Question

When aqueous KI is added gradually to mercury(II) nitrate, an orange precipitate forms. Continued addition of KI causes the precipitate to dissolve. Write balanced equations to explain these observations. (Hint: ${\mathrm{Hg}}^{2+}$ reacts with ${\mathrm{I}}^{-}$ to form ${\mathrm{HgI}}_4{}^{2-}$.) Would you expect ${\mathrm{HgL}}_4{}^{2-}$ to form colored solutions? Explain.

Short answer

The two balanced chemical equations are: 1) $2KI(aq)+Hg(N{O}_3{)}_2(aq)\to Hg{I}_2(s)+2KN{O}_3(aq)$ 2) $Hg{I}_2(s)+2KI(aq)\to Hg{I}_4^{2-}(aq)+2K^{+}(aq)$ HgL₄⁠^{2⁠−} may form colored solutions if the halide (L) has similar properties to iodide ions when attached to Hg²⁺. More information is needed to confidently predict the color formation of HgL₄⁠^{2⁠−} complexes.

Step-by-step solution

Write the first balanced chemical equation

Initially, aqueous KI is added to mercury(II) nitrate, and an orange precipitate of mercury(II) iodide forms. The balanced chemical equation for this reaction is: $$2KI(aq)+Hg(N{O}_3{)}_2(aq)\to Hg{I}_2(s)+2KN{O}_3(aq)$$

Write the second balanced chemical equation

Upon further addition of KI, the orange precipitate dissolves because Hg⁠^{2+} ions combine with I⁠^− ions to form the HgI₄⁠^{2⁠−} complex ion. The balanced chemical equation for this reaction is: $$Hg{I}_2(s)+2KI(aq)\to Hg{I}_4^{2-}(aq)+2K^{+}(aq)$$

Determine if Hg⁠L₄⁠^{2⁠−} would form colored solutions

The HgI₄⁠^{2⁠−} complex ion forms colored solutions because it absorbs certain wavelengths of light, resulting in the observed orange color. If the halides (L) in the HgL₄⁠^{2⁠−} complex ions have similar properties to iodide ions when attached to Hg⁠²⁺, then they would also form colored solutions. Without more information about the specific ligand (L), we cannot confidently predict if Hg⁠L₄⁠^{2⁠−} would form colored solutions, although it is likely due to the similar behavior with iodide ions.

Key concepts

Aqueous Solution

An aqueous solution is a solution in which water acts as the solvent. In the exercise, the mercury(II) nitrate and potassium iodide are both dissolved in water, forming an aqueous solution. This allows the ions to freely move and interact with each other.
When substances dissolve in water, they dissociate into their respective ions. For example:

• Potassium iodide ($KI$) dissociates into potassium ($K^{+}$) and iodide ($I^{-}$) ions.
• Mercury(II) nitrate ($Hg(N{O}_3{)}_2$) dissociates into mercury ($H{g}^{2+}$) and nitrate ($N{O}_3^{-}$) ions.

The aqueous environment facilitates the interaction between these ions, leading to chemical reactions, such as the formation of a precipitate.

Complex Ion Formation

Complex ion formation occurs when a central metal ion binds with surrounding ligands, creating a compound with a charge. In this case, the $H{g}^{2+}$ ion from mercury(II) nitrate reacts with iodide ions ($I^{-}$) from potassium iodide to form a complex ion:

• The initial reaction forms a precipitate of mercury(II) iodide ($Hg{I}_2$), as seen by the orange color.
• Adding more potassium iodide results in the $Hg{I}_2$ precipitate dissolving as $Hg{I}_4^{2-}$ complex ions form.

This happens because the mercury(II) ion can coordinate with multiple iodide ions, forming the tetrahedral complex $Hg{I}_4^{2-}$. This complex remains dissolved in the aqueous solution due to its negative charge, stabilizing the ions.

Precipitation Reaction

A precipitation reaction is a type of chemical reaction where two solutions react, resulting in the formation of a solid called a precipitate. In our exercise, the reaction between $KI$ and $Hg(N{O}_3{)}_2$ leads to the formation of the orange precipitate $Hg{I}_2$:

• The ions $H{g}^{2+}$ and $I^{-}$ combine and exceed the solubility product, creating mercury(II) iodide, a insoluble compound in water.

When the concentration of these ions reaches a certain level, they form solid particles that fall out of solution, visible as the orange precipitate.
This signifies a shift in equilibrium from dissolved ions to a solid form. Precipitation reactions are important in isolating substances and understanding solubility limits in solutions.

Colored Solutions

Colored solutions occur due to specific ions or compounds absorbing particular wavelengths of light. The color we perceive is from the wavelengths of light not absorbed by the solution. In our example, the formation of the $Hg{I}_4^{2-}$ complex ion creates a colored solution:

• The absorbed light relates to electronic transitions within the metal-ligand complex.
• The orange hue arises because the complex ion absorbs light predominantly in the blue region of the spectrum, reflecting and transmitting the complementary color—orange.

Understanding complex ions and their colored characteristics helps explain phenomena in various fields, such as analytical chemistry, where color changes signify reaction endpoints or compound identification. The model of color change in complex ions also extends to other ligands that show electronic activity similar to $I^{-}$ ions when forming complexes with metals.