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Q5.13:
Aqueous copper sulphate solution (blue in colour) gives: (i) a green precipitate with aqueous potassium fluoride and (ii) a bright green solution with aqueous potassium chloride. Explain these experimental results.

Solution :

Initial Theoretical Foundation: Coordination Environment

Aqueous copper sulphate solution consists of the hydrated complex ion $[Cu(H_2O)_6]^{2+}$. This complex is responsible for the characteristic blue color. The reactions described are ligand exchange reactions where the weak field water ligands are replaced by halide ions, leading to changes in the coordination geometry and the splitting energy ($\Delta$), which governs the observed color change [Per Crystal Field Theory (CFT)].

Step 1: Analysis of Reaction with Aqueous Potassium Fluoride

When $KF$ is added to the aqueous $CuSO_4$ solution, the fluoride ions ($F^-$) act as ligands. $F^-$ is a smaller and more electronegative ion compared to $Cl^-$. It displaces the water molecules to form a fluoro-complex:

$[Cu(H_2O)_6]^{2+} + 4F^- \rightarrow [CuF_4]^{2-} + 6H_2O$

The resulting complex $[CuF_4]^{2-}$ is a coordination compound with a lower coordination number. In aqueous media, this species is relatively insoluble or forms a suspension of a tetrafluorocuprate(II) salt, which manifests as a green precipitate. The color arises due to the $d-d$ transition influenced by the fluoride ligand field splitting energy.

Step 2: Analysis of Reaction with Aqueous Potassium Chloride

When $KCl$ is added to the aqueous $CuSO_4$ solution, the chloride ions ($Cl^-$) displace the water molecules to form the tetrachlorocuprate(II) complex ion:

$[Cu(H_2O)_6]^{2+} + 4Cl^- \rightarrow [CuCl_4]^{2-} + 6H_2O$

[Justification: Chloride is a weak field ligand, leading to a tetrahedral geometry for the $[CuCl_4]^{2-}$ ion].

Unlike the fluoride case, the resulting tetrachlorocuprate(II) complex is highly soluble in water, resulting in a homogeneous, bright green solution. The color change from blue to green is attributed to the replacement of the aqua ligand ($H_2O$) with the chloro ligand ($Cl^-$), which results in a smaller crystal field splitting energy ($\Delta_t$), shifting the absorption frequency to a lower energy (longer wavelength) within the visible spectrum.

Comparison Table of Coordination Processes

Reagent Product Formed Physical State Ligand Change
$KF$ $[CuF_4]^{2-}$ Green Precipitate $H_2O \rightarrow F^-$
$KCl$ $[CuCl_4]^{2-}$ Bright Green Solution $H_2O \rightarrow Cl^-$

Visual Representation of Coordination Geometry

Octahedral [Cu(H2O)6]2+ Tetrahedral [CuX4]2-

Final Solution: The experimental observations are explained by ligand exchange reactions where the aqua ligands of $[Cu(H_2O)_6]^{2+}$ are displaced by $F^-$ ions to form the insoluble green precipitate $[CuF_4]^{2-}$ and by $Cl^-$ ions to form the soluble green complex $[CuCl_4]^{2-}$, respectively.


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