Question

The reagent/s) that can selectively precipitate \(\mathrm{S}^{2-}\) from a mixture of \(\mathrm{S}^{2-}\) and \(\mathrm{SO}_{4}{ }^{2-}\) in aqueous solution is(are) (A) \(\mathrm{CuCl}_{2}\) (B) \(\mathrm{BaCl}_{2}\) (C) \(\mathrm{Pb}\left(\mathrm{OOCCH}_{3}\right)_{2}\) (D) \(\mathrm{Na}_{2}\left[\mathrm{Fe}(\mathrm{CN})_{5} \mathrm{NO}\right]\)

Short answer

Pb(OOCCH3)2 selectively precipitates S2- from a mixture of S2- and SO42-.

Step-by-step solution

Investigate the Solubility Rules

Use the solubility rules to determine which compounds will precipitate when certain reagents are added. Sulfides are generally insoluble except for those of alkali metals, alkaline earth metals, and ammonium. Sulfates are generally soluble, with exceptions such as BaSO4, PbSO4, and SrSO4.

Examine the Reagents Against the Solubility Rules

Evaluate each reagent to see if it would form an insoluble compound with S2- or SO42-. CuCl2 forms an insoluble copper(II) sulfide, BaCl2 forms an insoluble barium sulfate, Pb(OOCCH3)2 forms an insoluble lead(II) sulfide, and Na2[Fe(CN)5NO] does not lead to precipitation of either sulfide or sulfate.

Determine the Selective Precipitant

Select a reagent that precipitates S2- but not SO42-. BaCl2 should be avoided because it will precipitate both anions. CuCl2 is not selective as it also forms insoluble sulfates. Na2[Fe(CN)5NO] does not precipitate either anion. Pb(OOCCH3)2, however, will form a precipitate with sulfide but not sulfate, making it the selective reagent.

Key concepts

Solubility Rules

Understanding the solubility rules is crucial when working with chemical solutions and encountering selective precipitation challenges.
Solubility rules are a set of guidelines that predict whether a compound will dissolve in water to form an aqueous solution or form a precipitate. They are not absolute laws but offer a good prediction based on experimental evidence.

General Solubility Trends

Most nitrates (o3^-)) and alkali metal compounds are soluble. Compounds containing the ammonium ion (oh4^+)) generally dissolve readily in water. On the contrary, carbonates (cO3^2-)), phosphates (PO4^3-)), sulfides (S^2-)), and hydroxides often form precipitates.
There are exceptions to these rules though. For sulfides, for instance, those of alkali and alkaline earth metals are soluble, indicating the nuanced nature of these guidelines. Sulfates (SO4^2-)) are another example; they are usually soluble, except for a few like barium sulfate (BaSO4), lead sulfate (PbSO4), and strontium sulfate (SrSO4).
For the JEE Advanced exam, understanding these nuances could give an edge in solving related problems, such as selecting the appropriate reagent for selective precipitation.

Precipitate Formation

Precipitate formation occurs when two solutions containing different ions are mixed and an insoluble compound is formed. This is a result of the product of ion concentrations exceeding the solubility product (Ksp), leading to precipitation.

Understanding Precipitation Reactions

In the reaction, cations and anions combine to form a solid precipitate. This process is governed by the solubility product constant, with a lower Ksp representing less soluble compounds. In the context of JEE Advanced, being able to predict this can help solve complex reactions such as selective precipitation where only a specific ion, like S^2-, is targeted to precipitate while leaving others, like SO4^2-, in solution.
In a classroom scenario, this concept is often demonstrated using qualitative analysis, where students combine different ionic solutions to witness the formation of precipitates, helping connect theoretical solubility rules to practical laboratory observations.

Chemical Reagents

Chemical reagents are substances used to cause a chemical reaction. They are essential in selective precipitation since the choice of reagent can dictate which ion precipitates.

Role of Reagents in Selective Precipitation

Considering the exercise, it's important to choose a reagent that will react with S^2- to form an insoluble compound but not with SO4^2-. This underscores the essence of reagents' selectivity for achieving desired outcomes in reactions.
In the context of competitive exams like the JEE Advanced, understanding the properties of various reagents and their reaction outcomes is pivotal. For instance, Pb(OOCCH3)2 is selective for precipitating S^2- without affecting SO4^2-, which is a key insight in solving the given problem. Tailoring reactions through the adept selection of chemical reagents is a skill that can significantly benefit students in both theoretical and practical chemistry applications.