Problem 63 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In the reaction \(\mathrm{I}^{-}+\mathrm{I}_{2} \rightarrow \mathrm{I}_{3}^{-}\), the Lewis acid is \(\ldots \ldots \ldots .\)

Step-by-Step Solution

Answer

The Lewis acid is \( \mathrm{I}_{2} \).

Step 1: Identify the reactants and products

In the given chemical reaction, the reactants are iodide ion \( \mathrm{I}^{-} \) and iodine \( \mathrm{I}_{2} \). The product is the triiodide ion \( \mathrm{I}_{3}^{-} \).

Step 2: Understand the Lewis acid-base concept

A Lewis acid is a substance that can accept a pair of electrons. Conversely, a Lewis base is a substance that can donate a pair of electrons.

Step 3: Determine electron pair donor and acceptor

In this reaction, \( \mathrm{I}^{-} \) acts as an electron pair donor, making it a Lewis base. The \( \mathrm{I}_{2} \), which is neutral and capable of accepting an electron pair, acts as the Lewis acid.

Step 4: Conclude which is the Lewis acid

Since \( \mathrm{I}_{2} \) accepts an electron pair from \( \mathrm{I}^{-} \) to form \( \mathrm{I}_{3}^{-} \), \( \mathrm{I}_{2} \) is the Lewis acid in this reaction.

Key Concepts

Electron Pair Donor

In chemistry, the term "electron pair donor" is often used to describe a Lewis base. A Lewis base donates an electron pair to a Lewis acid. This is a central concept in the Lewis acid-base theory, which is broader than the Brønsted-Lowry theory of acids and bases. In the given chemical reaction, the iodide ion \( \mathrm{I}^{-} \) acts as an electron pair donor.

• Iodide ion \( \mathrm{I}^{-} \) has extra electrons and is negatively charged.
• By donating an electron pair, it interacts with another species capable of accepting these electrons.
• The resulting interaction allows the formation of a new molecule or ion.

It is important to understand that in the case of \( \mathrm{I}^{-} \), despite being simply a negatively charged ion, it plays a key role in forming the product \( \mathrm{I}_{3}^{-} \). The ability to donate an electron pair makes \( \mathrm{I}^{-} \) a vital participant in this Lewis acid-base reaction. Remember, identifying an electron pair donor involves looking at the species donating electrons in a chemical interaction.

Electron Pair Acceptor

The concept of an "electron pair acceptor" is crucial in identifying a Lewis acid in a reaction. According to the Lewis acid-base theory, a Lewis acid is a species that accepts an electron pair. In our focusing reaction, \( \mathrm{I}_{2} \), or diatomic iodine, acts as an electron pair acceptor.

• \( \mathrm{I}_{2} \) is neutral naturally, with no electrical charge.
• Despite its neutrality, \( \mathrm{I}_{2} \) can accept additional electrons.
• The acceptance of electrons by \( \mathrm{I}_{2} \) facilitates a bond formation with \( \mathrm{I}^{-} \).

When \( \mathrm{I}_{2} \) acts as an electron pair acceptor, it transforms the iodine molecule into a more complex form; part of the reason it functions as a Lewis acid. Identifying an electron pair acceptor (Lewis acid) involves looking at which molecule or ion takes up extra electrons, showing its capacity to engage in forming more substantial molecular structures.

Triiodide Ion Formation

Triiodide ion formation is a fascinating process that results from the interaction of iodine with the iodide ion under the framework of Lewis acid-base theory. When the iodide ion \( \mathrm{I}^{-} \) donates an electron pair to molecular iodine \( \mathrm{I}_{2} \), the result is the formation of the triiodide ion \( \mathrm{I}_{3}^{-} \).

• This reaction is an example of a Lewis acid-base reaction in action.
• The \( \mathrm{I}^{-} \) ion plays the role of the Lewis base, supplying the electron pair.
• The \( \mathrm{I}_{2} \) molecule functions as the Lewis acid, accepting this electron pair.
• Through the donation and acceptance of electrons, \( \mathrm{I}_{3}^{-} \) ions are formed, which possess a linear geometry.

Understanding triiodide ion formation requires recognizing the initial roles of these substances in electron transfer. Such reactions illustrate the richness of chemical interactions that go beyond traditional concepts, like merely acids donating protons. The example of \( \mathrm{I}_{3}^{-} \) showcases how influential electron interactions can be within molecular and ionic formation.