Problem 133 - A Complete Resource Book in Chemistry for JEE Main

Question

An aqueous solution containing \(\mathrm{S}^{2-}\) ions will not give (a) White precipitate with \(\mathrm{CaCO}_{3}\) suspension (b) Purple colour with sodium thiosulphate solution (c) A yellow precipitate with the suspension of \(\mathrm{CdCO}_{3}\) in water (d) Black precipitate with lead acetate solution

Step-by-Step Solution

Answer

(a) An aqueous solution containing \(\mathrm{S}^{2-}\) ions will not give a white precipitate with \(\mathrm{CaCO}_{3}\) suspension.

Step 1: Identify Reaction with CaCO3

Examine if \(\text{S}^{2-}\) ions form a white precipitate with a suspension of \(\text{CaCO}_{3}\). When \(\text{S}^{2-}\) ions react with \(\text{CaCO}_{3}\), they do not form a white precipitate, indicating that the reaction does not occur in this case.

Step 2: Examine Reaction with Sodium Thiosulphate

Check if \(\text{S}^{2-}\) ions react to form a purple color with sodium thiosulphate solution. \(\text{S}^{2-}\) does not react with sodium thiosulphate to form a colored solution; thus, no purple color is produced in this scenario.

Step 3: Investigate Reaction with CdCO3

Analyze if \(\text{S}^{2-}\) ions form a yellow precipitate with a suspension of \(\text{CdCO}_{3}\) in water. The reaction between \(\text{S}^{2-}\) and \(\text{CdCO}_{3}\) can result in a yellow precipitate when sulfur ions react with cadmium ions to form cadmium sulfide (CdS).

Step 4: Check Reaction with Lead Acetate

Determine if \(\text{S}^{2-}\) ions form a black precipitate with lead acetate solution. \(\text{S}^{2-}\) ions can react with lead (Pb) ions to form lead sulfide (PbS), a black precipitate, indicating that this reaction does occur.

Key Concepts

Sulfide Ion Reactions

Understanding sulfide ion reactions is crucial in predicting the behavior of these ions in different chemical environments. Sulfide ions, represented as \(\text{S}^{2-}\), are known for their reactivity, especially with metal ions. When sulfide ions are introduced to solutions containing metals like lead or cadmium, distinct reactions occur. For example:

• With cadmium ions, sulfide ions react to form cadmium sulfide (CdS), which is characterized by a yellow precipitate.
• With lead ions, the reaction results in lead sulfide (PbS), a black precipitate, indicating a successful reaction.
• However, sulfide ions do not react with all substances, as seen with calcium carbonate (CaCO\(_3\)) suspension or sodium thiosulphate, where no precipitate or color change is observed.

These reactions highlight the selective nature of sulfide ions and their ability to form specific compounds under the right conditions.

Precipitation Reactions in Aqueous Solutions

Precipitation reactions occur when two soluble substances in solution react to form an insoluble product, known as a precipitate. In aqueous solutions, it's important to consider the solubility rules to predict these outcomes. For instance:

• When \(\text{S}^{2-}\) ions interact with metal ions such as Cd\(^{2+}\) or Pb\(^{2+}\), a precipitate forms due to the low solubility of cadmium sulfide and lead sulfide in water.
• Conversely, certain combinations like \(\text{S}^{2-}\) with calcium ions in calcium carbonate do not form a precipitate because the resulting compounds remain soluble in the solution.

By understanding these principles, one can effectively predict when a precipitate will form, aiding in the identification of materials present in a reaction.

Chemical Reaction Predicting

Predicting chemical reactions involves understanding the reactivity of ions and compounds involved, as well as the solubility of potential products. Factors to consider include:

• The types of ions present, such as \(\text{S}^{2-}\), and their known reactions with various metal ions.
• The solubility of potential reaction products, which requires knowledge of solubility rules. For example, sulfide compounds with metals like lead and cadmium are notoriously insoluble, leading to precipitation.
• The conditions of the reaction, such as pH and temperature, which can influence the solubility and precipitation of products.

By integrating these factors, chemists can anticipate the formation of precipitates or lack thereof, allowing for accurate predictions about chemical reactions in aqueous solutions.