Problem 78 - A Complete Resource Book in Chemistry for JEE Main

Question

Mark the correct statement. (a) V group basic radicals precipitate as carbonates (b) IV group basic radicals precipitate as sulphides (c) I group basic radicals precipitate as chlorides (d) all the above statements are correct

Step-by-Step Solution

Answer

(d) all the above statements are correct

Step 1: Identifying Precipitation Groups

We need to identify the type of salt with which different groups of basic radicals (cations) precipitate. In qualitative analysis, certain groups of cations are precipitated using specific reagents to separate them.

Step 2: Analyzing Group I Radicals

Group I basic radicals precipitate as chlorides. This group includes cations like Ag⁺, Pb²⁺, and Hg₂²⁺, which form insoluble chlorides when reacted with dilute hydrochloric acid (HCl). This makes statement (c) true.

Step 3: Reviewing Group IV Radicals

Group IV basic radicals typically include cations such as Zn²⁺, Ni²⁺, Co²⁺, and Mn²⁺. These cations precipitate as sulphides in the presence of H₂S in an alkaline medium (ammonium sulfide solution). Hence, statement (b) is also true.

Step 4: Understanding Group V Radicals

Group V basic radicals usually consist of cations like Ba²⁺, Sr²⁺, and Ca²⁺. These cations precipitate as carbonates when treated with ammonium carbonate in the presence of ammonia. Therefore, statement (a) is true as well.

Step 5: Conclusion

Since each specific group of radicals precipitates in the manner described in statements (a), (b), and (c), all the statements provided are correct. Thus, option (d) is the correct choice.

Key Concepts

Precipitation of Cations

Qualitative analysis in chemistry often involves the separation and identification of cations by converting them into insoluble compounds, known as precipitates. This process, called precipitation, is essential for analyzing the composition of unknown samples.

Precipitation occurs when a cation reacts with an anion in a solution to form an insoluble salt. The particular reagents used for precipitation help distinguish between different groups of cations. Each group of basic radicals, or cations, precipitates with specific inorganic or organic reagents, allowing chemists to separate them from other ions in the mixture.

• Precipitation is determined by the solubility of the ionic compounds formed.
• This method is crucial in separating ions before further identification tests.

Understanding the precipitation of cations forms the foundation of classical qualitative analysis techniques used in analytical chemistry laboratories.

Group I Basic Radicals

Group I basic radicals are among the first cations to be identified in qualitative analysis, primarily due to their reactions with dilute hydrochloric acid (HCl). These cations include silver (Ag⁺), lead (Pb²⁺), and mercury (Hg₂²⁺).

When subjected to dilute HCl, these cations form insoluble chlorides, such as AgCl, PbCl₂, and Hg₂Cl₂. This reaction is quick and efficient, resulting in visible precipitates:

• The silver chloride precipitate is white and turns purple upon exposure to light, due to the reduction of silver ions.
• Lead chloride emerges as a white precipitate, soluble in hot water.
• Mercurous chloride forms a white precipitate as well, which further reacts in ammonia solutions to create a complex mixture.

Because these chlorides are fairly insoluble in water, this step helps isolate Group I cations from other ions present in the solution.

Group IV Basic Radicals

Group IV basic radicals include cations like zinc (Zn²⁺), nickel (Ni²⁺), cobalt (Co²⁺), and manganese (Mn²⁺). Unlike Group I cations, these radicals are precipitated as sulphides, primarily in the presence of hydrogen sulfide (H₂S) gas under alkaline conditions.

Using an alkaline medium such as ammonium sulfide solution allows for the formation of insoluble metal sulfides, each with distinct colors:

• Zinc sulfide is white and slightly soluble, appearing in alkaline medium due to its amphoteric nature.
• Nickel sulfide precipitates as a black solid, indicative of the low solubility typical for transition metal sulfides.
• Cobalt sulfide similarly forms a black precipitate, challenging to differentiate visually from nickel sulfide.
• Manganese sulfide appears as a pinkish-buff precipitate, adding a bit of color variety to these reactions.

The precipitation of these sulphides helps separate Group IV radicals, distinguishing them from cations of other groups based on their unique precipitation behaviours.