Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Passing \(\mathrm{H}_{2} \mathrm{~S}\) gas into a mixture of \(\mathrm{Mn}^{2+}, \mathrm{Ni}^{2+}, \mathrm{Cu}^{2+}\) and \(\mathrm{Hg}^{2+}\) ions in an acidified aqueous solution precipitates (a) \(\mathrm{CuS}\) and \(\mathrm{HgS}\) (b) \(\mathrm{MnS}\) and \(\mathrm{CuS}\) (c) \(\mathrm{MnS}\) and \(\mathrm{NiS}\) (d) NiS and \(\mathrm{HgS}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{CuS} \) and \( \mathrm{HgS} \) precipitate.

Step 1: Understanding Precipitation in Acidic Solutions

In an acidic aqueous solution, only sulfides of less soluble metals precipitate out. Among the given ions, the solubility of different sulfides determines which compounds will precipitate when \\( \text{H}_{2}\text{S} \) gas is passed through the solution. Under acidic conditions, some metal sulfides are less soluble and will precipitate.

Step 2: Identifying Ions that Form Insoluble Sulfides

In acidic aqueous solution:- \( \text{Cu}^{2+} \) ions form \( \text{CuS} \), which is insoluble.- \( \text{Hg}^{2+} \) ions form \( \text{HgS} \), which is also highly insoluble.- \( \text{Mn}^{2+} \) and \( \text{Ni}^{2+} \) tend to remain in the solution as their sulfides are more soluble in acidic conditions.

Step 3: Conclusion Based on Precipitation Behavior

Based on the solubility of their sulfides in acidic conditions, heavy metal sulfides like \( \text{CuS} \) and \( \text{HgS} \) precipitate. Therefore, when \( \text{H}_{2}\text{S} \) is passed, only CuS and HgS form precipitates.

Key Concepts

Precipitation Reactions

Precipitation reactions occur when two solutions or substances react, forming a solid that settles out from the mixture. This solid is known as the precipitate.
In the context of metal ions and sulfides, a precipitation reaction will produce an insoluble compound that falls out of the solution as a solid.
Precipitation occurs when the product formed is insoluble or only slightly soluble in water. To identify such reactions:

• Two ionic compounds dissolved in water are mixed.
• A solid, known as the precipitate, forms in the solution.
• The formation of precipitates is crucial in separating different ions in solution.

In acidic solutions, only certain metal sulfides will precipitate. This depends largely on their solubility in such environments. Understanding these reactions helps in predicting the outcome when reactants come in contact with each other.

Metal Sulfides

Metal sulfides are compounds formed by metals combining with sulfur. They are important in various chemical processes, especially in precipitation.
Sulfides vary in solubility based on the metal ions involved.

• Heavy metals like copper (Cu) and mercury (Hg) tend to form sulfides that are quite insoluble.
• In contrast, lighter metals like manganese (Mn) and nickel (Ni) often form more soluble sulfides.

Sulfides such as CuS and HgS are notable for their low solubility, especially under acidic conditions. This low solubility makes them easy to separate from a solution as precipitates.
The insolubility of metal sulfides is utilized in various applications like ore refinement and chemical analysis.

Solubility Rules

Solubility rules help predict whether a compound will dissolve in water or form a precipitate. These rules derive from empirical observations but provide a reliable guide in chemical reactions.

• Soluble compounds generally do not form precipitates and stay dissolved in the solution.
• Insoluble compounds do not dissolve, instead forming a solid precipitate when reacted.
• Some metals, particularly heavy metals, tend to form sulfides that are typically insoluble in water.
• In acidic solutions, the solubility of sulfides decreases, favoring the precipitation of some heavy metal sulfides.

In chemistry, understanding these rules is crucial for predicting the outcomes of reactions, especially in systems involving metal ions and varying pH levels.
In acidic environments, specific sulfate ions will have reduced solubility, enabling precipitation.