Problem 36 - Chemistry in 30 days

Question

The hydroxide of which metal ion is soluble in excess of sodium hydroxide solution (a) \(\mathrm{Fc}^{3+}\) (b) \(\mathrm{Cr}^{3+}\) (c) \(\mathrm{Sn}^{2+}\) (d) \(\mathrm{Cu}^{2+}\)

Step-by-Step Solution

Answer

The hydroxide of Cr^{3+} is soluble in excess NaOH.

Step 1: Understanding the solubility behavior

When a metal ion reacts with sodium hydroxide (NaOH), it generally forms a metal hydroxide precipitate. Some metal hydroxides are soluble in excess NaOH, forming complex ions, while others remain insoluble. The objective is to identify the metal ion whose hydroxide dissolves in excess NaOH.

Step 2: Analyze the given ions

Examine each given metal ion to consider its solubility behavior with NaOH: - Cr^{3+}: Chromium forms hydroxides like Cr(OH)₃, which is initially insoluble but dissolves in excess NaOH to form a soluble complex ion like [Cr(OH)₆]^{3-}. - Fc^{3+}: Iron(III) hydroxide ( Fe(OH)₃) is insoluble in water and excess NaOH. - Sn^{2+}: Tin(II) forms Sn(OH)₂ which is soluble in excess NaOH forming complexes, like [Sn(OH)₄]^{2-}. - Cu^{2+}: Copper(II) hydroxide ( Cu(OH)₂) is insoluble in excess NaOH.

Step 3: Conclusion based on analysis

Based on the behavior in NaOH, Cr^{3+} and Sn^{2+} form hydroxides that dissolve in excess NaOH. Since chromium ( Cr^{3+}) is among the options given, it is confirmed that Cr^{3+} forms a soluble complex in excess NaOH.

Key Concepts

Sodium Hydroxide Reactions

Sodium hydroxide (NaOH) is a powerful base often used in chemical reactions involving metal ions. When metal ions are introduced to NaOH, a reaction occurs that typically forms a metal hydroxide precipitate. This is an important reaction because it helps determine the solubility of the metal ion in water and other solutions.
The interaction of sodium hydroxide with different metal ions can lead to different outcomes:

• Some metal hydroxides remain insoluble and form solid precipitates.
• Others dissolve in excess NaOH, forming complex ions which are soluble.

Understanding these reactions is crucial for determining the behavior of specific metals in solution and predicting their properties in various chemical processes.

Complex Ion Formation

Complex ions are formed when a central metal ion binds to surrounding molecules or ions, known as ligands. In the case of reactions with sodium hydroxide, some metal hydroxides can dissolve further by forming complex ions when excess NaOH is added.
This occurs because the hydroxide ions ( ext{OH}^{-}) from NaOH can act as ligands, surrounding the metal ion and allowing for the formation of a larger, soluble molecule. For example, chromium ( ext{Cr}^{3+}) when reacted with NaOH, first forms an insoluble ext{Cr(OH)₃} precipitate. With excess NaOH, ext{Cr(OH)₃} transforms into the soluble complex ion ext{[Cr(OH)₆]^{3-}}.
These complex ions significantly impact the solubility and behavior of metal ions in solutions, which is utilized in various industries including water treatment and metal plating.

Metal Ions

Metal ions are atoms of metals that have lost electrons and thus carry a positive charge. They are critical in various chemical reactions due to their ability to form bonds with negatively charged ions or molecules.
In reactions involving sodium hydroxide, different metal ions display varying behaviors. For instance:

• Chromium ( ext{Cr}^{3+}) forms a precipitate initially but can dissolve in excess NaOH by making a complex ion.
• Iron ( ext{Fe}^{3+}) and Copper ( ext{Cu}^{2+}) form insoluble hydroxides that are stable even in the presence of excess NaOH.

Understanding how these ions interact with bases like sodium hydroxide is essential for predicting how they behave in different chemical settings, which has practical applications in material science and chemistry.

Hydroxide Precipitate

A hydroxide precipitate forms when a metal ion reacts with hydroxide ions from bases such as sodium hydroxide, resulting in an insoluble compound that separates from the solution. Usually, this compound appears as a solid that settles out of the liquid mixture.
The formation of a hydroxide precipitate is an important method for identifying and separating metal ions in a solution. When the initial reaction occurs with NaOH, the metal ions pair with hydroxide ions to create a compound like ext{Cr(OH)₃}.
Different metal ions have varying tendencies to form these precipitates:

• Tin ( ext{Sn}^{2+}) and Chromium ( ext{Cr}^{3+}) form hydroxide precipitates which can dissolve upon the addition of more NaOH, due to complex ion formation.
• Iron ( ext{Fe}^{3+}) and Copper ( ext{Cu}^{2+}) form precipitates that don't dissolve in excess NaOH, thereby indicating their insolubility.

Recognizing and understanding precipitate formation is important in both lab environments and industrial applications, where separation and purification of metal ions are required.