Problem 98 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

What happens when : (i) aqueous ammonia is added dropwise to a solution of copper sulphate till it is in excess. (ii) \(\mathrm{CrCl}_{3}\) solution is treated with sodium hydroxide and then with hydrogen peroxide.

Step-by-Step Solution

Answer

(i) A copper ammonia complex forms, changing the solution from light blue to deep blue. (ii) Chromium(III) becomes yellow chromate after treatment.

Step 1: Understanding the Reaction

When aqueous ammonia (NH₃) is added to a copper sulfate (CuSO₄) solution, the ammonia acts as a ligand and interacts with copper ions.

Step 2: Initial Reaction with Ammonia

Initially, when ammonia is added dropwise to copper sulfate, a light blue precipitate of copper hydroxide ( ext{Cu(OH)₂}) forms due to the reaction between copper ions ( ext{Cu}^{2+}) and hydroxide ions from the ammonia dissociating in water:\[ ext{Cu}^{2+} + 2 ext{OH}^- ightarrow ext{Cu(OH)}_2 \\]

Step 3: Ammonia in Excess

As ammonia is added in excess, the light blue precipitate dissolves, forming a deep blue complex ion, tetraamminecopper(II) ( ext{Cu(NH}_3 ext{)}_4^{2+}):\[ ext{Cu(OH)}_2 + 4 ext{NH}_3 ightarrow ext{Cu(NH}_3 ext{)}_4^{2+} + 2 ext{OH}^- \\]

Step 4: Reaction of CrCl₃ with NaOH

When chromium(III) chloride ( ext{CrCl}_3) solution is treated with sodium hydroxide ( ext{NaOH}), a green precipitate of chromium(III) hydroxide ( ext{Cr(OH)}_3) is formed:\[ ext{Cr}^{3+} + 3 ext{OH}^- ightarrow ext{Cr(OH)}_3 \\]

Step 5: Addition of Hydrogen Peroxide

Upon adding hydrogen peroxide ( ext{H}_2 ext{O}_2) to the mixture containing chromium(III) hydroxide, the hydroxide is oxidized to chromate ( ext{CrO}_4^{2-}), which is yellow in color:\[2 ext{Cr(OH)}_3 + 3 ext{H}_2 ext{O}_2 ightarrow 2 ext{CrO}_4^{2-} + 2 ext{OH}^- + 4 ext{H}_2 ext{O} \\]

Key Concepts

Ligand interaction

In coordination chemistry, ligand interaction plays a crucial role in the formation of complex ions. A ligand is an ion or molecule that can donate a pair of electrons to a central atom, often a metal, to form a coordination complex. For example, ammonia (\(\text{NH}_3\)) acts as a ligand when it interacts with copper ions (\(\text{Cu}^{2+}\)). As ammonia is introduced to a copper sulfate solution, it begins to donate electrons to the copper ions.
Initially, when ammonia is added, a precipitate of copper hydroxide (\(\text{Cu(OH)}_2\)) is formed. However, when ammonia is added in excess, it further acts as a ligand, forming a complex ion.

• Ammonia, being a neutral ligand with a lone pair of electrons, coordinates with the metal ion.
• The coordination number of the metal ion defines how many ligands can bind to it simultaneously, in this case resulting in tetraamminecopper(II) (\(\text{Cu(NH}_3\text{)}_4^{2+}\)).

This complex ion interaction highlights the importance of ligand concentration, as it can significantly impact the overall chemical properties and stability of the complexes formed.

Complex ion formation

Complex ion formation is a significant process in coordination chemistry where central metal ions bind with multiple ligands to form a new structure with unique properties. When aqueous ammonia is added to copper sulfate and excess is reached, ammonia manages to dissolve the previously formed copper hydroxide precipitate, leading to the formation of a deep blue complex ion.
These complexes arise from specific binding interactions:

• Ligands like ammonia donate their lone pair of electrons to the metal center.
• This results in the formation of new coordinate covalent bonds.
• The metal ion becomes the central atom in a polyatomic ionic compound—the complex ion.

In this specific instance, the reaction is described by the solid precipitate transition into a soluble deep blue solution. The copper ion, surrounded by four ammonia molecules, forms a new species: the tetraamminecopper(II) ion. Complex ion formation has profound implications in both chemical behavior and industrial applications, where it can alter solubility, color, and reactivity.

Oxidation reactions in transition metals

Transition metals exhibit variable oxidation states, which allow them to engage in multiple types of redox reactions. When chromium(III) chloride is treated sequentially with sodium hydroxide and hydrogen peroxide, it highlights the dynamic oxidation reactions of transition metals.
Initially, chromium ions (\(\text{Cr}^{3+}\)) react with hydroxide ions to form a green precipitate of chromium(III) hydroxide (\(\text{Cr(OH)}_3\)). The addition of hydrogen peroxide leads to the oxidation of the chromium hydroxide:

• This oxidation results in the conversion of chromium(III) to chromate, a higher oxidation state.
• The chromate ion (\(\text{CrO}_4^{2-}\)) is distinctly yellow, indicating a change not just in oxidation state but also in chemical properties.
• Oxidation involves the loss of electrons, and the peroxide acts as the oxidizing agent.

Through these reactions, transition metals like chromium can transition from one state to another, allowing them to participate in a diverse range of chemical processes. Understanding these transitions is vital in fields like catalysis and materials science, emphasizing the versatility and reactivity characteristic of transition metal chemistry.