Question

If \(\mathrm{NaOH}\) is added to an aqueous solution of \(\mathrm{Zn}^{2+}\) ions a white precipitate appears and on adding excess NaOH the precipitate dissolves. In this solution, zinc exists in the (1) Cationic part (2) Anionic part (3) Both in cationic and anionic parts (4) There is no zinc left in the solution

Short answer

Anionic part

Step-by-step solution

- Reaction with NaOH

When \(\mathrm{NaOH}\) is added to an aqueous solution of \(\mathrm{Zn}^{2+}\) ions, a white precipitate of \(\mathrm{Zn(OH)_2}\) forms. The reaction can be written as: \[\mathrm{Zn^{2+} + 2NaOH \rightarrow Zn(OH)_2 (s) + 2Na^+}\]

- Reaction with Excess NaOH

Adding excess \(\mathrm{NaOH}\) to the precipitate \(\mathrm{Zn(OH)_2}\) causes it to dissolve, forming \(\mathrm{Na_2[Zn(OH)_4]}\). The reaction can be shown as: \[\mathrm{Zn(OH)_2 (s) + 2NaOH \rightarrow Na_2[Zn(OH)_4] (aq)}\]

- Identifying the Ionic Species

In \(\mathrm{Na_2[Zn(OH)_4]}\), zinc exists as part of the anionic complex \(\mathrm{[Zn(OH)_4]^{2-}}\). This confirms that zinc is in the anionic part of the solution.

Key concepts

Precipitation Reaction

When a chemical reaction causes a solid to form in a solution, we call this a precipitation reaction. Here, when \( \text{NaOH} \) is added to a solution containing \( \text{Zn}^{2+} \) ions, a white solid, \( \text{Zn(OH)}_2 \), precipitates from the mixture.

This occurs due to the following reaction:
\[ \text{Zn}^{2+} + 2 \text{NaOH} \rightarrow \text{Zn(OH)}_2 (s) + 2 \text{Na}^+ \]

The solid \( \text{Zn(OH)}_2 \) forms because zinc hydroxide is not very soluble in water. Precipitation reactions are important for separating components of a mixture and for identifying specific ions in a solution.

Complex Ion Formation

Complex ion formation refers to the creation of a complex ion from a metal ion and other molecules or ions. Here, when we add excess \( \text{NaOH} \) to the \( \text{Zn(OH)}_2 \) precipitate, we dissolve the solid to form a new, soluble complex ion:
\[ \text{Zn(OH)}_2 (s) + 2 \text{NaOH} \rightarrow \text{Na}_2[\text{Zn(OH)}_4] (aq) \]

The compound \( \text{Na}_2[\text{Zn(OH)}_4] \) is soluble in water. The zinc now exists as part of the complex anion \( [\text{Zn(OH)}_4]^{2-} \).

This process helps in the easy identification and isolating of zinc ions as the complex formed changes the solubility properties of the original precipitate.

Zinc Hydroxide Solubility

Zinc hydroxide's solubility depends on the presence of additional reagents. Initially, zinc hydroxide \( \text{Zn(OH)}_2 \) is not very soluble in water, which is why it precipitates out of solution upon the addition of \( \text{NaOH} \).

However, when we add excess \( \text{NaOH} \), \( \text{Zn(OH)}_2 \) dissolves, forming the soluble complex \( \text{Na}_2[\text{Zn(OH)}_4] \). This is due to the fact that zinc hydroxide behaves amphoteric, meaning it can react with both acids and bases.

In this reaction, the zinc forms a complex ion \( [\text{Zn(OH)}_4]^{2-} \), which is soluble in water. This transformation highlights zinc hydroxide's ability to remain in solution in different chemical environments.