Question

Aqueous copper(II) sulfate solution is blue in color. When aqueous potassium fluoride is added, a green precipitate is formed. When aqueous potassium chloride is added instead, a bright-green solution is formed. Explain what is happening in these two cases.

Short answer

Adding potassium fluoride: forms a green precipitate. Adding potassium chloride: forms a bright-green solution.

Step-by-step solution

Understanding Copper(II) Sulfate

Copper(II) sulfate are these ions in solution: \( ext{Cu}^{2+} \) and \( ext{SO}_4^{2-} \). The blue color arises from the copper(II) ions in solution.

Adding Potassium Fluoride

When potassium fluoride is added to the solution, the fluoride ions \( ext{F}^- \) precipitate with the \( ext{Cu}^{2+} \) ions to form copper(II) fluoride \( ext{CuF}_2 \), which is insoluble and thus forms a green precipitate.

Reaction Equation for Potassium Fluoride

The equation for the reaction with potassium fluoride is: \[ ext{CuSO}_4 + 2 ext{KF} \rightarrow ext{CuF}_2 (s) + ext{K}_2 ext{SO}_4 \] where \( ext{CuF}_2 \) is the green precipitate.

Adding Potassium Chloride

When potassium chloride is added, chloride ions \( ext{Cl}^- \) form a complex with \( ext{Cu}^{2+} \) ions, resulting in \( ext{CuCl}_4^{2-} \). This complex is soluble and gives the bright-green coloration in the solution.

Reaction Equation for Potassium Chloride

The equation for the reaction with potassium chloride is: \[ ext{CuSO}_4 + 4 ext{KCl} \rightarrow ext{K}_2[ ext{CuCl}_4] + ext{K}_2 ext{SO}_4 \] where \( ext{K}_2[ ext{CuCl}_4] \) forms the bright-green solution.

Key concepts

Aqueous Solution

An aqueous solution is a mixture where a substance is dissolved in water. In this scenario, copper(II) sulfate ( CuSO_4 ) dissolves in water, and you’ll find copper ions ( Cu^{2+} ) and sulfate ions ( SO_4^{2-} ) floating in the solution. The water acts as the solvent, meaning it's the part of the solution that dissolves other substances. Copper(II) sulfate solutions are typically blue because the copper ions reflect that color when dissolved in water.
The ability of these ions to interact with water and remain free is what makes an aqueous solution different from simply having a solid substance.

• Water is the solvent.
• Substances dissolved are solutes.
• Copper ions give the blue color.

Precipitate Formation

A precipitate forms when two dissolved substances react to create an insoluble compound. In the case of copper(II) sulfate and potassium fluoride, the fluoride ions ( F^- ) react with copper ions to form copper(II) fluoride ( CuF_2 ). Copper(II) fluoride doesn’t dissolve in water, so it separates from the solution as a solid, appearing as a green precipitate.
Think of it like making a solid out of liquid ingredients. The moment these ions meet and form an insoluble compound, they "fall out" of the solution.

• Insoluble compound forms a solid.
• Visible as a change in solution color.
• Here, green precipitate of CuF_2 appears.

Complex Ion

Complex ions form when simple ions combine with other ions or molecules to create a new, coordinated structure. When potassium chloride is added to the copper sulfate solution, chloride ions ( Cl^- ) surround the copper ions to form a new species, CuCl_4^{2-} . This is a complex ion.
Complex ions often result in distinct color changes in solutions because they reflect light differently than their component ions would on their own.

• Complexes change color of solution.
• Formation involves coordinate bonds.
• New ionic structure is CuCl_4^{2-} .

Color Change in Reactions

Color changes in chemical reactions indicate a shift in the arrangement or type of particles present. In these copper sulfate reactions, adding different substances causes the color to shift away from the original blue.
Adding potassium fluoride leads to a green precipitate. This happens because the copper changes its state by reacting with fluoride ions.
On adding potassium chloride, the solution turns bright green due to the formation of the CuCl_4^{2-} complex ion, altering the way light interacts with the solution.

• Color reflects chemical changes.
• Different ions produce different colors.
• Visual cue of reaction completion.