Problem 40 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Gradual addition of KI solution to \(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)\) solution initially produces a dark brown precipitate which dissolves in excess of KI to give a clear yellow solution. Write chemical equations for the above reactions. [1996-2 Marks]

Step-by-Step Solution

Answer

1. \(\text{Bi(NO}_3\text{)}_3 + 3\text{KI} \rightarrow \text{BiI}_3 + 3\text{KNO}_3\). 2. \(\text{BiI}_3 + \text{KI} \rightarrow \text{KBiI}_4\).

Step 1: Identifying the Initial Reaction

When potassium iodide (KI) is added to bismuth nitrate, a dark brown precipitate of bismuth(III) iodide (BiIe3) forms. This reaction can be expressed as follows:\[\text{Bi(NO}_3\text{)}_3 + 3\text{KI} \rightarrow \text{BiI}_3 + 3\text{KNO}_3\]

Step 2: Dissolution in Excess KI

Upon adding excess KI, the previously formed bismuth(III) iodide (BiIe3) dissolves to form a complex ion resulting in a clear yellow solution. The reaction can be represented by:\[\text{BiI}_3 + \text{KI} \rightarrow \text{KBiI}_4\]

Key Concepts

Precipitation Reaction

When we talk about precipitation reactions, we're diving into the process where two solutions, upon mixing, form an insoluble solid known as a precipitate. In the given example, when potassium iodide (KI) is added to bismuth nitrate oindent(Bi(NO}_{3})_{3} , a reaction occurs that leads to the formation of a new compound: bismuth(III) iodide (BiI_{3}) , which is insoluble in water. This causes the appearance of a dark brown precipitate. Precipitation reactions are well-known for:

• Quickly forming a solid from a liquid solution.
• Often changing the color and appearance of the solution.
• Occurring due to the low solubility of the new compound formed.

These reactions are quite common in labs and are essential for processes like purification and separation of compounds.

Complex Ion Formation

After our environment is filled with the striking dark precipitate of bismuth(III) iodide, something interesting happens when more KI is introduced. The bismuth(III) iodide doesn't just sit there—it transforms! It dissolves again through a process called complex ion formation. In this reaction, excess potassium iodide causes the formation of a new species: (KBiI_{4}) . This complex ion formation results from the additional iodide ions surrounding the bismuth ion, forming a stable complex that dissolves in the solution, making it appear clear and yellow. Key characteristics of complex ion formation include:

• Transforming an insoluble compound into a soluble form through additional reactants.
• Stabilization of ions by forming ionic complexes.
• Often altering the color of the solution.

This behavior is crucial in various fields, including biochemistry and materials science, where molecular interactions play a pivotal role.

Bismuth(III) Iodide Dissolution

The dissolution of bismuth(III) iodide in excess KI is a fascinating combination of chemistry concepts resulting in the disappearance of a solid precipitate as a complex ion forms. Initially insoluble in water, bismuth(III) iodide (BiI_{3}) has a hard time staying dissolved in normal conditions. However, when there's an abundance of potassium iodide (KI), the situation changes. Here, the bismuth(III) iodide interacts with the excess iodide ions to form a complex ion, specifically KBiI_{4} , thanks to which we see the brown precipitate completely dissolve, leaving behind a clear yellow solution. Understanding this dissolution process is vital because it demonstrates:

• How precipitates can be manipulated through chemical interactions.
• The ability to dissolve ionic compounds with tailored conditions.
• Concepts of solubility and stability in chemical solutions.

Such knowledge is essential not only on a theoretical level but also in practical applications like analytical chemistry and industrial processes.