Problem 46 - A Complete Resource Book in Chemistry for JEE Main

Question

Ions of \(\mathrm{Ca}\), Ba and Sr precipitate in \(\mathrm{V}\) group as their (a) sulphides (b) sulphates (c) oxides (d) carbonates

Step-by-Step Solution

Answer

Ions of Ca, Ba, and Sr precipitate in the V group as sulphates.

Step 1: Understanding Precipitation in the V Group

The V group in qualitative analysis refers to the group of alkaline earth metal ions, specifically calcium (Ca), barium (Ba), and strontium (Sr). These ions are known to form precipitates when certain reagents are added, helping in their identification.

Step 2: Analyzing the Precipitation Options

The ions can form precipitates with different types of anions or reagents. Sulphides form when these ions react with sulphide ions, sulphates are formed with sulphate ions, oxides with oxide ions, and carbonates with carbonate ions. We need to determine under which reaction condition Ca, Ba, and Sr precipitate in the V group.

Step 3: Evaluating Solubility Rules

According to solubility rules, sulphates of calcium, barium, and strontium are generally insoluble in water and tend to form precipitates. Conversely, their sulphides and oxides are usually soluble under normal conditions. That makes sulphates the correct group-forming precipitates.

Step 4: Concluding the Answer Based on Analysis

Given the above analysis, ions of calcium, barium, and strontium form insoluble precipitates with sulphate ions, which are typically used to identify and remove these ions from solutions in qualitative analysis.

Key Concepts

Alkaline Earth Metals

The alkaline earth metals are a group of elements found in Group 2 of the periodic table. This group includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). These metals have common properties that justify their grouping. Firstly, alkaline earth metals have two valence electrons, making them highly reactive, although less reactive than their neighbors, the alkali metals. They typically form divalent cations with a charge of +2. Some of their notable characteristics include:

• Shiny appearance and silver-white color
• Good conductors of electricity
• Malleability and ductility
• React with water to form alkaline solutions and hydrogen gas

Calcium, strontium, and barium play significant roles in chemical reactions involving precipitation because of their tendency to form insoluble compounds. This characteristic is crucial in qualitative analysis tests where specific compounds are precipitated out of solution to help identify the presence of these metal ions.

Solubility Rules

Understanding solubility rules is crucial for predicting the precipitate formation in chemical reactions. These rules are guidelines that tell us which compounds dissolve in water and which ones form precipitates or remain insoluble. Solubility rules are especially relevant in qualitative analysis because they help chemists predict the outcomes of reactions and identify substances. Key points about solubility rules include:

• Most nitrate ( O_3^-) and acetate (CH_3COO^-) salts are soluble.
• Sulphates ( SO_4^{2-}) are generally soluble, but calcium, strontium, and barium sulphates are exceptions and tend to be insoluble in water.
• Alkali metals and ammonium (NH_4^+) compounds are almost always soluble.
• Halide salts are mostly soluble except when combined with silver, lead, and mercury ions.

For calcium, barium, and strontium, specifically, the solubility of their sulphates is a significant factor that contributes to their identification via precipitation reactions. This means if you add a sulphate ion source to a solution containing these ions, you can expect to form a precipitate, confirming their presence.

Precipitation Reactions

Precipitation reactions are a type of chemical reaction where two solutions are mixed, resulting in the formation of an insoluble product called a precipitate. This is an important concept in qualitative analysis, as it allows chemists to separate and identify specific ions in a mixture.In the context of precipitation reactions involving alkaline earth metals:When solutions containing calcium, barium, or strontium are mixed with a source of sulphate ions, such as sulphuric acid or a soluble sulphate salt (like sodium sulphate), these metal ions will react to form their respective sulphate precipitates. This process can be represented by the general equation:\[ M^{2+} (aq) + SO_4^{2-} (aq) \rightarrow MSO_4 (s)\]Where \(M\) represents the metal ion (Ca, Sr, or Ba). The appearance of a solid in an otherwise clear solution indicates the formation of a precipitate, confirming the presence of these ions.Such reactions are not only critical for analytical purposes but also have practical applications in industries, such as water treatment, where specific ions are removed from solutions by forming an insoluble precipitate.