Problem 85 - A Complete Resource Book in Chemistry for JEE Main

Question

When \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through the \(\mathrm{HCl}\) containing aqueous solution of \(\mathrm{CuCl}_{2}, \mathrm{HgCl}_{2}, \mathrm{BiCl}_{3}\) and \(\mathrm{CoCl}_{2}\), it does not precipitate out (a) \(\operatorname{CoS}\) (b) \(\mathrm{HgS}\) (c) \(\mathrm{CuS}\) (d) \(\mathrm{Bi}_{2} \mathrm{~S}_{3}\)

Step-by-Step Solution

Answer

\(\mathrm{H}_2\mathrm{S}\) does not precipitate \(\mathrm{CoS}\).

Step 1: Analyze the Chemical Reaction

When hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)) gas is passed through a hydrochloric acid (\(\mathrm{HCl}\)) containing aqueous solution of the given chlorides, sulfides of these metals may precipitate out. This occurs because \(\mathrm{H}_2\mathrm{S}\) can react with metal ions to form metal sulfides, which are often insoluble in water.

Step 2: Evaluate the Solubility of Metal Sulfides

The solubility of different metal sulfides in water varies depending on the ionic product and solubility product (\(K_{sp}\)). Among the given options, the sulfides of \(\mathrm{Cu}^{2+}, \mathrm{Hg}^{2+},\) and \(\mathrm{Bi}^{3+}\) are known to be insoluble in water and will form precipitates.

Step 3: Identify the Soluble Sulfide

\(\mathrm{CoS}\) (Cobalt sulfide) is relatively more soluble in an acidic medium like hydrochloric acid compared to \(\mathrm{CuS}, \mathrm{HgS},\) and \(\mathrm{Bi}_2\mathrm{S}_3\). Therefore, it does not precipitate out when \(\mathrm{H}_2\mathrm{S}\) is passed through the solution.

Key Concepts

Metal Sulfides

When hydrogen sulfide (\( \mathrm{H}_2\mathrm{S} \)) encounters metal ions, one of the key reactions that occur is the formation of metal sulfides. This is an important process because many metal sulfides are insoluble in water. When you think of metal sulfides, it's helpful to picture them as the result of a bond between metal ions and sulfide ions. This bond is generally quite strong and forms a solid that does not easily dissolve. Some common metal sulfides include copper sulfide (\( \mathrm{CuS} \)), mercury sulfide (\( \mathrm{HgS} \)), and bismuth sulfide (\( \mathrm{Bi}_2\mathrm{S}_3 \)). They readily precipitate out of the solution when formed, illustrating their low solubility.

• Insolubility: Most metal sulfides are water-insoluble.
• Precipitation Process: Precipitation occurs when these compounds form a solid from a liquid solution.
• Result of Reaction: Metal ions combine with sulfide from \( \mathrm{H}_2\mathrm{S} \) to form the insoluble sulfide.

These compounds are crucial in various chemical processes and can serve as a method for isolating or identifying specific metals through precipitation.

Ionic Product

The ionic product is a term used to describe the product of the concentrations of the ions in solution. It is usually denoted for the dissociation of ions. When \( \mathrm{H}_2\mathrm{S} \) is passed through an ionic solution containing metal ions, the ionic product involves both the metal and sulfide ions.
For a more complex reaction like this where metal sulfides precipitate, the ionic product becomes an essential part of predicting and understanding whether precipitation will occur.

• Definition: It is the multiplication of ionic concentrations in a solution.
• Use in Precipitation: Helps to check if the product surpasses the solubility product.
• Comparative Assessment: Determines if a solution is unsaturated, saturated, or supersaturated with respect to the metal sulfide.

In simpler terms, if the ionic product of the metal and sulfide ions in a solution is larger than the solubility product, precipitation occurs as the solution becomes supersaturated.

Solubility Product

The solubility product, often represented as \( K_{sp} \), is a measurement used to determine the solubility of ionic compounds in solution. It is the product of the maximum concentrations of ions that can dissolve in a solution without forming a precipitate. This concept is essential for predicting the behavior of metal sulfides when exposed to hydrogen sulfide gas.
For instance, in an aqueous solution, \( \mathrm{CoS} \) has a higher \( K_{sp} \) than the other metal sulfides mentioned, such as \( \mathrm{CuS} \), \( \mathrm{HgS} \), and \( \mathrm{Bi}_2\mathrm{S}_3 \), which is why it doesn’t precipitate as easily in acidic conditions.

• Significance: It is significant in defining the solubility limit of substances in solutions.
• Comparative Solubility: Lower \( K_{sp} \) means lower solubility, leading to precipitation of that particular compound.
• Precipitation Prediction: If the ionic product exceeds the \( K_{sp} \), the compound will precipitate out of the solution.

Understanding solubility product helps in systematically controlling and predicting the formation and dissolution of insoluble metal sulfides in different chemical scenarios.