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: Understanding the Initial Reaction

When NaOH (sodium hydroxide) is added to a solution containing zinc ions (e.g., Zn²⁺), a reaction occurs to form a white precipitate of zinc hydroxide, Zn(OH)₂. This happens because zinc ions react with hydroxide ions (OH⁻) from NaOH: \[ \text{Zn}^{2+} + 2\text{OH}^- \rightarrow \text{Zn(OH)}_2 \]

Step 2: Effect of Excess NaOH

Upon adding excess NaOH, the white precipitate of Zn(OH)₂ dissolves. This is because zinc hydroxide is amphoteric and can react with additional hydroxide to form a soluble complex: \[ \text{Zn(OH)}_2 + 2\text{OH}^- \rightarrow [\text{Zn(OH)}_4]^{2-} \] where zinc now exists in the complex anion \([\text{Zn(OH)}_4]^{2-}\).

Step 3: Conclusion on Zinc's Presence

Initially, Zn²⁺ is in the cationic form in the solution. Once excess NaOH is added, zinc forms a complex anion \([\text{Zn(OH)}_4]^{2-}\). Thus, in the final solution, zinc exists in the anionic part.

Key Concepts

Amphoteric Behavior

The term "amphoteric" refers to a substance that can react with both acids and bases. Zinc hydroxide (Zn(OH)₂) is a perfect example of an amphoteric compound. Its amphoteric nature becomes evident when interacting with added substances like sodium hydroxide (NaOH).

When you add NaOH to a solution containing zinc ions, zinc hydroxide initially forms as a white precipitate. This happens because zinc ions (Zn²⁺) react with hydroxide ions (OH⁻) generated from NaOH. Yet, upon further addition of NaOH, the precipitate dissolves. Why, you ask?

This is where the amphoteric character of zinc hydroxide comes into play:

• With moderate hydroxide ion presence: Acts as a traditional base, leading to precipitation.
• With excess hydroxide ions: Acts as an acid, dissolving to form a complex ion.

This dual nature makes zinc hydroxide unique, able to change its behavior based on surrounding conditions. It's like having the best of both world—regarding chemical reactions!

Complex Ion Formation

Complex ions are formed when central metal ions bind to surrounding molecules or ions. With zinc, complex ion formation is particularly intriguing, especially in solutions with excess hydroxide ions. When NaOH is added to a zinc-ion solution, an initial white zinc hydroxide precipitate forms.

If more NaOH is introduced, this situation changes. Zinc hydroxide dissolves in this excess, forming a soluble complex ion

1. The complex ion here is \[\text{Zn(OH)}_4^{2-} \].
2. It contains a zinc center bonded to four hydroxide ions.

This transformation is possible because zinc hydroxide reflects amphoteric qualities, and hydroxide acts as a ligand. Ligands like hydroxide essentially "attach" to the zinc, creating the complex ion.

Such transformations are vital in chemical reactions, affecting solubility and ion concentration. They illustrate how substances adjust their chemical frameworks to align with environmental conditions. In essence, zinc efficiently adapts to its surroundings!

Precipitation Reaction

Precipitation reactions are all about forming a solid, or precipitate, when two solutions mix. In the case of zinc ions in an aqueous solution, once NaOH is added, hydroxide ions from the NaOH react with zinc ions. This leads to the formation of zinc hydroxide (\[ Zn(OH)_2 \]), a white precipitate.

Precipitation reactions are usually driven by the product's low solubility in the solution. Significant points to keep in mind with this type of reaction include:

• The appearance of a solid from a previously clear solution indicates a chemical transformation.
• Insoluble compounds, like zinc hydroxide, fall out as solids.
• Such precipitation leads to easy separation of components within a solution.

Understanding the fundamentals of precipitation reactions offers valuable insight into separation techniques in chemistry. Plus, it illustrates the dynamic nature of chemical interactions—how substances physically manifest when chemical environments shift.

In our specific case, the formation of a zinc hydroxide precipitate reveals critical information about zinc and hydroxide behavior, further paving the way for deeper exploration into zinc's amphoteric nature and subsequent transformations.