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

CoS does not precipitate.

Step 1: Understand the Context

The problem involves the reaction of hydrogen sulfide gas (\(\mathrm{H_{2}S}\)) with a solution containing metal chlorides in hydrochloric acid (\(\mathrm{HCl}\)). We need to determine which metal sulfide will not precipitate under these conditions.

Step 2: Recall Solubility Rules

The solubility of metal sulfides in acidic solutions depends on the metal ions involved. Generally, sulfides of heavy metals like \(\mathrm{Hg^{2+}}\), \(\mathrm{Bi^{3+}}\), and \(\mathrm{Cu^{2+}}\) are insoluble in an acidic medium, whereas sulfides of transition metals like \(\mathrm{Co^{2+}}\) are more soluble.

Step 3: Analyze Each Metal Sulfide

- **\(\mathrm{CuS}\):** Typically, insoluble in acidic solutions, especially in the presence of \(\mathrm{HCl}\).- **\(\mathrm{HgS}\):** Extremely insoluble and will precipitate even in acidic solutions.- **\(\mathrm{Bi}_{2} \mathrm{~S}_{3}\):** Also insoluble and precipitates in acidic solutions.- **\(\mathrm{CoS}\):** Soluble in acidic solutions due to the presence of \(\mathrm{HCl}\), so it does not precipitate.

Step 4: Conclude the Reaction

Given the solubility differences, in a \(\mathrm{HCl}\) acidic environment, \(\mathrm{CoS}\) will remain dissolved and not precipitate, while others like \(\mathrm{CuS}\), \(\mathrm{HgS}\), and \(\mathrm{Bi}_{2} \mathrm{~S}_{3}\) will precipitate.

Key Concepts

Solubility Rules

Solubility rules are a set of guidelines that help predict whether a compound will dissolve in water. For instance, certain compounds, such as nitrates (\( ext{NO}_3^-\)), are usually soluble in water. However, sulfides (\( ext{S}^{2-}\)) have a special case where the solubility depends heavily on the presence of metal ions and other conditions like acidity.
In the context of sulfides, heavy metal sulfides like those of mercury, bismuth, and copper generally do not dissolve in acidic solutions due to their low solubility product constants (\( K_{sp} \)).
Therefore, in hydrochloric acid solutions, these sulfides will precipitate, while transition metal sulfides may behave differently.

• Heavy Metals: Often form insoluble sulfides.
• Transition Metals: May form more soluble sulfides in acidic environments.

Transition Metals

Transition metals occupy the central block of the periodic table and are known for their variable oxidation states and ability to form colored compounds. Examples include copper (\( ext{Cu} \)) and cobalt (\( ext{Co} \)). These metals often form coordination complexes and their chemistry is crucial in reactions involving metal sulfides.
When considering sulfide reactions:\( \)

• Transition metals can exhibit varying behavior, from forming insoluble compounds like cobalt sulfide (\( ext{CoS} \)) to acting differently in varying environmental conditions like acidity.
• Coordination compounds with transition metals can significantly alter solubility characteristics.

Recognizing these variations helps predict which metal sulfides will precipitate in specific conditions.

Acidic Solutions

Acidic solutions, such as those containing hydrochloric acid (\( ext{HCl} \)), can impact solubility due to their influence on the chemical environment. The hydrogen ions (\( ext{H}^+ \)) present in acidic solutions will often promote the dissolution of some metal sulfides more than others.
Heavy metal sulfides like those of mercury and bismuth tend not to dissolve in acids. These remain stable and precipitate out of solution as solid particles.

• Hydrogen ions neutralize the negatively charged sulfide ions, preventing dissolution.
• Some metal sulfides remain soluble due to specific interactions with hydrogen ions.

This interaction is crucial for determining whether a sulfide will precipitate or stay dissolved.

Sulfide Solubility

Sulfide solubility is a concept that depends largely on the metal ion involved and the surrounding conditions like pH. In general, sulfides of metals such as sodium and potassium are water-soluble, whereas those involving transition and heavy metals have varied behaviors.
In acidic environments, it's notable that:

• Heavy metal sulfides are typically insoluble, precipitating out when \( ext{H}_2 ext{S} \) gas is bubbled through acidic solutions.
• Transition metal sulfides often have higher solubility, although this can be reversed under different conditions.

Understanding these unique solubilities is key when predicting which compounds will form solids and which will remain in solution.

Heavy Metal Sulfides

Heavy metal sulfides are compounds made with heavy metal ions and sulfide ions. Examples include copper sulfide (\( ext{CuS} \)), mercury sulfide (\( ext{HgS} \)), and bismuth sulfide (\( ext{Bi}_2 ext{S}_3 \)). These compounds typically exhibit very low solubility in water and are almost always insoluble in acidic solutions.
Their strong tendency to precipitate can be attributed to:

• Low solubility products that make them less likely to dissolve.
• Stable lattice structures that do not readily break apart in solution.

Because of these characteristics, heavy metal sulfides will precipitate when mixed with \( ext{H}_2 ext{S} \) gas in hydrohalic acid solutions like hydrochloric acid.