Problem 109 - A Complete Resource Book in Chemistry for JEE Main

Question

If \(\mathrm{NaOH}\) is added to an aqueous solution of \(\mathrm{Zn}^{2+}\) ions, a white precipitate appears and on adding excess \(\mathrm{NaOH}\), the precipitate dissolves. In this solution zinc exists in the (a) anionic part (b) cationic part (c) both in anionic and cationic parts (d) there is no zinc left in the solution

Step-by-Step Solution

Answer

(a) anionic part

Step 1: Identify the Reaction with Zinc Ions and NaOH

When sodium hydroxide (NaOH) is added to a solution containing zinc ions (Zn²⁺), a chemical reaction occurs that produces zinc hydroxide, Zn(OH)₂, as a precipitate. The reaction can be represented as: \[ \text{Zn}^{2+} + 2\text{OH}^- \rightarrow \text{Zn(OH)}_2 \downarrow \] This results in the formation of a white precipitate of zinc hydroxide.

Step 2: Effect of Excess NaOH on the Precipitate

Upon adding excess NaOH to the precipitate Zn(OH)₂, it dissolves to form a complex ion. Zinc hydroxide reacts with additional hydroxide ions (OH⁻) to form a soluble zincate ion: \[ \text{Zn(OH)}_2 + 2\text{OH}^- \rightarrow [\text{Zn(OH)}_4]^{2-} \] This reaction indicates that zinc now exists in the solution as part of the anionic zincate ion, [Zn(OH)₄]²⁻.

Step 3: Determine the Part in Solution

After the excess NaOH is added and the precipitate dissolves, zinc exists in the anionic part of the solution as part of the zincate ion, [Zn(OH)₄]²⁻. Therefore, in the final solution, zinc is found as an anion.

Key Concepts

Zinc hydroxide precipitation

When sodium hydroxide (\(\mathrm{NaOH}\)) is added to a solution containing zinc ions (\(\mathrm{Zn}^{2+}\)), a fascinating transformation occurs.
This initial reaction results in the formation of zinc hydroxide, \(\mathrm{Zn(OH)}_2\), which is characteristically a white, solid precipitate.
The reaction that takes place can be explained simply as follows:
The zinc ions react with hydroxide ions present in the solution to produce zinc hydroxide:
- Zinc ions (\(\mathrm{Zn}^{2+}\)) combine with hydroxide ions (\(\mathrm{OH}^-\))
- The result is zinc hydroxide (\(\mathrm{Zn(OH)}_2\)) as a precipitate:
\[\text{Zn}^{2+} + 2\text{OH}^- \rightarrow \text{Zn(OH)}_2 \downarrow\]Zinc hydroxide precipitation is crucial in this context as it provides a visual clue in different chemical reactions involving zinc. Many laboratory and industrial processes take advantage of this simple and observable reaction mechanism.

Complex ions formation

An interesting development occurs when excess sodium hydroxide (\(\mathrm{NaOH}\)) is continually added to the zinc hydroxide precipitate.
The precipitate dissolves, indicating the formation of complex ions.
In chemistry, complex ions are entities formed from a metal ion bonded to one or more molecules or ions, called ligands.
In this reaction, the zinc hydroxide reacts with additional hydroxide ions to form a soluble complex ion.
What takes place here is a transformation from the simple zinc hydroxide compound into a compound called zincate:
- Zinc hydroxide (\(\mathrm{Zn(OH)}_2\)) reacts with additional hydroxide ions
- This results in the formation of the zincate ion, (\([\text{Zn(OH)}_4]^{2-}\)).
The chemical equation representing this process is:
\[\text{Zn(OH)}_2 + 2\text{OH}^- \rightarrow [\text{Zn(OH)}_4]^{2-}\]The formation of complex ions is a significant aspect of chemistry as it serves as the basis for understanding reactions in various fields like biochemistry, metallurgy, and environmental sciences.

Solubility in excess NaOH

The solubility of zinc hydroxide in excess \(\mathrm{NaOH}\) is a prime example of the complex interplay within chemical reactions.
Initially appearing as an insoluble precipitate, zinc hydroxide dissolves with the addition of further hydroxide ions.
This transformation alters the solubility dynamics, showcasing that the presence of excess reactants can alter solubility.
Here's what happens step-by-step:
- Zinc hydroxide is initially insoluble, resulting in a white precipitate.
- On the addition of excess \(\mathrm{NaOH}\), zinc hydroxide changes into a soluble zincate ion, (\([\text{Zn(OH)}_4]^{2-}\)).
- This transition from insoluble to soluble form marks the zinc transfer from a simple solid to a more complex aqueous solution form.This phenomenon is critical for applications in various fields:

• Industries use similar processes in refining metals and treating wastewater.
• Understanding these properties helps chemists engineer solutions for complex chemical syntheses.
• Zinc's chemical properties are also significant in nutrient formulations and medical treatments.