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

Question

When \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through the 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

(a) CoS does not precipitate out.

Step 1: Understanding Sulfide Precipitation

When \(1H_{2}S\) gas is passed through a solution, it can cause metal ions to form metal sulfides. The solubility of these sulfides in water determines whether a solid precipitate will form.

Step 2: Analyzing Ion Precipitation with HCl

In solutions containing \(1H^+\) ions from HCl, only those metal ions which form insoluble sulfides will precipitate. Generally, only the sulfides of lead, copper, silver, mercury, bismuth, and cadmium precipitate under acidic conditions.

Step 3: Evaluating Insoluble Sulfides

The relevant metal ions are \(Cu^{2+}\), \(Hg^{2+}\), \(Bi^{3+}\), and \(Co^{2+}\). The sulfides of copper (CuS), mercury (HgS), and bismuth (Bi2S_3) are insoluble in acidic conditions, thus they will precipitate out.

Step 4: Identifying Soluble Sulfide

Only \(CoS\) is relatively more soluble in acidic solutions compared to the others, so it does not precipitate in the presence of HCl.

Key Concepts

Sulfide Precipitation

Sulfide precipitation is an important concept in chemistry, particularly for separating metal ions in a solution. When hydrogen sulfide gas, \(\mathrm{H}_2\mathrm{S} \), is introduced into a solution containing various metal ions, it can precipitate those ions as metal sulfides. The key factor that determines whether a precipitate will form is the solubility of the resulting metal sulfide. Generally, metal sulfides with low solubility in water will precipitate out of the solution as solids.
In the context of the given problem, when \(\mathrm{H}_2\mathrm{S} \) is bubbled through an acidic solution containing metal ions like \(\mathrm{Cu}^{2+}\), \(\mathrm{Hg}^{2+}\), \(\mathrm{Bi}^{3+}\), and \(\mathrm{Co}^{2+}\), we observe different behaviors based on the nature of the metal sulfides formed. Most of these metals will form insoluble sulfides, allowing them to precipitate out of the solution. However, if the sulfide is soluble in the given conditions, no precipitation will occur.

Metal Sulfides Solubility

The solubility of metal sulfides plays a crucial role in determining whether they will precipitate out of solution. Each metal sulfide has a characteristic solubility product (K_sp), which is a constant at a given temperature.

• When \(\text{K}_{sp}\) is low, the sulfide is less soluble, and the metal ion is more likely to form a solid precipitate.
• Conversely, a higher \(\text{K}_{sp}\) indicates greater solubility and lesser chance of precipitation.

In an acidic environment, such as one with hydrochloric acid (HCl), many metal sulfides are further influenced by the concentration of hydrogen ions. Certain metal sulfides like those of copper (\(\text{CuS}\)), mercury (\(\text{HgS}\)), and bismuth (\(\text{Bi}_2\mathrm{~S}_3\)) are known for their very low solubility in acidic conditions, hence they precipitate readily. On the other hand, cobalt sulfide (\(\text{CoS}\)) is relatively more soluble in comparison, which means it does not precipitate under these conditions.

Acidic Conditions Effect on Metal Ions

Acidic conditions significantly impact the precipitation reactions of metal ions with sulfide ions. When \(\mathrm{HCl} \) or other acids are present in the solution, they increase the concentration of hydrogen ions (\({H}^{+}\)), which has two main effects:

• It decreases the solubility of certain metal sulfides, which enhances precipitate formation for ions such as \(\mathrm{Cu}^{2+}\), \(\mathrm{Hg}^{2+}\), and \(\mathrm{Bi}^{3+}\).
• It shifts the equilibrium of \( \mathrm{H}_2\mathrm{S} \) dissociation, affecting the availability of \(\mathrm{S}^{2-}\) ions for reaction.

In this scenario, while \(\text{CuS, HgS,}\) and \(\text{Bi}_2\mathrm{~S}_3\) precipitate since they are insoluble in acidic solutions, \(\text{CoS}\) remains soluble. The higher acidity decreases the concentration of free \(\mathrm{S}^{2-}\) ions, making \(\text{CoS}\) less likely to precipitate compared to sulfides of metals that form less soluble solids.