typically form such complexes.

Step 4: Identifying the Dissolved Precipitate

Among the given options, \( \mathrm{Zn}( ext{OH})_{2} \) and \( \mathrm{Al}( ext{OH})_{3} \) can dissolve in \( \mathrm{NH}_{4} \mathrm{OH} / \mathrm{NH}_{4} \mathrm{Cl} \). However, \( \mathrm{Cl} \)-ions normally do not have an effect on \( \mathrm{Zn} \) but can cause aluminum to form a gelatinous precipitate. Therefore, the precipitate that completely dissolves is \( \mathrm{Zn}( ext{OH})_{2} \).

Key Concepts

Metal Hydroxides

When a clear solution is diluted and boiled, it often leads to the formation of a white precipitate. This indicates the presence of a metal hydroxide. Metal hydroxides are compounds composed of metal cations and hydroxide ions (OH^−). Common candidates for white hydroxide precipitates include zinc hydroxide ( Zn(OH)_{2} ), aluminum hydroxide ( Al(OH)_{3} ), magnesium hydroxide ( Mg(OH)_{2} ), and calcium hydroxide ( Ca(OH)_{2} ).

These metals react with water to form hydroxides based on their properties:

• Zinc, aluminum, magnesium, and calcium are metals capable of forming hydroxide precipitates in aqueous solutions.
• Each forms a white or slightly gelatinous precipitate under the right conditions.

Understanding the specific behavior of these metal hydroxides when reacted with various components is crucial in identifying the correct compounds involved in precipitation reactions.

Complexation with Ammonium

In chemistry, complexation refers to the formation of a complex between molecules or ions. When ammonium chloride ( NH_{4}Cl ) and ammonium hydroxide ( NH_{4}OH ) are added to the solution containing the metal hydroxide precipitate, a reaction occurs. Some of the initial precipitate dissolves, indicating partial dissolution by complexation.

Ammonium ions can interact with certain metal hydroxides to form complexes, which may result in partial dissolution of the precipitate, leaving behind a gelatinous residue. This effect can be explained by the atom's ability to interact with the ligands:

• Ammonium ions can cause zinc hydroxide and aluminum hydroxide to partially dissolve, forming different complex species.
• The presence of excess NH_{4}Cl helps maintain the ionic balance, affecting solubility and interaction of molecules involved.

Understanding this concept helps in predicting how different metals will behave in similar conditions, influencing the prolongation or reduction of the precipitate in a solution.

Ammine Complexes

Ammine complexes are formed when metal ions bind with ammonia molecules. These complexes have a varying degree of solubility and can affect the presence of precipitates in a solution. An ammine complex occurs in this context when zinc hydroxide dissolves in the ammonium solution.

An ammine complex formation for zinc is Zn(NH_{3})_{4}^{2+} . This is when ammonia molecules interact directly with the zinc ion and form a soluble complex.

• Zinc hydroxide can form stable ammine complexes, thereby reducing the precipitate's volume as it becomes soluble in water.
• Ammonia acts as a ligand, binding to the zinc ion and creating this soluble complex.
• This contrasts with the behavior of calcium or magnesium, which do not readily form such complexes.

By forming these ammine complexes, the zinc ions become part of a solution, transforming their previous solid state into ions in the aqueous medium, thus explaining the decrease in the precipitate's volume.