Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Mercuric iodide dissolves in excess of KI and forms (a) \(\mathrm{Hg}_{2} \mathrm{I}\) (b) \(\mathrm{Hg}\) (c) \(\mathrm{K}_{2}\left[\mathrm{HgI}_{4}\right]\) (d) \(\mathrm{K}\left[\mathrm{HgI}_{2}\right]\)

Step-by-Step Solution

Answer

(c) \( \mathrm{K}_2\left[\mathrm{HgI}_4\right] \)

Step 1: Understanding the Compounds

Mercuric iodide is \( \text{HgI}_2 \). When it reacts with potassium iodide \( \text{KI} \), it forms a complex salt. We need to determine what complex is formed given the excess of \( \text{KI} \).

Step 2: Reaction Mechanism

When \( \text{HgI}_2 \) reacts with \( \text{KI} \), the iodide ions \( \text{I}^- \) can form a complex with mercury(II). The key is that in excess \( \text{KI} \), further iodide ions will associate with \( \text{HgI}_2 \) to form a more complex ion \( \left[\text{HgI}_4\right]^{2-} \).

Step 3: Complex Formation

The complex \( \text{K}_2\left[\text{HgI}_4\right] \) is a result of the iodide ions surrounding the mercuric iodide. The \( \text{K}^+ \) ions from the excess \( \text{KI} \) will balance the charge of the \( \left[\text{HgI}_4\right]^{2-} \) complex ion.

Step 4: Final Answer

The correct answer is that mercuric iodide dissolves in excess \( \text{KI} \) to form \( \text{K}_2\left[\text{HgI}_4\right] \). Thus, the correct option is (c).

Key Concepts

Complex Ions

Complex ions are fascinating chemical species formed when simple ions and molecules come together in a solution. These formations involve coordination between a central metal ion and surrounding molecules or ions known as ligands. The metal ion serves as the core, which attracts and binds to the ligands through coordinate covalent bonds. This type of bonding occurs when the ligand donates a pair of electrons to the metal ion.

• Central Metal Ion: Typically a metal ion like mercury (Hg), acts as the centerpiece of the complex ion.
• Ligands: In the case of iodide complex formation, these are iodide ions (I^-).
• Coordinate Covalent Bond: Occurs when a ligand donates a pair of electrons to the metal ion.

These bonds form more intricate chemical species displaying unique properties such as solubility and stability differences, quite different from their individual components.

Iodide Complex Formation

Iodide complex formation is a specific type of coordination chemistry that involves the reaction between iodide ions and a metal ion. In this context, mercury(II) ions interact with iodine atoms to form a stable complex ion.

• Mercury Interaction: The mercury ion, typically in a +2 oxidation state, can bond with multiple iodide ions.
• Complex Species: The excess iodide ions around mercuric iodide transform it into a tetraiodomercurite(II) complex, represented as \(\text{[HgI}_4\text{]}^{2-} \).
• Stability Impact: The formation of the iodide complex enhances the solubility of mercuric iodide in the solution.

This process reduces the apparent concentration of free iodine ions, shifting the equilibrium and allowing more mercuric iodide to dissolve.

Chemical Reactions

In chemistry, reactions occur when two or more molecules interact and the atoms rearrange to form new substances. For understanding complex ions, consider the starting compounds and products that result from these intricate chemical processes.

• Reactants: Here, mercuric iodide \(\text{(HgI}_2\text{)} \) reacts with excess potassium iodide \(\text{(KI)}\).
• Products: The reaction produces a complex compound, \(\text{K}_2[\text{HgI}_4]\).
• Ionic Changes: Iodide ions from the potassium iodide combine with the mercury to create a fully coordinated complex ion.

This type of chemical reaction showcases the transformation and reformation of ions, leading to the stability of the complex ion compared to simplistic interactions.

Mercuric Iodide

Mercuric iodide (\(\text{HgI}_2 \)) is a binary compound comprised of mercury and iodine. In its normal state, it appears as a red or yellow solid under standard conditions, showcasing its unique properties.

• Appearance: It can change from a stable red form to a yellow form upon heating.
• Chemical Composition: Consists of 2 iodine atoms bonded directly to a mercury atom.
• Reactivity: When in the presence of additional iodide ions, it can transform, showcasing its ability to form complex ions.

Mercuric iodide's reactivity, particularly with iodide ions, makes it a fundamental species for studying complex formation in coordination chemistry.

Potassium Iodide

Potassium iodide \(\text{(KI)}\) is an inorganic salt that readily dissociates in solution. It serves as a rich source of iodide ions which play a critical role in forming complex ions with metals like mercury.

• Solubility: Potassium iodide is highly soluble in water, providing a significant supply of iodide ions for reactions.
• Role in Complex Formation: It helps in forming the \(\text{[HgI}_4\text{]}^{2-}\) complex ion by providing the extra iodide ions needed.
• Buffering Action: The presence of KI in excess helps in stabilizing the complex ion in solution.

Overall, potassium iodide serves a dual purpose by enhancing the reaction environment and actively participating in complex ion formation.