Problem 21 - Chemistry in 30 days

Question

The hydroxide which is soluble in excess of \(\mathrm{NaOH}\) solution is (a) \(\mathrm{Cu}(\mathrm{OH})_{2}\) (b) \(\mathrm{Fe}(\mathrm{OH})_{1}\) (c) \(\mathrm{Cr}(\mathrm{OH})_{3}\) (d) \(\mathrm{Zn}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

(d) Zn(OH)_2, as it is amphoteric and soluble in excess NaOH.

Step 1: Understanding Solubility in NaOH

To determine which hydroxide is soluble in excess NaOH, we need to recognize which compounds are amphoteric. Amphoteric hydroxides dissolve in both acids and bases (like NaOH) to form soluble complex ions.

Step 2: Analyzing the Options

- Cu(OH)_2 is not amphoteric; it does not dissolve in NaOH. - Fe(OH)_1 does not exist, as iron forms Fe(OH)_3; however, iron's hydroxide is not amphoteric. - Cr(OH)_3 is amphoteric, meaning it can dissolve in excess NaOH, forming complex ions. - Zn(OH)_2 is also amphoteric and dissolves in NaOH to form soluble sodium zincate (Na_2ZnO_2).

Step 3: Conclusion

Both Cr(OH)_3 and Zn(OH)_2 are likely answers because they are amphoteric, but based on the common reactions in laboratory settings, Zn(OH)_2 is prominently known for its solubility in NaOH.

Key Concepts

Solubility in NaOH

When dealing with hydroxides and their solubility in sodium hydroxide ( NaOH ), it is crucial to understand that not all hydroxides behave the same way. The key to determining solubility is recognizing amphoteric behavior. Amphoteric hydroxides have the unique ability to react with both acids and bases, like NaOH . This means they can dissolve in NaOH to form other chemical species. For instance, amphoteric hydroxides will form soluble complex ions when they dissolve in NaOH .

• Zn(OH) _2 is a classic amphoteric hydroxide which dissolves readily in excess NaOH . It forms soluble sodium zincate ( Na_2ZnO_2 ).
• Cr(OH) _3 is another example, forming soluble complex ions when dissolved in NaOH .

Understanding which compounds are amphoteric will help you predict solubility behavior effectively. Notably, amphoteric hydroxides don't follow the typical 'insoluble in bases' rule, making them unique in chemical reactions involving NaOH .

Complex Ions

Complex ions are fascinating entities resulting from the interaction of simple ions and molecules. When amphoteric hydroxides dissolve in excess NaOH , they often form complex ions. These ions consist of a central metal atom bonded to surrounding molecules or ions called ligands.

• The formation of complex ions significantly affects the solubility of hydroxides in NaOH .
• For example, when Zn(OH) _2 dissolves in NaOH , it forms the complex ion sodium zincate, Na_2ZnO_2 .
• For Cr(OH) _3 , dissolving in NaOH leads to the formation of complex ions as well, typically chromium oxo or chromium hydroxide complexes.

The existence of complex ions underlines the amphoteric nature of compounds, allowing these substances to behave unexpectedly when exposed to basic environments.

Amphoteric Compounds

Amphoteric compounds stand out because of their ability to act both as acids and bases. This dual capability is what enables them to dissolve in solutions like NaOH , which is a base.

• Not all hydroxides are amphoteric. This property mainly belongs to certain metals such as zinc, aluminum, and chromium.
• Amphoteric hydroxides dissolve readily in both acidic and basic environments, thus forming complex ions in the process.
• Understanding if a compound is amphoteric involves recognizing its chemical behavior in different environments.

This property is especially exploited in chemical processes to separate or identify amphoteric compounds given their unique reactivity pattern. For instance, both Zn(OH) _2 and Cr(OH) _3 show amphoteric behavior, forming readily soluble compounds in NaOH , which allows for differentiation from non-amphoteric compounds.