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

Zinc exists in the anionic part of the solution.

Step 1: Understand the Reaction

When sodium hydroxide \((\mathrm{NaOH})\) is added to a solution containing zinc ions \((\mathrm{Zn}^{2+})\), a white precipitate of zinc hydroxide \((\mathrm{Zn(OH)_2})\) forms initially. This is because zinc ions react with hydroxide ions to form an insoluble compound.

Step 2: Reaction Equation Formation

The initial reaction that forms the precipitate can be written as: \[\mathrm{Zn}^{2+} (aq) + 2\mathrm{OH}^- (aq) \rightarrow \mathrm{Zn(OH)_2} (s)\] This reaction shows the formation of the white precipitate, zinc hydroxide.

Step 3: Excess NaOH Addition

Upon adding excess \(\mathrm{NaOH}\), the zinc hydroxide precipitate dissolves, forming a soluble complex ion. This is due to the formation of the zincate complex: \[\mathrm{Zn(OH)_2} (s) + 2\mathrm{OH}^- (aq) \rightarrow \mathrm{[Zn(OH)_4]^{2-}} (aq)\] This shows the zinc now exists as a tetrahydroxozincate anion in solution.

Step 4: Final Analysis

The dissolution of the \(\mathrm{Zn(OH)_2}\) precipitate in excess \(\mathrm{NaOH}\) indicates that zinc transitions from being part of the precipitate to existing as a complex ion in the anionic part of the solution. The zinc is now part of the \(\mathrm{[Zn(OH)_4]^{2-}}\) ion, confirming its presence in the anionic section of the solution.

Key Concepts

Precipitation Reaction

A precipitation reaction is a process in which two soluble salts mix in a solution to form one or more insoluble products, called precipitates. When sodium hydroxide (\(\text{NaOH}\)) is added to a solution containing zinc ions (\(\text{Zn}^{2+}\)), a solid white precipitate forms.
This precipitate is zinc hydroxide (\(\text{Zn(OH)}_2\)).

• A key feature of this type of reaction is that the product forms because it is insoluble in the solution it is found in.
• The reaction can be described by the chemical equation: \(\text{Zn}^{2+} (\text{aq}) + 2\text{OH}^- (\text{aq}) \rightarrow \text{Zn(OH)}_2 (\text{s})\)
• In this equation, the compounds on the left are aqueous, meaning they are dissolved in water, and the product on the right is a solid precipitate.

Recognizing precipitation reactions is essential for predicting outcomes when mixing solutions, as well as for applications in chemical analysis and separating mixture components.

Formation of Complex Ions

In chemistry, complex ions are formed when central metal ions combine with surrounding molecules or ions. This process often involves a central metal ion bonding with several other groups, typically involving coordinate covalent bonds.
In the reaction involving zinc hydroxide, if excess \(\text{NaOH}\) is added, zinc hydroxide (\(\text{Zn(OH)}_2\)) dissolves and forms a different compound known as a complex ion.

• The new complex ion formed is called tetrahydroxozincate and is represented as \(\text{[Zn(OH)}_4\text{]}^{2-}\).
• This occurs as the excess hydroxide ions further interact with the zinc hydroxide precipitate. The reaction can be expressed as: \(\text{Zn(OH)}_2 (\text{s}) + 2\text{OH}^- (\text{aq}) \rightarrow \text{[Zn(OH)}_4\text{]}^{2-} (\text{aq})\)
• The formation of a complex ion like \(\text{[Zn(OH)}_4\text{]}^{2-}\) signifies that the zinc ion is now stabilized further by additional hydroxide ions in the solution.

This phenomenon is crucial for understanding how metals behave in chemical environments and has practical applications in fields like coordination chemistry and industrial processes.

Solubility of Hydroxides in Alkali Solutions

The solubility of metal hydroxides, such as zinc hydroxide (\(\text{Zn(OH)}_2\)), often changes in the presence of alkali solutions. Initially, \(\text{Zn(OH)}_2\), a sparingly soluble compound, forms as a precipitate when an alkali like \(\text{NaOH}\) is introduced to a zinc ion solution. However, in excess alkali, such as when more \(\text{NaOH}\) is added, zinc hydroxide’s behavior changes, transitioning from an insoluble state to a soluble form.

• This occurs as more hydroxide ions from the alkali solution interact with zinc hydroxide, creating soluble complex ions (\(\text{[Zn(OH}_4\text{]}^{2-}\)).
• Factors influencing this solubility change include the concentration of hydroxide ions and the nature of the metal cation involved (in this case, zinc).
• Understanding these solubility principles helps predict the behavior of hydroxides in various pH environments, which is useful in areas like waste treatment and materials science.

Thus, the ability of hydroxides to dissolve in alkalis like \(\text{NaOH}\) is an important aspect of inorganic chemistry that explains the transformation of zinc during the reaction.