Problem 55 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

During the qualitative analysis of a mixture containing \(\mathrm{Cu}^{2+}\) and \(\mathrm{Zn}^{2+}\) ions, \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through an acidified solution containing these ions in order to test \(\mathrm{Cu}^{2+}\) alone. Explain briefly.

Step-by-Step Solution

Answer

Passing \(\mathrm{H}_{2} \mathrm{~S}\) in acidic conditions precipitates \(\mathrm{Cu}^{2+}\) as \(\mathrm{CuS}\), leaving \(\mathrm{Zn}^{2+}\) in solution.

Step 1: Understanding the Problem

In this exercise, we need to determine why passing \(\mathrm{H}_{2} \mathrm{~S}\) gas through an acidified solution helps test for \(\mathrm{Cu}^{2+}\) alone. We'll analyze how these ions react in acidic conditions.

Step 2: Reactivity of Copper (II) Ion

When \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through an acidified solution, \(\mathrm{Cu}^{2+}\) ions can react with \(\mathrm{S}^{2-}\) ions formed from \(\mathrm{H}_{2} \mathrm{~S}\) to produce a black precipitate of copper(II) sulfide, \(\mathrm{CuS}\). This reaction is feasible even in acidic conditions.

Step 3: Behavior of Zinc (II) Ion

In the presence of an acidic solution, \(\mathrm{Zn}^{2+}\) ions do not react with \(\mathrm{H}_{2} \mathrm{~S}\) to form \(\mathrm{ZnS}\) because it requires a basic to neutral environment to precipitate. Therefore, \(\mathrm{Zn}^{2+}\) remains in solution and does not interfere with the test for \(\mathrm{Cu}^{2+}\).

Step 4: Conclusion on Selectivity

By adding \(\mathrm{H}_{2} \mathrm{~S}\) gas in an acidic environment, \(\mathrm{Cu}^{2+}\) ions selectively precipitate as \(\mathrm{CuS}\), allowing the test to specifically indicate the presence of \(\mathrm{Cu}^{2+}\) ions while \(\mathrm{Zn}^{2+}\) ions remain unaffected.

Key Concepts

Copper(II) Sulfide Precipitation

Copper(II) sulfide precipitation is a fascinating chemical reaction used to test for the presence of copper ions, specifically \( \mathrm{Cu}^{2+} \). In a qualitative analysis, when hydrogen sulfide gas \( \mathrm{H}_2\mathrm{~S} \) is bubbled through the solution, it dissociates to produce sulfide ions \( \mathrm{S}^{2-} \). These sulfide ions react readily with \( \mathrm{Cu}^{2+} \) ions to form a solid precipitate of copper(II) sulfide \( \mathrm{CuS} \), which is black.
This reaction is particularly useful because \( \mathrm{CuS} \) has a very low solubility product, meaning it does not dissolve easily in water. As a result, when \( \mathrm{CuS} \) forms, it settles to the bottom of the test tube, making it easy to identify. Copper(II) sulfide precipitation is a clear indicator due to its distinct color, allowing for quick visual confirmation of copper ion presence.

• The formed precipitate is noticeable, making detection straightforward.
• The reaction occurs even under acidic conditions, which is key in selective precipitation.

Acidic Conditions

Understanding the role of acidic conditions is important for selective ion testing. Acidic environments affect the reactivity of metal ions with hydrogen sulfide gas. In an acidified solution, the dissociation of \( \mathrm{H}_2\mathrm{~S} \) gas is limited, producing a lower concentration of \( \mathrm{S}^{2-} \), but still enough for certain ions like copper to form a precipitate.
This limited ion production is why zinc ions \( \mathrm{Zn}^{2+} \) do not react with \( \mathrm{S}^{2-} \) to form zinc sulfide \( \mathrm{ZnS} \) because \( \mathrm{ZnS} \) requires a higher sulfide concentration, typically found in neutral to basic conditions. The acidic environment thus effectively prevents the precipitation of zinc sulfide, ensuring that only copper ions react. This specificity in reaction based on environment is an essential concept in chemical selective testing. Acidic conditions are favored in such tests to prevent unwanted reactions and false positives in ion testing.

Selectivity of Ion Reactions

The selectivity of ion reactions is a crucial aspect of qualitative analysis. It enables chemists to target specific ions for identification or separation from a mixture. Using hydrogen sulfide gas in an acidified medium perfectly illustrates this selectivity.
The primary reason \( \mathrm{Cu}^{2+} \) ions are selectively precipitated over \( \mathrm{Zn}^{2+} \) ions is due to their unique chemical properties under these conditions. While copper ions readily form insoluble copper sulfide precipitate, zinc ions do not due to the need for basic conditions to do so. This selective precipitation lets chemists differentiate between the two ions without interference from one another.

• Selectivity is essential for accurate testing and analysis in solutions containing multiple ions.
• By controlling the environment, chemists can dictate which ions react and which do not.

The selective approach helps maintain precision in chemical tests, ensuring accurate results in the presence of interfering substances.