Question

In qualitative analysis, in order to detect second group basic radical, \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed in the presence of dil. \(\mathrm{HCl}\) to (a) decrease the dissociation of \(\mathrm{H}_{2} \mathrm{~S}\) (b) increase the dissociation of salt solution (c) decrease the dissociation of the salt solution (d) increase the dissociation of \(\mathrm{H}_{2} \mathrm{~S}\)

Short answer

(a) Decrease the dissociation of \( \mathrm{H}_2 \mathrm{~S} \).

Step-by-step solution

Understand the Role of HCl

When detecting second group basic radicals, it is essential to control the availability of sulfide ions in the solution. The role of dil. HCl is to affect the dissociation of hydrogen sulfide gas, \( \mathrm{H}_2 \mathrm{S} \), and to focus on the reaction environment needed for proper precipitation.

Analyzing H2S Dissociation

The dissociation of \( \mathrm{H}_2 \mathrm{S} \) in solution is a reversible process. It dissociates into \( \mathrm{H}^+ \) and \( \mathrm{S}^{2-} \) ions. However, the presence of \( \mathrm{HCl} \) provides additional \( \mathrm{H}^+ \) ions into the solution.

Effect of Additional H+ Ions

Since \( \mathrm{HCl} \) dissociates completely in solution, releasing \( \mathrm{H}^+ \) ions, this increases the concentration of these protons in the solution. According to Le Chatelier's principle, the increased concentration of \( \mathrm{H}^+ \) ions will shift the equilibrium of \( \mathrm{H}_2 \mathrm{S} \) dissociation towards the undissociated molecule to restore balance.

Conclusion on Dissociation Control

The additional \( \mathrm{H}^+ \) ions provided by the dil. \( \mathrm{HCl} \) effectively decreases the dissociation of \( \mathrm{H}_2 \mathrm{S} \). This ensures that less \( \mathrm{S}^{2-} \) is available in solution, which is critical when only specific second group metal ions (Group II elements like \( \mathrm{Cu}^{2+}, \mathrm{Pb}^{2+} \)) are to be precipitated as their sulfides under these conditions.

Key concepts

Dissociation Control

In qualitative analysis, controlling the dissociation of substances in a solution is key to understanding chemical behavior. Specifically, for the detection of second group basic radicals, the dissociation of hydrogen sulfide (\( \mathrm{H}_2 \mathrm{S} \)) must be precisely managed. When \( \mathrm{H}_2 \mathrm{S} \) is dissolved in water, it splits into hydrogen ions (\( \mathrm{H}^+ \)) and sulfide ions (\( \mathrm{S}^{2-} \)). However, in the presence of dilute hydrochloric acid (\( \mathrm{HCl} \)), additional hydrogen ions are introduced into the solution. This introduction increases the overall \( \mathrm{H}^+ \) concentration, directly impacting the dissociation process of \( \mathrm{H}_2 \mathrm{S} \). By manipulating the dissociation process, chemists can control the availability of \( \mathrm{S}^{2-} \) ions in the solution, which is fundamental for achieving desired outcomes in chemical reactions, especially when precipitating specific metal ions.

Hydrogen Sulfide Dissociation

Hydrogen sulfide, \( \mathrm{H}_2 \mathrm{S} \), is a weak acid that dissociates in water, though not completely. This means it splits into hydrogen ions (\( \mathrm{H}^+ \)) and sulfide ions (\( \mathrm{S}^{2-} \)), establishing an equilibrium between the dissociated and undissociated forms. The dissociation process in the presence of \( \mathrm{HCl} \) can be represented as:\[ \mathrm{H}_2 \mathrm{S} \rightleftharpoons \mathrm{H}^+ + \mathrm{S}^{2-} \]The addition of \( \mathrm{HCl} \) provides extra \( \mathrm{H}^+ \), shifting the equilibrium to favor the formation of more undissociated \( \mathrm{H}_2 \mathrm{S} \). This ultimately means fewer \( \mathrm{S}^{2-} \) ions are available. This method is strategically used in qualitative analysis to precipitate only certain metal ions by limiting the concentration of sulfide ions, allowing chemists to isolate target substances while preventing the unwanted formation of other metal sulfides.

Le Chatelier's Principle

Le Chatelier's Principle is a cornerstone in understanding chemical equilibrium. It states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change and restore balance. This principle is crucial for controlling reactions like the dissociation of \( \mathrm{H}_2 \mathrm{S} \).When \( \mathrm{HCl} \) is added to a solution containing \( \mathrm{H}_2 \mathrm{S} \), the increased concentration of \( \mathrm{H}^+ \) ions disturbs the equilibrium. In response, according to Le Chatelier's Principle, the system shifts to favor the reverse reaction, reducing the dissociation of \( \mathrm{H}_2 \mathrm{S} \) and promoting the restoration of the undissociated form. This controlled shift ensures fewer \( \mathrm{S}^{2-} \) ions are present, which is crucial for specific precipitation reactions in qualitative analysis. By understanding and applying Le Chatelier's Principle, chemists can predict and influence the direction of chemical processes to achieve desired outcomes.