Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

The ion that cannot be precipitated by both \(\mathrm{HCl}\) and \(\mathrm{H}_{2} \mathrm{~S}\) is (a) \(\mathrm{Ag}^{+}\) (b) \(\mathrm{Cu}^{+}\) (c) \(\mathrm{Sn}^{2+}\) (d) \(\mathrm{Pb}^{2+}\)

Step-by-Step Solution

Answer

The ion that cannot be precipitated by both HCl and H₂S is Sn²⁺.

Step 1: Understanding precipitation with HCl

Analyze which ions can form insoluble chlorides with the addition of hydrochloric acid (HCl). - When HCl is added, ions that form insoluble chlorides and get precipitated include silver ion (Ag⁺) and lead ion (Pb²⁺) as they form silver chloride (AgCl) and lead chloride (PbCl₂), respectively. Cu⁺ can also form a precipitate as cuprous chloride (CuCl). Tin ion (Sn²⁺) does not form a precipitate with HCl.

Step 2: Analyzing precipitation with H2S

Determine which ions form sulfides that are insoluble in dilute HCl when hydrogen sulfide (H₂S) is bubbled through a solution. - In acidic conditions, H₂S will precipitate metal ions as metal sulfides. Ag⁺ and Cu⁺ can be precipitated as Ag₂S and Cu₂S, respectively. Pb²⁺ can also form lead sulfide (PbS) which is insoluble. Sn²⁺ gets precipitated as stannous sulfide (SnS) in acidic solutions.

Step 3: Final analysis and conclusion

Combine the analysis from the first two steps to find the ion that cannot be precipitated by both methods. - From the analysis, Ag⁺ and Pb²⁺ are precipitated by both HCl and H₂S. Cu⁺ is also precipitated by both reagents. Sn²⁺, however, does not precipitate with HCl, even though it does with H₂S.

Key Concepts

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where soluble ions in different solutions are mixed to form an insoluble compound known as a precipitate. When studying precipitation reactions, it is critical to understand which ions can combine to form these insoluble compounds.
In our specific problem, the ions tested against different reagents include silver ion (Ag⁺), cuprous ion (Cu⁺), tin ion (Sn²⁺), and lead ion (Pb²⁺). When hydrochloric acid (HCl) is added in these cases:

• Ag⁺ forms a precipitate as silver chloride (AgCl).
• Cu⁺ forms a precipitate as cuprous chloride (CuCl).
• Pb²⁺ forms a precipitate as lead chloride (PbCl₂).
• Sn²⁺ does not form a precipitate.

When hydrogen sulfide (H₂S) is used in acidic conditions, each of these ions undergo different reactions to form sulfides:

• Ag⁺ forms silver sulfide (Ag₂S).
• Cu⁺ forms cuprous sulfide (Cu₂S).
• Pb²⁺ forms lead sulfide (PbS).
• Sn²⁺ forms stannous sulfide (SnS).

Understanding these reactions helps determine the solubility and stability of ions in different chemical settings.

Metal Ions

Metal ions are positively charged atoms that may participate in chemical reactions by forming various compounds. The charge on these ions, denoted by the symbols like Ag⁺, Cu⁺, Sn²⁺, and Pb²⁺, determines their reactivity and the types of compounds they will form in both acidic and neutral environments.
The behavior of metal ions in precipitation reactions can be predicted by considering their interactions with other ions:

• Ag⁺ is known for forming insoluble salts such as AgCl and Ag₂S.
• Cu⁺ is somewhat less common but also forms less soluble compounds like CuCl and Cu₂S.
• Sn²⁺, though reactive, is unique because it does not precipitate with HCl, but does with sulfide ions.
• Pb²⁺ creates insoluble precipitates such as PbCl₂ and PbS, even in dilute solutions.

Knowledge about metal ions and their specific reactions is crucial in predicting outcomes of precipitation reactions and understanding their occurrence in chemical analysis.

Chemical Analysis

Chemical analysis involves a variety of techniques and methods to determine the composition of substances. Precipitation reactions play a significant role in qualitative chemical analysis by helping to identify the presence of specific ions in solution.
Through controlled chemical reactions, such as those observed with HCl and H₂S in our problem, chemists can isolate and identify unknown ionic species. In a laboratory setting, precipitation is often used to:

• Test for specific ions by observing the formation of a precipitate.
• Separate ions based on their reactivity and formation of insoluble products.
• Confirm the identity of ions by comparing the observed precipitate with known reactions.

By understanding the solubility rules and reactivity characteristics of different metal ions, chemists can effectively use precipitation as a diagnostic tool. This enhances the precision and accuracy of chemical analysis in both research and applied chemistry contexts.