Question

Upon treatment with ammoniacal \(H_{2} S\), the metal ion that precipitates as a sulfide is (A) \(\mathrm{Fe}(\mathrm{III})\) (B) \(A l(I I I)\) (C) \(M g(\mathrm{II})\) (D) \(Z n(\mathrm{II})\)

Short answer

\(Fe(III)\) precipitates as \(FeS\) when treated with ammoniacal \(H_{2} S\).

Step-by-step solution

- Understanding the Chemical Process

When exposed to ammoniacal hydrogen sulfide \(H_{2}S\text{ (ammoniacal)}\), various metal ions can form precipitates depending on their ability to react with the sulfur ion \(S^{2-}\) from the \(H_{2}S\). In an ammoniacal environment, metal sulfides form readily if they have lower solubility products (\(K_{sp}\)) in the presence of excess \(S^{2-}\) ions.

- Identifying Precipitates from Given Options

We need to find which of the given metal ions readily forms a sulfide precipitate in an ammonia-rich solution. The solubility of the sulfides decreases down the group in the periodic table. so metal sulfides like \(FeS\), \(ZnS\), \(Al_{2}S_{3}\), and \(MgS\) may form, but only the one with a significantly lower solubility will precipitate.

- Comparing the Solubilities

Aluminum sulfide \(Al_{2}S_{3}\) reacts with water to form \(Al(OH)_{3}\) and \(H_{2}S\), meaning it does not precipitate as a sulfide in aqueous solutions. \(MgS\) is soluble in ammoniacal solutions. Among \(Fe(III)\) and \(Zn(II)\), \(FeS\) is relatively insoluble and, thus, will precipitate. So, \(FeS\) is expected to precipitate as it has a lower solubility product in comparison to other metal sulfide options provided.

Key concepts

Sulfide Precipitation

Sulfide precipitation is a process commonly used in qualitative inorganic analysis, where certain metal ions form insoluble sulfides upon treatment with sulfide ions (\(S^{2-}\)). When an ammoniacal solution containing hydrogen sulfide (\(H_2S\text{ (ammoniacal)}\)) is added to a solution of metal ions, not all metal sulfides will precipitate out at the same rate or extent. This is because each metal sulfide has a unique solubility product constant (\(K_{sp}\)), which quantitatively expresses its solubility in the solution.

In our example, among iron (\(Fe^{3+}\)), aluminum (\(Al^{3+}\)), magnesium (\(Mg^{2+}\)), and zinc (\(Zn^{2+}\)) ions, iron (\(Fe^{3+}\)) forms iron(III) sulfide (\(FeS\)), which is notably less soluble than the other metal sulfides in the ammoniacal solution. Thus, it precipitates out as a solid, leaving the other ions in solution. Factors affecting sulfide precipitation in addition to the solubility product include pH level, concentration of sulfide ions, and the presence of complexing agents like ammonia.

Solubility Product (Ksp)

The solubility product, often abbreviated as \(K_{sp}\), is an important concept in chemistry that defines the extent to which a compound can dissolve in water. It is a constant at a given temperature for a particular substance and represents the product of the ion concentrations that a solid can produce in a saturated solution, each raised to the power of its stoichiometric coefficient.

For a general compound \(A_{m}B_{n}\), the solubility product is expressed as:\[\begin{equation}\(K_{sp} = [A^{n+}]^{m}[B^{m-}]^{n}\)\end{equation}\]For instance, the insolubility of \(FeS\) in an ammoniacal solution is indicated by its low \(K_{sp}\) value. The lower the \(K_{sp}\), the less soluble the compound is, leading to precipitate formation even at low concentrations. Understanding \(K_{sp}\) values helps chemists predict whether a precipitate will form when two solutions are mixed.

Ammoniacal Hydrogen Sulfide (H2S)

Ammoniacal hydrogen sulfide refers to \(H_{2}S\) in the presence of ammonia (\(NH_{3}\)). In analytical chemistry, it plays a critical role in the selective precipitation of metal sulfides. The ammoniacal environment alters the solubility of metal sulfides.

Ammonia acts as a complexing agent, which means it can form complexes with metal ions and therefore increase the solubility of some metal sulfides. However, in the case of \(FeS\), ammonia does not form a significant complex with iron ions, which promotes the precipitation of iron(III) sulfide. Sulfide ions produced from \(H_{2}S\) by the addition of ammonia will readily react with metal ions that form insoluble sulfides, resulting in a solid precipitate, as seen with \(FeS\), aiding in the separation and identification of specific metal ions in analytical procedures.