Question

H₂S gas when passed through a solution of cations containing HCl precipitates the cations of second group of qualitative analysis but not those belonging to the fourth group. It is because:

• A. presence of HCl decreases the sulphide ion concentration
• B. presence of HCl increases the sulphide ion concentration
• C. solubility product of group II sulphides is more than that of group IV sulphides
• D. sulphides of group IV cations are unstable in HCl

Answer 1

The Correct Option is A

In qualitative inorganic analysis, cations are categorized and precipitated based on their reactions with specific reagents under controlled conditions. The problem involves understanding why hydrogen sulfide gas (H_2S) in the presence of hydrochloric acid (HCl) precipitates cations of the second group but not those of the fourth group.

1. Effect of HCl on Sulfide Ion Concentration: When H_2S is passed through a solution, it dissociates as follows: H_2S \rightleftharpoons 2H^+ + S^{2-}. The presence of HCl adds more H^+ ions to the solution, pushing the equilibrium toward the left according to Le Chatelier's Principle, which results in a decrease in the concentration of S^{2-} ions.
2. Precipitation of Cations: In group II analysis, cations such as Pb^{2+}, Cu^{2+}, Cd^{2+} require a lower concentration of sulfide ions to form sulfide precipitates because their solubility products (K_sp) are low. Hence they precipitate even with the decreased sulfide ion concentration. For example, the formation of PbS: Pb^{2+} + S^{2-} \rightarrow PbS \, (s).
3. Solubility in Group IV: Cations of group IV, such as Zn^{2+}, Mn^{2+}, have a much higher solubility product and therefore require a higher concentration of sulfide ions to precipitate as sulfides. Under acidic conditions, provided by HCl, the concentration of S^{2-} is insufficient for these ions to reach saturation and form a precipitate, thus they remain in solution.
4. Conclusion: The correct reasoning is that the presence of HCl decreases the sulfide ion concentration, allowing the precipitation of group II cations while preventing the precipitation of group IV cations.

By understanding the effect of HCl and the solubility products of the various cations, it's clear why the cations in group II precipitate while those in group IV do not.