Problem 31 - Chemistry in 30 days

Question

When \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through the salt solution in the presence of \(\mathrm{HCl}\), which of the following is not precipitated? (a) Nis (b) \(\mathrm{CdS}\) (c) \(\mathrm{CuS}\) (d) \(\mathrm{Sb}_{2} \mathrm{~S}_{3}\)

Step-by-Step Solution

Answer

NiS is not precipitated.

Step 1: Understand the Context

In qualitative analysis, passing hydrogen sulfide gas through salt solutions in the presence of hydrochloric acid helps in precipitating certain metal sulfides. The solubility of these sulfides varies, and only those that are less soluble will precipitate under acidic conditions.

Step 2: Determine the Solubility

Identify which of the given sulfides (NiS, CdS, CuS, Sb2S3) precipitate in acidic medium. Note that metal sulfides like CuS and CdS are less soluble and tend to precipitate, whereas NiS generally does not.

Step 3: Consider Solubility Rules

Under acidic conditions, sulfides of Group 2 metals (like Ni) are typically more soluble compared to those of other groups, especially transition metals like copper and cadmium. Thus, NiS will not precipitate.

Step 4: Analyze the Exception

Identify which compound is the exception to the precipitation rule under these conditions. NiS is more soluble in acidic conditions and hence does not precipitate when H2S is passed in the presence of HCl.

Key Concepts

Solubility Rules

Solubility rules are essential guidelines that help us predict whether a solute will dissolve or form a precipitate in a solution under specific conditions. These rules are particularly useful in qualitative analysis—a type of chemical analysis dealing with the identification of substances based on their chemical properties. The solubility rules can be simplified into a few key principles:

• Most nitrates (NO₃^-) and acetates (C₂H₃O₂^-) are soluble.
• Salts containing alkali metal cations and ammonium (NH₄⁺) are generally soluble.
• Sulfides of transition metals are usually insoluble, except in acidic conditions where some can dissolve more readily.
• Salts of chloride (Cl^-), bromide (Br^-), and iodide (I^-) are typically soluble, except with silver (Ag⁺), lead (Pb²⁺), and mercury (Hg₂²⁺).

In acidic environments, solubility can change, especially with respect to sulfide ions. This is why understanding and applying these rules efficiently is crucial in predicting the outcome of chemical experiments.

Metal Sulfides

Metal sulfides are compounds composed of metals combined with sulfur. In the context of chemistry and qualitative analysis, they are noteworthy because they possess unique solubility characteristics in different environments. Many metal sulfides are insoluble in water, but this changes significantly in acidic conditions when hydrogen sulfide (H2S) gas is introduced.

• Metal sulfides, such as CdS and CuS, are typically insoluble in water and precipitate in acidic solutions.
• However, sulfides of metals like Ni, when in the presence of hydrochloric acid (HCl) and H₂S, tend to remain soluble and do not form precipitates.
• This unique solubility behavior of metal sulfides is crucial in separation techniques used in analytical chemistry, particularly during the transition metal analysis.

Transition Metals

Transition metals are elements found in the center of the periodic table, typically in groups 3 through 12. They are characterized by their ability to form various oxidation states and colorful compounds due to electronic transitions between d-orbitals. These properties make transition metals significant in qualitatively analyzing metal ions in a solution.

• Transition metals like copper (Cu) and cadmium (Cd) are known to form insoluble sulfides, such as CuS and CdS, in acidic conditions, leading to their easy precipitation.
• This behavior helps in distinguishing transition metal ions from ions of more soluble metals, especially when conducting precipitation reactions with H2S gas.
• Understanding the chemistry of transition metals is essential for predicting the formation of complex ions and their reactivity in various solvents and solutions.

Group 2 Metals

Group 2 metals, also known as alkaline earth metals, include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These metals are characterized by their full s-orbital electron configuration, which makes them less reactive than Group 1 metals but still highly relevant in reaction dynamics.

• In general, sulfides of alkaline earth metals are more soluble in water compared to those of transition metals. This solubility aspect is a critical distinction in qualitative analysis.
• In acidic environments, Group 2 metal sulfides can sometimes remain soluble where others do not, such as in the case of nickel sulfide (NiS) which does not precipitate in the presence of HCl.
• This solubility rule is particularly important in identifying and separating Group 2 metals from transition metals using selective precipitation methods.