Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{H}_{2} \mathrm{~S}\) in the presence of \(\mathrm{HCl}\) precipitates II group but not IV group because (a) HCl increases concentration of \(\mathrm{Cl}^{-}\) (b) HCl lowers the solubility of \(\mathrm{H}_{2} \mathrm{~S}\) in solution (c) HCl activates \(\mathrm{H}_{2} \mathrm{~S}\) (d) HCl decrease concentration of \(\mathrm{S}^{2-}\)

Step-by-Step Solution

Answer

Option (d): HCl decreases concentration of S²⁻.

Step 1: Understanding Group Precipitation

In qualitative analysis, different groups of metals are separated by precipitating them with specific reagents. The II group precipitates with HCl, which forms insoluble chlorides with certain metal ions, while IV group metals do not form precipitates in the presence of HCl and H2S.

Step 2: Role of HCl in Qualitative Analysis

HCl, when added to a solution containing H2S, increases the concentration of Cl⁻ ions. This is crucial for the precipitation of Group II metal ions, which form insoluble chlorides (like PbCl2, BiCl3, etc.) in the presence of HCl and H2S.

Step 3: Understanding Solubility Changes

HCl also affects the solubility of hydrogen sulfide (H2S). While it does not directly precipitate metals of Group IV, it affects the equilibrium concentration of S²⁻ ions, which are responsible for forming metal sulfides.

Step 4: Analyzing S²⁻ Ion Concentration

HCl decreases the concentration of bisulfide ions (HS⁻) and sulfide ions (S²⁻) in the solution because the acidity (caused by HCl) shifts the equilibrium between H2S, HS⁻, and S²⁻ towards H2S. Lower concentrations of S²⁻ means metals requiring sulfide ion for precipitation (like those in Group IV) do not precipitate as effectively.

Step 5: Conclusion and Answer Selection

Therefore, HCl precipitates Group II by forming insoluble chlorides and does not readily precipitate Group IV, as the concentration of S²⁻ is too low for sulfide precipitation due to the acidic shift caused by HCl. This matches explanation (d), that HCl decreases the concentration of S²⁻.

Key Concepts

Group Precipitation

In qualitative analysis, group precipitation is a useful method for separating metal ions based on their differing chemical properties. This technique depends on using specific reagents that selectively react with particular metal ions to form solid precipitates.

The concept of group precipitation functions by utilizing reagents that are unique to a group of metals, leading to the formation of insoluble compounds. In the context of this analysis, adding \( ext{HCl}\) results in the precipitation of metals from specific groups, like Group II, which form insoluble chlorides in the presence of \( ext{H}_2 ext{S}\) and \( ext{HCl}\).

Group precipitation helps in simplifying the vast pool of metal ions into more manageable sub-groups for easier identification. However, each metal's distinct chemical behavior must be understood, as seen with Group IV metals that do not easily precipitate with \( ext{HCl}\) and \( ext{H}_2 ext{S}\), unlike the Group II metals.

Role of HCl in Analysis

Hydrochloric acid (HCl) plays a vital role in the separation and identification of Group II metal ions in qualitative analysis. When added to a solution containing hydrogen sulfide (H\(_2\)S), it serves several functions:

- **Increases Cl⁻ Concentration:** HCl dissociates to provide a high concentration of chloride ions (Cl⁻), promoting the formation of insoluble metal chlorides with Group II ions. Examples include lead chloride (PbCl\(_2\)) and bismuth chloride (BiCl\(_3\)).

- **Enhances Precipitation with H\(_2\)S:** The high chloride ion concentration assists in the selective precipitation of these chlorides, helping to remove unwanted ions from the mixture.

A deeper understanding of HCl's role in analysis shows how it can manipulate chemical environments to facilitate the separation of desired metal ions through strategic ion concentration changes.

Solubility Changes

Understanding solubility changes is key in predicting which ions precipitate from solution during analysis. The introduction of \( ext{HCl}\) affects solubility equilibria substantially, with noticeable impact on hydrogen sulfide (H\(_2\)S).

When \( ext{HCl}\) is added, the solubility of H\(_2\)S can decrease due to acid dissociation, which then alters the concentration of other ions. In solutions, H\(_2\)S can dissociate into hydrosulfide (HS⁻) and sulfide (S²⁻) ions.

The solubility product principle helps us to understand that higher acidity lowers the dissociation of H\(_2\)S, reducing the availability of S²⁻ ions. This change in solubility limits the formation of metal sulfides needed for the precipitation of some metal groups, like Group IV, thereby preventing their precipitation.

Sulfide Ion Concentration

The concentration of sulfide ions (S²⁻) in solution is a pivotal factor in the precipitation of metal ions, particularly for those in the IV group. These ions form metal sulfides that rely on sufficient S²⁻ in the solution.

HCl, when added to a mixture, influences the sulfide ion concentration. The acidity from HCl shifts the equilibrium of the dissociation of hydrogen sulfide (H\(_2\)S \rightarrow HS⁻ \rightarrow S²⁻) towards the un-dissociated H\(_2\)S. This shift leads to:

• A decrease in the available S²⁻ ions.
• Reduced potential for sulfide-based precipitates.

For metal ions that require excess S²⁻ for precipitation, such as those in Group IV, this decreased concentration means they remain dissolved and unprecipitated, illustrating HCl's role in controlling precipitation through complex ion equilibrium.