Problem 122 - A Complete Resource Book in Chemistry for JEE Main

Question

A solution, when diluted with \(\mathrm{H}_{2} \mathrm{O}\) 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 in \( \text{NH}_4 \text{OH}/\text{NH}_4 \text{Cl} \) is \( \text{Zn(OH)}_2 \).

Step 1: Understand the context

We have a solution that, when diluted with \( \text{H}_2\text{O} \) and boiled, forms a white precipitate. Further addition of \( \text{NH}_4 \text{Cl} \) and \( \text{NH}_4 \text{OH} \) causes this precipitate to decrease in volume, leaving behind a white gelatinous precipitate. Our task is to identify the precipitate that dissolves in the \( \text{NH}_4 \text{OH} \)/\( \text{NH}_4 \text{Cl} \) mix.

Step 2: Analyze precipitate behavior

The precipitate that decreases in volume upon addition of \( \text{NH}_4 \text{Cl} \) and \( \text{NH}_4 \text{OH} \) likely dissolves in this mixture. This behavior is characteristic of amphoteric hydroxides, which can dissolve in excess hydroxide ions.

Step 3: Match options with known reactions

Among the given options, \( \text{Zn(OH)}_2 \) and \( \text{Al(OH)}_3 \) are amphoteric and known to dissolve in solutions containing excess hydroxide ions, provided by \( \text{NH}_4 \text{OH} \). \( \text{Zn(OH)}_2 \) and \( \text{Al(OH)}_3 \) can form complex ions, such as \( [\text{Zn} \text{(NH}_3\text{)}_4]^{2+} \) and \( [\text{Al} \text{(OH)}_4]^{-} \), respectively, in such a mixture.

Step 4: Arrive at a conclusion

Since we observe that the volume of the precipitate decreases, \( \text{Zn(OH)}_2 \) dissolves in \( \text{NH}_4 \text{OH} \)/\( \text{NH}_4 \text{Cl} \). But since we still see a white gelatinous precipitate, which is typical for \( \text{Al(OH)}_3 \), the one that dissolves is \( \text{Zn(OH)}_2 \), while \( \text{Al(OH)}_3 \) remains.

Key Concepts

Precipitation Reaction

A precipitation reaction involves the formation of a solid, known as a precipitate, when two aqueous solutions are mixed. In such reactions, one of the products becomes insoluble in water and separates out as a solid. This is what occurs when the solution in the original exercise is diluted with water and boiled, resulting in a white precipitate. This initial step indicates that an insoluble compound is being formed.

• The formation of a precipitate is a result of low solubility of the product in water.
• Boiling enhances the reaction rate, which quickly leads to the formation of the precipitate.

Knowing how to predict such reactions is crucial in understanding which combinations of ions will result in a solid formation. Examples often include sulfates or carbonates forming limited soluble compounds.

Amphoteric Behavior

Amphoteric substances have the unique ability to react both as an acid and as a base. In the context of hydroxides like Zn(OH)_2 and Al(OH)_3, they might behave in amphoteric manners by dissolving in both acidic and basic solutions. This is the core of the problem in the original exercise.

• In an acidic medium: Amphoteric hydroxides can react with an acid to form salts and water.
• In a basic medium: They can dissolve by reacting with a base to form soluble complexes.

This means that when subjected to an alkaline solution of NH_4OH, the precipitate's expected behavior is to either dissolve or form a different compound due to its amphoteric nature.

Complex Ion Formation

The formation of complex ions is crucial to understanding why certain hydroxides dissolve in excess ammonia solution. Complex ions consist of a central metal atom bonded to one or more ligands. Here, Zn(OH)_2 can dissolve by forming a complex with ammonia, which ensures its solubility in the solution.

• Ammonia acts as a ligand by donating electron pairs to the metal ion, forming a stable complex ion.
• This reaction shifts the equilibrium, decreasing the concentration of free metal ions in the solution.

For example, Zn(OH)_2 reacts to form the complex ion [Zn(NH_3)_4]^{2+}. This process plays a significant role in separation techniques and analytical chemistry.

Ammonium Salt Reaction

Ammonium salts, such as NH_4Cl, when added to a solution, participate in a variety of reactions. One of the notable reactions is with amphoteric hydroxides which can further demonstrate complex behaviors. Ammonium salts generate NH_4OH in solution by releasing NH_3, which reacts with water.

• By increasing the basicity of the solution, it facilitates the dissolution of some hydroxides.
• This reaction is employed to selectively dissolve or precipitate specific components within a mixture.

The addition of ammonium salts can lead to the dissolution or transformation of certain hydroxides, impacting the volume and nature of any precipitates left in the solution, as was conducted in the exercise. Understanding this can help in selectively controlling which precipitates dissolve or remain.