Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

If \(\mathrm{NaOH}\) is added to an aqueous solution of zinc ions, a white precipitate appears and on adding excess of \(\mathrm{NaOH}\), the precipitate dissolves. In this solution, zinc exists in the (a) anionic part (b) cationic part (c) both in cationic and anionic parts (d) there is no zinc in the solution

Step-by-Step Solution

Answer

(a) anionic part

Step 1: Identify Initial Reaction

When \( ext{NaOH}\) is added to an aqueous solution containing zinc ions \(( ext{Zn}^{2+})\), a white precipitate of zinc hydroxide \( ext{Zn(OH)}_2\) forms. This reaction is represented by the equation: \( ext{Zn}^{2+} + 2 ext{OH}^- ightarrow ext{Zn(OH)}_2(s)\).

Step 2: Examine Reaction with Excess NaOH

When excess \( ext{NaOH}\) is added, the \( ext{Zn(OH)}_2 \) precipitate dissolves, forming the soluble complex ion zincate \([ ext{Zn(OH)}_4]^{2-}\). This reaction is represented by the equation: \( ext{Zn(OH)}_2(s) + 2 ext{OH}^- ightarrow [ ext{Zn(OH)}_4]^{2-}\).

Step 3: Determine Zinc's form in the Solution

With excess \( ext{NaOH} \), the zinc ions convert into the anionic complex \( [ ext{Zn(OH)}_4]^{2-} \). Therefore, zinc exists in the anionic part of the solution.

Key Concepts

Zinc Complex Ions

In chemical reactions, zinc can form complex ions by combining with other ions or molecules. Typically, complex ions consist of a central metal ion surrounded by molecules or anions, creating a stable structure. For zinc, common ligands include hydroxide ions, which lead to the formation of zinc complex ions when in an aqueous solution.

In the presence of excess hydroxide ions, zinc can form a stable complex known as zincate ion, denoted as \[\text{Zn(OH)}_4\]^{2-}\. This transformation showcases zinc's ability to become part of complex ions. The hydroxide ions coordinate with the zinc, and the metal becomes part of the anionic structure. This transition highlights the versatility of zinc in forming stable complexes within a solution, enhancing the behavior and properties of the resulting compound.

Understanding zinc complex ions is crucial in chemistry as they affect the solubility, reactivity, and color of zinc-containing solutions. This concept provides foundational knowledge essential in fields like inorganic chemistry and applied chemical sciences.

Precipitation Reactions

Precipitation reactions occur when two soluble ions in aqueous solutions combine to form an insoluble compound, called a precipitate. In the context of zinc chemistry, zinc ions \(\text{Zn}^{2+}\) react with hydroxide ions \(\text{OH}^{-}\) to form zinc hydroxide \[\text{Zn(OH)}_2(s)\]. This reaction results in the appearance of a white precipitate, indicating that the product does not dissolve in the existing solution.

The chemistry behind precipitation involves the concept of solubility product and the equilibrium between the dissolved ions and the solid precipitate. The phase shift from soluble ions to an insoluble product forms the basis of many chemical separations, analytical techniques, and industrial processes. Recognizing precipitation reactions allows chemists to isolate specific compounds, monitor reaction progress, and create pure chemical samples.

Precipitation is a valuable tool in laboratory practices, giving insight into solubility, reaction dynamics, and compound interactions in aqueous solutions.

Dissolution in Excess Reagents

The phenomenon of dissolution with excess reagents describes how certain precipitates can dissolve when additional reagents convert them into soluble forms. In our zinc example, adding excess \(\text{NaOH}\) leads to the dissolution of zinc hydroxide precipitate \[\text{Zn(OH)}_2(s)\] by forming the soluble zincate ion \[\text{Zn(OH)}_4^{2-}\].

This conversion happens because the higher concentration of hydroxide ions offers more molecular interactions, stabilizing the zinc as a soluble, complex ion. The shift from a solid precipitate to a soluble complex changes the physical state of the compound without altering its elemental composition.

Being able to manipulate solubility through excess reagents is beneficial in both analytical chemistry and industrial applications. It allows for the fine-tuning of chemical processes, enabling the purification, separation, and identification of components in a mixture. Students can appreciate how adjustments in reagent quantities can pivot the course of a reaction, transforming even the most stubborn precipitates into clear solutions. This understanding is crucial for mastering solution equilibria and reaction manipulations.