Problem 23 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which one among the following pairs of ions cannot be separated by \(\mathrm{H}_{2} \mathrm{~S}\) in dilute hydrochloric acid? (a) \(\mathrm{Bi}^{3+}, \mathrm{Sn}^{4+}\) (b) \(\mathrm{Al}^{3+}, \mathrm{Hg}^{2+}\) (c) \(\mathrm{Zn}^{2+}, \mathrm{Cu}^{2+}\) (d) \(\mathrm{Ni}^{2+}, \mathrm{Cu}^{2+}\)

Step-by-Step Solution

Answer

(c) Zn^2+ and Cu^2+; (d) Ni^2+ and Cu^2+.

Step 1: Understand the role of H2S in dilute HCl

Hydrogen sulfide (H2S) in dilute hydrochloric acid is used to precipitate metal ions as their sulfide salts. This works effectively under acidic conditions where the solubility of certain metal sulfides allows for separation.

Step 2: Examine the behavior of ions in acidic medium

In dilute HCl, metal ions that form insoluble sulfides will precipitate, while those with soluble or no sulfides will stay in solution. These ions fall into different groups: Group II ions, such as Hg^2+, and Group III ions, like Bi^3+, Sn^4+, Al^3+, Zn^2+, Ni^2+, and Cu^2+. Group II ions precipitate with H2S in dilute HCl while others do not.

Step 3: Analyze the separation of each ion pair

- Bi^3+ and Sn^4+: No separation; both can precipitate with H2S. - Al^3+ and Hg^2+: Hg^2+ precipitates, but Al^3+ does not, allowing separation. - Zn^2+ and Cu^2+: Both form precipitates; thus, they cannot be separated by H2S in dilute HCl. - Ni^2+ and Cu^2+: Both form precipitates, so they cannot be separated.

Step 4: Identify the correct answer

Reviewing the above analysis, the ion pairs that cannot be separated by H2S in dilute HCl are those where both ions precipitate. The correct answer is (c) Zn^2+ and Cu^2+ and (d) Ni^2+ and Cu^2 +.

Key Concepts

Precipitation Reactions

Precipitation reactions are processes where soluble ions in separate solutions combine to form an insoluble compound, known as a precipitate. This typically occurs when the product of the reaction of two ionic compounds is not soluble in water. The formation of a precipitate confirms that a reaction occurred.
These reactions are essential in chemistry, particularly in qualitative analysis, as they help identify the presence of specific ions within a solution.

• When two aqueous solutions are mixed, and an insoluble substance forms, a precipitation reaction has taken place.
• The insoluble substance formed is often a solid that can be seen settling out of the solution, especially if given time to stand.
• Precipitation can be predicted using solubility rules, which indicate whether a compound will dissolve in water or form a solid.

Understanding precipitation reactions is crucial for comprehending how compounds interact, especially for determining the composition of elements in a mixture.

Sulfide Precipitation

Sulfide precipitation involves the addition of hydrogen sulfide ( H_{2}S ) to a solution to form metal sulfides. This is a particular type of precipitation reaction used for separating metal ions.

• In acidic conditions, such as in the presence of dilute hydrochloric acid (HCl), sulfide ions will readily react with metal ions to form metal sulfide precipitates.
• This method is especially useful because sulfide ions can cause specific metal ions to precipitate, allowing for effective separation.

The sulfide precipitation method allows for selective precipitation of metal ions based on their sulfide solubility, a critical factor in chemists' toolkit for identifying and separating elements from each other.

Ion Separation by H2S

The process of ion separation by H _{2}S is grounded on the differential solubility of metal sulfides. Under acidic conditions, H _{2}S can precipitate certain metal ions from a solution as insoluble sulfides. This selective precipitation offers a means of separating ions.

• Different metal ions will have varying tendencies to precipitate as sulfides based on their solubility products in the presence of H_{2}S.
• Some ions will form precipitates readily, like Hg ^{2+} , while others, especially those in weaker acidic conditions or more complex ions like Al ^{3+} , will remain soluble.

Ion separation by H _{2}S is particularly valuable in qualitative analysis, allowing for targeted tests to identify specific ions present in complex mixtures.

Metal Sulfide Solubility

The solubility of metal sulfides is a key factor determining the outcome of separation processes like precipitation. It relies on specific conditions such as pH and concentration of the sulfide ion.

• Metal sulfide solubility often decreases as the solution becomes more acidic, allowing different metals to be separated based on how readily they form precipitates.
• Sulfides like ZnS and CuS have low solubility in acidic conditions, aiding in the formation of solid precipitates when H _{2}S is introduced.

Understanding these solubility differences is crucial for employing techniques that hinge on selective precipitation, ensuring that methods achieve the desired separation or identification of metals.

Qualitative Analysis in Chemistry

Qualitative analysis in chemistry aims to determine the presence of specific ions or compounds within a mixture. This approach contrasts with quantitative analysis, which measures the amount of a substance.

• Qualitative analysis techniques utilize reactions like precipitation to confirm the presence of ions.
• Precipitation reactions, like those involving H _{2}S , are central to separating ions into distinct groups based on their properties and reactivity.

By using methods such as ion separation and precipitation, qualitative analysis helps chemists identify substance components easily and accurately, which is essential in various fields like environmental testing, pharmaceuticals, and forensic investigations, among others.