Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is formed, when \(\mathrm{H}_{2} \mathrm{~S}\) is passed through the solution of sodium zincate? (a) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (b) \(\mathrm{ZnS}\) (c) \(\mathrm{Na}_{2}\left[\mathrm{Zn}(\mathrm{OH})_{4}\right]\) (d) \(\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

(b) \(\mathrm{ZnS}\)

Step 1: Understanding the Reaction

When hydrogen sulfide \(\mathrm{H}_{2} \mathrm{S}\) is passed through a solution of sodium zincate \(\mathrm{Na}_{2} [\mathrm{Zn} (\mathrm{OH})_{4}]\), a chemical reaction occurs. Sodium zincate is a complex ion compound containing zinc, while hydrogen sulfide contains sulfide ions \(\mathrm{S}^{2-}\). To solve this exercise, we need to determine which potential product is formed during this reaction.

Step 2: Chemical Reaction Equation

The passage of \(\mathrm{H}_{2} \mathrm{~S}\) through sodium zincate \(\mathrm{Na}_{2}[\mathrm{Zn}(\mathrm{OH})_{4}]\) precipitates zinc sulfide (\(\mathrm{ZnS}\)). The chemical reaction can be written as: \[\mathrm{Na}_{2}[\mathrm{Zn}(\mathrm{OH})_{4}] + \mathrm{H}_{2} \mathrm{S} \rightarrow \mathrm{ZnS} + 2\mathrm{NaOH} + 2\mathrm{H}_2\mathrm{O}\] This reaction shows that zinc from the zincate complex reacts with sulfide ions to form an insoluble precipitate, zinc sulfide \(\mathrm{ZnS}\).

Step 3: Determining the Precipitate

Among the given options, zinc sulfide \(\mathrm{ZnS}\) is the compound that forms as a precipitate. This occurs because the sulfide ion from \(\mathrm{H}_{2} \mathrm{S}\) combines with the zinc ion from sodium zincate to form \(\mathrm{ZnS}\). Other options are not correct because they either do not involve this reaction path or result in other chemical products.

Key Concepts

Chemical Reactions

Inorganic chemistry often involves the study of chemical reactions where two or more substances interact to form new compounds. In the context of this exercise, a chemical reaction occurs when hydrogen sulfide (\(\mathrm{H}_2\mathrm{S}\)) is introduced to a solution of sodium zincate (\(\mathrm{Na}_2\left[\mathrm{Zn}(\mathrm{OH})_4\right]\)). This results in the formation of a new compound, zinc sulfide (\(\mathrm{ZnS}\)), alongside other byproducts such as sodium hydroxide (\(\mathrm{NaOH}\)) and water (\(\mathrm{H}_2\mathrm{O}\)). This reaction showcases how initial reactants transform into different compounds by rearranging their atoms and forming new chemical bonds.

• Reactants: substances undergoing the reaction (\(\mathrm{H}_2\mathrm{S}\) and \(\mathrm{Na}_2\left[\mathrm{Zn}(\mathrm{OH})_4\right]\)).
• Products: new substances formed during the reaction (\(\mathrm{ZnS}\), \(\mathrm{NaOH}\), and \(\mathrm{H}_2\mathrm{O}\)).

Understanding chemical reactions is crucial for predicting the behavior of compounds in various conditions. This allows us to comprehend the process as a transfer and transformation of energy and matter from reactants to products.

Complex Ion

Complex ions play a central role in many inorganic chemistry reactions and are a key component of this exercise. A complex ion consists of a central metal atom or ion, bound to one or more molecules or ions, typically through coordinate covalent bonds. In our exercise, sodium zincate (\(\mathrm{Na}_2\left[\mathrm{Zn}(\mathrm{OH})_4\right]\)) features the zinc ion at the core of the complex ion.

• The zinc atom is surrounded by four hydroxide ions (\(\mathrm{OH}^-\)).
• The formation of such complexes stabilizes the metal in solution.

Complex ions are often highly reactive and undergo chemical reactions that simplify them, such as the breakdown of sodium zincate when reacted with hydrogen sulfide. The dissociation of the complex releases the zinc ion, allowing it to combine with sulfide ions to form an insoluble precipitate. These interactions are fundamental to understanding complex ion behavior and their reactivity patterns.

Zinc Sulfide Precipitation

Zinc sulfide precipitation is an outcome of the reaction between hydrogen sulfide and sodium zincate. Precipitation describes the process wherein soluble ions in solution interact to form a solid, insoluble compound that separates from the liquid. In this scenario, zinc ions meet sulfide ions, leading to the formation of zinc sulfide (\(\mathrm{ZnS}\)), a white solid that appears as the precipitate.

• Precipitates form when the product of the ions' concentrations exceeds their solubility product (Ksp).
• \(\mathrm{ZnS}\) is a common example of an inorganic compound formed through precipitation.

Understanding precipitation is essential for predicting outcomes related to solubility and insolubility in chemical reactions. It highlights how insoluble compounds can be isolated from solutions, a principle widely used in chemical separation techniques and analysis.