Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Upon treatment with ammoniacal \(\mathrm{H}_{2} \mathrm{~S}\), the metal ion that precipitates as a sulfide is (a) \(\mathrm{Fe}(\mathrm{III})\) (b) Al(III) (c) \(\mathrm{Mg}(\mathrm{II})\) (d) \(\mathrm{Zn}(\mathrm{II})\)

Step-by-Step Solution

Answer

(d) \( \mathrm{Zn(II)} \) precipitates as a sulfide.

Step 1: Understanding the Question

The exercise asks which metal ion forms a sulfide precipitate when treated with ammoniacal \( \mathrm{H}_{2} \mathrm{~S} \). To solve this, we need to identify which metal ion reacts with \( \mathrm{H}_{2} \mathrm{~S} \) to form an insoluble sulfide.

Step 2: Sulfur Precipitation Basics

Metal ions that form insoluble sulfides will precipitate out of solution when \( \mathrm{H}_{2} \mathrm{~S} \) is added. Typically, this includes ions of Group II (zinc group) and below in the periodic table.

Step 3: Evaluate Each Option

1. \( \mathrm{Fe(III)} \) forms \( \mathrm{Fe}_{2}\mathrm{S}_{3} \), which can precipitate in acidic medium but not effectively in neutral or basic conditions like ammoniacal \( \mathrm{H}_{2} \mathrm{~S} \).2. \( \mathrm{Al(III)} \) and \( \mathrm{Mg(II)} \) do not form sulfides in these conditions.3. \( \mathrm{Zn(II)} \) forms \( \mathrm{ZnS} \), which is insoluble and thus precipitates.

Step 4: Conclusion

The metal ion \( \mathrm{Zn(II)} \) precipitates as \( \mathrm{ZnS} \) upon treatment with ammoniacal \( \mathrm{H}_{2} \mathrm{~S} \), making option (d) the correct answer.

Key Concepts

Metal Sulfides

Metal sulfides are compounds composed of metal ions and sulfide ions. Many metal ions can form sulfides, and these often have distinct properties. In particular, sulfides are known for their variable solubility, which plays a key role in their chemical behavior.
In the context of precipitation, metal ions that form insoluble sulfides precipitate out of a solution. This occurs because the solid form of the sulfide compound is not soluble in the liquid medium. For example:

• Zinc (Zn) forms zinc sulfide ( ZnS ), which is an insoluble metal sulfide.
• Iron ( Fe ) may form iron sulfide ( FeS ), but its precipitation depends on the pH environment.

Metal sulfides are crucial in various industrial applications, including metallurgy and pigment production, due to their distinctive properties.
Recognizing which metal ions form insoluble sulfides is essential in chemical processes, as it aids in separating and identifying specific metals.

Ammoniacal H2S

Ammoniacal H2S is a solution containing hydrogen sulfide ( H2S ) in an ammonia ( NH3 ) medium, typically basic in nature. This solution is often employed in analytical chemistry to test for the presence of certain metal ions.
The presence of ammonia modifies the behavior of H2S by altering the pH of the solution. Since H2S acts as a weak acid, the ammonia helps create a slightly basic environment. This environment can influence which metal ions precipitate as sulfides.

• For metals like zinc, the slightly basic conditions enable the formation of an insoluble sulfide precipitate.
• Conversely, many metal ions, such as Fe(III) , may require acidic conditions for effective sulfide formation.

Utilizing ammoniacal H2S , chemists can discern between different metal ions based on their sulfide precipitate behavior, ensuring accurate analyses.

Insoluble Compounds

Insoluble compounds do not dissolve in a solvent, usually water, resulting in a solid precipitate. When metal ions form insoluble compounds, such as sulfides, they separate from the solution. This property is a fundamental principle in precipitation reactions.
Insolubility is determined by factors like ionic size, charge, and the nature of the solvent. When a metal sulfide is formed and remains undissolved, it indicates that the sulfide compound is insoluble in the water.

• Zinc sulfide ( ZnS ) is an example of an insoluble sulfide, easily precipitating from aqueous solutions.
• By contrast, magnesium ( Mg(II) ) and aluminium ( Al(III) ) ions do not typically form insoluble sulfides under the same conditions.

Knowing which compounds are insoluble is instrumental in numerous scientific applications. This includes wastewater treatment and mineral processing, where selective precipitation is leveraged for separation and purification processes.