Problem 156 - A Complete Resource Book in Chemistry for JEE Main

Question

How do we differentiate between \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\) in group III? \([2002]\) (a) by increasing \(\mathrm{NH}_{4}{\underline{\phantom{xx}}}^{+}\)ion concentration (b) by decreasing \(\mathrm{OH}^{-}\)ion concentration (c) by adding excess of \(\mathrm{NH}_{4} \mathrm{OH}\) solution (d) both (a) and (b)

Step-by-Step Solution

Answer

Both increasing \(\mathrm{NH}_{4}^{+}\) and decreasing \(\mathrm{OH}^{-}\) differentiate \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\) (Option d).

Step 1: Understand the Problem

We need to identify how to differentiate between two ions, \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\), in group III where they occur together as they both form hydroxides in water. The options provided hint at changes in \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{OH}^{-}\) ion concentrations.

Step 2: Evaluate Ion Solubility

\(\mathrm{Cr}^{3+}\) forms a precipitate of \(\mathrm{Cr(OH)}_3\), which is amphoteric and can dissolve in both acids and excess bases. The \(\mathrm{Fe}^{3+}\) forms \(\mathrm{Fe(OH)}_3\), which is insoluble in excess \(\mathrm{NH}_{4}\mathrm{OH}\), but \(\mathrm{Cr(OH)}_3\) can dissolve as a complex ion when \(\mathrm{NH}_{4}^{+}\) is increased.

Step 3: Effect of Increasing \(\mathrm{NH}_{4}^{+}\) Ion Concentration

Increasing \(\mathrm{NH}_{4}^{+}\) concentration can potentially form a complex with \(\mathrm{Cr}^{3+}\) while \(\mathrm{Fe}^{3+}\) remains unaffected, allowing differentiation based on solubility.

Step 4: Effect of Decreasing \(\mathrm{OH}^{-}\) Ion Concentration

Reducing \(\mathrm{OH}^{-}\) concentration can cause \(\mathrm{Cr(OH)}_3\) to dissolve partially or completely depending on the conditions, whereas \(\mathrm{Fe(OH)}_3\) remains a precipitate owing to its different solubility properties.

Step 5: Conclusion

Based on the effects observed, both increasing \(\mathrm{NH}_{4}^{+}\) ion concentration and decreasing \(\mathrm{OH}^{-}\) ion concentration can differentiate between \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\) ions due to solubility differences.

Key Concepts

Ion Differentiation

Differentiating between ions like \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\) in group III is a classic problem in qualitative analysis. Both are positively charged ions that can be found in similar environments but need specific conditions to separate. To distinguish these ions, we often rely on their unique chemical reactions and solubility properties.

These ions form hydroxides when they react with hydroxide ions. However, their solubility in water and complex formation differs. \(\mathrm{Fe}^{3+}\) tends to form \(\mathrm{Fe(OH)}_3\), a distinctly insoluble hydroxide, while \(\mathrm{Cr}^{3+}\) forms amphoteric \(\mathrm{Cr(OH)}_3\), which can dissolve in either acidic or basic environments.

By manipulating the concentrations of other ions in their solution, such as \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{OH}^{-}\) ions, we can effectively separate and identify these transition metal ions based on their specific reactions.

Solubility of Hydroxides

The solubility of metal hydroxides is key in qualitative analysis, especially when differentiating ions like \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\). \(\mathrm{Fe(OH)}_3\) is generally insoluble in water and does not dissolve even when you add excess \(\mathrm{NH}_{4}\) hydroxide.

On the other hand, \(\mathrm{Cr(OH)}_3\) showcases amphoteric behavior. This means that it can dissolve in strong acidic solutions, forming \(\mathrm{[Cr(OH)_4]^{-}}\), or react with excess bases to form complex ions. This unique solubility property is what allows chemists to identify and treat these ions differently.

Understanding these solubility principles is crucial. It helps predict whether a hydroxide will precipitate or remain dissolved in a chemical reaction.

Chemical Reactions with NH4OH

Chemical reactions involving \(\mathrm{NH}_{4}\) hydroxide (also known as ammonia solution) play an essential role in ion differentiation. \(\mathrm{NH}_{4}\) hydroxide reacts differently with \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\), highlighting their distinct chemistry.

When \(\mathrm{NH}_{4}\) hydroxide is added to a solution containing \(\mathrm{Fe}^{3+}\), \(\mathrm{Fe(OH)}_3\) immediately precipitates. It remains largely insoluble even in excess ammonia, demonstrating the robustness of this precipitate.

In contrast, adding \(\mathrm{NH}_{4}\) hydroxide to \(\mathrm{Cr}^{3+}\) forms \(\mathrm{Cr(OH)}_3\) which, upon further addition of \(\mathrm{NH}_{4}\) hydroxide, may start dissolving due to complex ion formation. This is a telltale property used to differentiate between the two ions effectively.

Properties of Transition Metal Ions

Transition metal ions like \(\mathrm{Fe}^{3+}\) and \(\mathrm{Cr}^{3+}\) are known for their distinct properties, that influence their reactivity and the types of complexes they form. These properties include variable oxidation states, formation of colored compounds, and the ability to form complexes.

For example, \(\mathrm{Fe}^{3+}\) often forms colorful compounds, exhibiting oxidation states that can facilitate a variety of chemical reactions. Such differences are attributed to the structure of their d-orbitals and electron configurations.

• \(\mathrm{Fe}^{3+}\) generally forms red or brown precipitates.
• \(\mathrm{Cr}^{3+}\) often forms complexes resulting in green or violet solutions.

These unique properties make transition metals an exciting subject of study in chemistry, while also providing practical pathways for identifying and utilizing these ions in both industrial and laboratory settings.