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: Understanding the Reaction

When \(abla\) \(\mathrm{NaOH}\) is added to an aqueous solution containing \(\mathrm{Zn}^{2+}\) ions, the \(\mathrm{OH}^{-}\) ions react with \(\mathrm{Zn}^{2+}\) ions to form a white precipitate of zinc hydroxide \(\mathrm{Zn(OH)_2}\). This can be represented by the equation: \[\mathrm{Zn^{2+} + 2OH^{-} \rightarrow Zn(OH)_2 \downarrow}\]. Here, the downward arrow indicates the formation of a precipitate.

Step 2: Recognizing Solubility Changes

Upon adding excess \(\mathrm{NaOH}\), the white precipitate of \(\mathrm{Zn(OH)_2}\) dissolves to form a soluble complex ion, \(\mathrm{Zn(OH)_4^{2-}}\). This is due to the amphoteric nature of zinc hydroxide, which can further react with \(\mathrm{OH^{-}}\) ions: \[\mathrm{Zn(OH)_2 + 2OH^{-} \rightarrow Zn(OH)_4^{2-}}\]. This reaction shows that the zinc is now part of a soluble complex, in the anionic form \(\mathrm{Zn(OH)_4^{2-}}\) with a net negative charge.

Step 3: Identifying Zinc's Form in Solution

In the resulting solution after excess \(\mathrm{NaOH}\) is added, zinc exists in the form of the complex anion \(\mathrm{Zn(OH)_4^{2-}}\). This indicates that the zinc is present in the ionic species of the anionic part of the solution, as the zinc is in the negatively charged complex.

Key Concepts

Precipitation Reactions

Precipitation reactions are a fascinating aspect of chemistry, occurring when two solutions react to form an insoluble solid known as a precipitate. When this happens, the ions from the reacting solutions combine to create a compound that is not soluble in water, causing it to separate out as a solid. This is highly common in reactions between solutions of ionic compounds. In the case of our example, when sodium hydroxide (\(\mathrm{NaOH}\)) is added to a solution of zinc ions (\(\mathrm{Zn}^{2+}\)), a white precipitate forms. This is due to the formation of zinc hydroxide (\(\mathrm{Zn(OH)_2}\)). The chemical equation representing this process is: \[\mathrm{Zn^{2+} + 2OH^{-} \rightarrow Zn(OH)_2 \downarrow}\]where the downward arrow symbolizes the emergence of the solid precipitate.Whenever you engage in laboratory experiments and see a powder or crystal form in the solution, you are likely observing a precipitation reaction. The beauty in these reactions comes from their utility in isolation techniques, where specific ions in a solution can be 'captured' as precipitates for further analysis.

Complex Ions

Complex ions are formed when a metal ion binds with one or more ligands. A ligand is a molecule or ion that can donate a pair of electrons to the metal ion, forming a coordinate bond. This ability to form complexes adds a layer of complexity to reactions, particularly when dealing with ions that can react with different ligands.In the exercise scenario, when excess \(\mathrm{NaOH}\) is added to a zinc precipitate of \(\mathrm{Zn(OH)_2}\), it dissolves because of the formation of a complex ion. The zinc now forms \(\mathrm{Zn(OH)_4^{2-}}\), an example of a complex ion where zinc acts as the central metal atom surrounded by hydroxide ligands. This occurs due to the amphoteric nature of zinc hydroxide, allowing it to react further with hydroxide ions to form this soluble species:\[\mathrm{Zn(OH)_2 + 2OH^{-} \rightarrow Zn(OH)_4^{2-}}\]This shows how the behavior of ions changes in solution depending on the conditions you provide. Complex ion formation is a cornerstone in understanding advanced chemistry topics, including metal extraction, catalysis, and bioinorganic processes.

Solubility Changes

Solubility refers to the maximum quantity of a substance that can dissolve in a solvent at a given temperature. Solubility changes are particularly notable in reactions involving complex ions and precipitation. Such changes can be the cornerstone of determining whether a substance remains in solution or forms a solid precipitate.In our example, the initial addition of \(\mathrm{NaOH}\) causes zinc ions to precipitate as \(\mathrm{Zn(OH)_2}\) due to limited solubility. However, an interesting transformation occurs upon adding excess \(\mathrm{NaOH}\). The zinc hydroxide dissolves again, revealing its unique ability to behave both as a base and an acid, known as amphoterism. This is why \(\mathrm{Zn(OH)_4^{2-}}\) emerges as a more soluble complex ion:\[\mathrm{Zn(OH)_2 + 2OH^{-} \rightarrow Zn(OH)_4^{2-}}\]Thus, the zinc ions transition from a precipitated to a soluble state, showcasing how manipulating solubility can drive chemical equilibrium and change the composition of a solution. Understanding solubility changes helps chemists predict and control the outcomes of reactions efficiently and effectively. This principle is fundamental in applications ranging from pharmaceutical formulations to environmentally safe waste treatments.