Question

Which of the following pair is not distinguished by passing \(\mathrm{H}_{2} \mathrm{~S} ?\) (a) As, Cd (b) \(\mathrm{Zn}, \mathrm{Mn}\) (c) \(\mathrm{Hg}, \mathrm{Pb}\) (d) \(\mathrm{Cd}, \mathrm{Pb}\)

Short answer

The pair As, Cd is not distinguished by passing H2S.

Step-by-step solution

Understand the role of H2S

Hydrogen sulfide (H_2S) is commonly used in qualitative analysis to precipitate metal sulfides. The ability of a metal ion to precipitate as a sulfide depends on the solubility of its sulfide form.

Identify metal sulfide solubilities

Metals can be organized into groups based on the solubility of their sulfides: Group I consists of metals whose sulfides are insoluble in both dilute and concentrated solutions, like As, Cd, and Hg. Group II includes metals whose sulfides are insoluble in dilute acid but soluble in concentrated solutions, like Zn and Mn, and Group III where no sulfides form.

Analyze each pair

For (a) As and Cd, both form insoluble sulfides with H_2S in acidic medium. For (b) Zn and Mn, Zn forms a precipitate, but Mn does not precipitate under the same conditions. In (c) Hg forms insoluble HgS, and Pb forms PbS, both are precipitated. Lastly, (d) Cd forms CdS (insoluble), and Pb forms PbS under similar conditions.

Determine which pair is not distinguished

Compare the pairs: In pair (b), Zn can be precipitated while Mn cannot, so they are distinguished. In pairs (a) and (c), both metals form sulfides in similar conditions, but they're not key for distinction. In pair (d), Cd forms a yellow sulfide precipitate and Pb forms a black one; visible distinction can arise. The only pair without clear separation based on sulfide formation alone in acidic medium is pair (a), As and Cd, as they both form similar precipitates.

Key concepts

Metal Sulfides

Metal sulfides are compounds that consist of a metal ion combined with sulfide ions (S²⁻). These compounds are vital in qualitative analysis, particularly in separating and identifying metal ions from a mixture. When hydrogen sulfide gas ( H₂S ) is passed through an aqueous solution, it reacts with metal ions to form metal sulfides. Some key points about metal sulfides:

• Solubility: The solubility of metal sulfides varies. Some, like cadmium sulfide ( CdS ), are insoluble in acidic conditions, leading to precipitate formation.
• Appearance: Metal sulfides often have characteristic colors, which aids in their identification. For instance, CdS is yellow, while lead sulfide ( PbS ) is black.
• Utility: They are used to identify metals in group analysis based on their formation as a precipitate.

By understanding the properties of metal sulfides, chemists can determine which metal ions are present in a solution.

Hydrogen Sulfide

Hydrogen sulfide ( H₂S ) is a colorless gas with a distinct smell of rotten eggs. It plays a crucial role in the qualitative analysis of metal ions by helping to form metal sulfides that can be easily observed. Here are some important aspects of H₂S :

• Reactivity: H₂S reacts with many metal ions to form sparingly soluble or insoluble metal sulfides.
• Medium: The reaction often occurs in acidic conditions, which influence the solubility of certain metal sulfides.
• Precipitation: When H₂S is mixed with metal ions in a solution, it can create a precipitate if the resulting metal sulfide is insoluble in the solution.

The use of H₂S is a traditional and straightforward method to help identify and separate metal ions based on the solubility properties of the resulting metal sulfides.

Group Analysis

Group analysis in chemistry refers to a systematic method of identifying metal ions based on their reactivity and properties, such as the formation of insoluble sulfides with hydrogen sulfide. This process helps in categorizing metals into different groups, which can be analyzed separately. Some important features of group analysis include:

• Sequential Approach: Metals are tested in a sequence, often by adding specific reagents like H₂S to create reactions that help distinguish them.
• Classification: Metals are classified into groups based on the solubility of their resulting sulfides. Group I metals form insoluble sulfides under all conditions, while Group II metals form sulfides that are insoluble in dilute acid but soluble in concentrated acid.
• Distinction: This method is effective for distinguishing between metals that form different colored precipitates or require specific conditions for sulfide formation.

By utilizing group analysis, chemists can efficiently identify metal ions present in a solution, using reactions of varying specificity and sensitivity.