Problem 50 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following dissolve in hot conc. \(\mathrm{NaOH}\) solutio (a) \(\mathrm{Fe}\) (b) \(\mathrm{Zn}\) (c) \(\mathrm{Cu}\) (d) \(\mathrm{Ag}\)

Step-by-Step Solution

Answer

Zinc (Zn) dissolves in hot concentrated NaOH.

Step 1: Identify Reactive Metals with NaOH

Metals that can dissolve in concentrated sodium hydroxide (NaOH) solutions are typically those that can react with hydroxide ions to form soluble complexes. Common reactive metals with NaOH are amphoteric metals, such as Zinc (Zn), Aluminium (Al), and Lead (Pb). Other metals like Iron (Fe), Silver (Ag), and Copper (Cu) do not usually react with NaOH.

Step 2: Analyze Each Metal for Reactivity

Examine the given list of metals. **(a)** Iron (Fe) does not react with NaOH as it is not amphoteric with respect to hydroxides. **(b)** Zinc (Zn) is amphoteric and can react with NaOH, forming soluble zincate ions. **(c)** Copper (Cu) does not react with NaOH. **(d)** Silver (Ag) does not react with NaOH.

Step 3: Conclusion on Solvable Metals

Among the options provided, only Zinc (Zn) forms soluble compounds with NaOH. Therefore, only zinc can dissolve in hot concentrated NaOH solution. Other metals listed, like Fe, Cu, and Ag, do not exhibit this property due to their lack of amphoteric nature in the presence of hydroxide ions.

Key Concepts

Amphoteric Metals

Amphoteric metals have the unique ability to react with both acids and bases. This means they can dissolve in solutions like concentrated sodium hydroxide (\(\text{NaOH}\)), which is a strong base. Examples of amphoteric metals include aluminum (Al), zinc (Zn), and lead (Pb).

These metals react with sodium hydroxide to form a metal hydroxide first, which might further dissolve to form soluble metal complexes when the conditions are right. Specifically for zinc, when it reacts with \(\text{NaOH}\), it forms zincate ions, a process that showcases its amphoteric nature.

Recognizing amphoteric behavior is essential in predicting how metals will react in different chemical environments, including both acidic and basic solutions.

Soluble Metal Complexes

Some metals can form complexes that are soluble in water, especially in the presence of strong bases like sodium hydroxide (\(\text{NaOH}\)). When amphoteric metals interact with hydroxide ions in a concentrated \(\text{NaOH}\) solution, they can form soluble metal complexes.

Take zinc, for example. It reacts with excess hydroxide ions to produce zincate ions which are soluble in the solution:

\[ \text{Zn}^{2+} + 4\text{OH}^- \rightarrow [\text{Zn(OH)}_4]^{2-} \]

This reaction exhibits the formation of a soluble complex, allowing the metal to effectively "dissolve" in the solution. These complexes can be crucial in various chemical processes, including industrial applications and analytical chemistry.

Chemical Reactions with Hydroxide Ions

Reactions with hydroxide ions are a hallmark of metal reactivity in bases, such as sodium hydroxide (\(\text{NaOH}\)). For these reactions to occur, metals often need to be amphoteric, enabling them to react and form metal hydroxides immediately.

• In the case of zinc in a \(\text{NaOH}\) solution, the initial formation of zinc hydroxide (\(\text{Zn(OH)}_2\)) can further react to form the zincate ion, \([\text{Zn(OH)}_4]^{2-}\).


• Treating these reactions involves understanding how hydroxide ions influence metal behavior, guiding educational and practical applications.

These reactions not only illustrate the fundamental chemistry of metals and bases but also reflect crucial concepts used in refining metals, removing impurities, and even in laboratory techniques.