Problem 23 - Chemistry in 30 days

Question

The hydroxide of which metal ion is soluble in excess of sodium hydroxide soluuou (a) \(\mathrm{Fe}^{t+t}\) (b) \(\mathrm{Cr}^{4+t}\) (c) \(\mathrm{Al}^{*+}\) (d) \(\mathrm{Cu}^{* *+}\)

Step-by-Step Solution

Answer

(c)  ext{Al}^{3+} forms a soluble hydroxide in excess sodium hydroxide.

Step 1: Identify Reactivity with Sodium Hydroxide

To solve this problem, we first identify which metal ions form hydroxides that can dissolve in excess sodium hydroxide (NaOH). Generally, amphoteric hydroxides can react with excess NaOH to form soluble complexes.

Step 2: Understand Amphoteric Hydroxides

Amphoteric hydroxides can behave as both acids and bases. Common examples include aluminum hydroxide Al(OH)_3 and chromium hydroxide Cr(OH)_3. These compounds can dissolve in excess NaOH to form complex ions.

Step 3: Evaluate Each Option

Consider each ion provided: (a) Fe^{3+}: Fe(OH)_3 is not soluble in excess NaOH. (b) Cr^{3+}: Cr(OH)_3 is amphoteric and dissolves in excess NaOH. (c) Al^{3+}: Al(OH)_3 is amphoteric and dissolves in excess NaOH. (d) Cu^{2+}: Cu(OH)_2 is not soluble in excess NaOH.

Step 4: Determine the Soluble Hydroxide

After evaluating, both Cr^{3+} and Al^{3+} form hydroxides that dissolve in excess NaOH. According to the options and their representation,  ext{(c) } alumnium ion Al^{3+} typically appears as Al^{3+}
9. Therefore, the answer provided corresponds to  ext{(c)}.

Key Concepts

Solubility

Solubility refers to the ability of a substance (solute) to dissolve in a solvent, forming a homogeneous solution. In the context of hydroxides, it pertains to their ability to dissolve in solutions such as sodium hydroxide ( NaOH ). The solubility of metal hydroxides can vary widely:

• Some hydroxides, like those of alkali metals, are very soluble in water.
• Other hydroxides, such as those formed by transition metals, may be insoluble.
• Amphoteric hydroxides, such as Al(OH)_3 and Cr(OH)_3 , can dissolve in both acids and bases, making them unique.

Understanding the behavior of metal hydroxides in solution is crucial, as it impacts their reactivity, uses in industrial processes, and environmental behavior.

Sodium Hydroxide Reaction

Sodium hydroxide ( NaOH ) is a strong base commonly used to test the solubility of metal hydroxides. When NaOH is added to metal ions in solution, it can lead to a variety of reactions. Here's what often happens:

• If a metal ion forms a soluble hydroxide, it will dissolve in the excess NaOH , forming a clear solution.
• For amphoteric metal ions like Al^{3+} or Cr^{3+} , they react with excess NaOH to form soluble complex ions.
• Non-amphoteric metal hydroxides, like Fe(OH)_3 and Cu(OH)_2 , might precipitate out and will not dissolve further in excess NaOH .

These reactions are pivotal in chemical tests used to identify compounds, and in various industrial applications ranging from water treatment to the production of cleaning agents.

Aluminum Hydroxide

Aluminum hydroxide ( Al(OH)_3 ) is an important example of an amphoteric hydroxide. Here's what you need to know:

• It can dissolve in excess NaOH , forming a soluble aluminate complex ion ( [Al(OH)_4]^- ).
• This property is harnessed in various fields such as water purification and in the manufacturing of aluminum products.
• Amphoteric nature allows it to react both as an acid and a base.

Because aluminum hydroxide can act in multiple ways, it plays a significant role in laboratory processes and industrial applications where its ability to neutralize acids or precipitate metals is valuable.

Chromium Hydroxide

Chromium hydroxide ( Cr(OH)_3 ) also showcases amphoteric behavior. It possesses intriguing properties worth noting:

• It dissolves in excess NaOH to form a soluble chromite complex ( [Cr(OH)_4]^- ).
• This reaction shows its dual nature, as it reacts both as a base and an acid.
• Chromium hydroxides are used in dyeing and tanning processes, capitalizing on their reactivity.

The behavior of chromium hydroxide in reactions with NaOH exemplifies its usefulness in chemical synthesis and industrial applications, where it can adjust pH levels and act as a catalyst.