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

Question

Mercuric iodide dissolves in excess of KI and forms (a) \(\mathrm{Hg}_{2} \mathrm{I}_{2}\) (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

Mercuric iodide in excess KI forms \(\mathrm{K}_2\left[\mathrm{HgI}_4\right]\).

Step 1: Understanding the Reaction

Mercuric iodide is denoted by the chemical formula \(\mathrm{HgI}_2\). When it is dissolved in an excess of potassium iodide \(\mathrm{KI}\), a reaction occurs involving the formation of complexes.

Step 2: Chemical Reaction Involved

The dissolution of \(\mathrm{HgI}_2\) in an excess of \(\mathrm{KI}\) involves a reaction that forms a complex ion. Specifically, the mercuric iodide reacts with iodide ions from \(\mathrm{KI}\) to form the complex \(\left[\mathrm{HgI}_4\right]^{2-}\).

Step 3: Combining with Potassium Ions

The newly formed complex ion \(\left[\mathrm{HgI}_4\right]^{2-}\) combines with potassium ions \(\mathrm{K}^+\) from \(\mathrm{KI}\) solution to form the compound \(\mathrm{K}_2\left[\mathrm{HgI}_4\right]\), a soluble compound.

Step 4: Conclusion of Reaction

Thus, when mercuric iodide is dissolved in excess \(\mathrm{KI}\), it forms the soluble complex compound \(\mathrm{K}_2\left[\mathrm{HgI}_4\right]\).

Key Concepts

Chemical Reactions

Chemical reactions are processes in which substances, known as reactants, are transformed into different substances, called products. This transformation involves breaking and forming chemical bonds. In the case of mercuric iodide (\(\mathrm{HgI}_2\)) dissolving in potassium iodide (\(\mathrm{KI}\)), the key reaction is the formation of a complex ion. A complex ion usually consists of a central metal atom bonded to surrounding anions or neutral molecules that have lone pairs.
In this reaction, additional \(\mathrm{I}^-\) ions from \(\mathrm{KI}\) react with \(\mathrm{HgI}_2\) to form a complex ion \(\left[\mathrm{HgI}_4\right]^{2-}\). The overall transformation can be represented in the following form:

• Reactants: \(\mathrm{HgI}_2\) + excess \(\mathrm{KI}\)
• Product: \(\mathrm{K}_2\left[\mathrm{HgI}_4\right]\)

This product represents a new chemical entity derived from the original reactants, showcasing the transformative power of chemical reactions.

Mercuric Iodide

Mercuric iodide is a chemical compound with the formula \(\mathrm{HgI}_2\). It is notable for its vibrant red or yellow color, depending on its crystalline form. This compound is of interest not only because of its distinct appearance but also due to its involvement in complex ion formation.
When mercuric iodide is placed in a solution with excess potassium iodide, it behaves as a reactant that undergoes a significant chemical transformation. The initially insoluble \(\mathrm{HgI}_2\) is converted into a soluble form through a reaction with iodide ions to create a complex ion, \(\left[\mathrm{HgI}_4\right]^{2-}\).

• Solubility: Normally low in water
• Reactivity: Can form complex ions with excess iodide
• Application: Used in pharmaceuticals and radiation detection

Understanding \(\mathrm{HgI}_2\)'s behavior is crucial for grasping complex ion chemistry and its applications in real-world scenarios.

Potassium Iodide

Potassium iodide, \(\mathrm{KI}\), is an ionic compound that readily dissolves in water, providing iodide ions \(\mathrm{I}^-\) essential for forming complex ions. It is not only integral to the chemical reaction with mercuric iodide but also widely used in various applications.
In the dissolution process of \(\mathrm{HgI}_2\), the iodide ions from \(\mathrm{KI}\) react with the mercuric iodide to form \(\left[\mathrm{HgI}_4\right]^{2-}\), a complex soluble in the solution. The presence of potassium ions \(\mathrm{K}^+\) ensures electrical neutrality, which leads to the final creation of the soluble compound \(\mathrm{K}_2\left[\mathrm{HgI}_4\right]\).

• Function: Source of iodide ions
• Applications: Food fortification, medical treatments
• Reactivity: Forms complexes in metal ion reactions

Recognizing \(\mathrm{KI}\)'s role and versatility enhances the understanding of both basic chemical reactions and specialized uses in industry.