Problem 72 - General Chemistry: Principles and Modern Applications

Question

In the following reactions indicate which is the Lewis acid and which is the Lewis base: (a) \(\mathrm{SOI}_{2}+\mathrm{BaSO}_{3} \longrightarrow \mathrm{Ba}^{2+}+2 \mathrm{I}^{-}+2 \mathrm{SO}_{2}\) (b) \(\mathrm{HgCl}_{3}^{-}+\mathrm{Cl}^{-} \longrightarrow \mathrm{HgCl}_{4}^{2}\)

Step-by-Step Solution

Answer

In reaction (a) the Lewis acid is BaSO3 and the Lewis base is SOI2. In reaction (b) the Lewis acid is HgCl3- and the Lewis base is Cl-.

Step 1: Identifying the Lewis Acid and Base in Reaction (a)

In reaction (a), SOI2 is interacting with BaSO3 to produce Ba2+, I-, and SO2. The Lewis base, SOI2, donates an iodine ion which accepts an electron pair from BaSO3, the Lewis acid. Thus, BaSO3 is the Lewis acid, because it receives the electron pair, and SOI2 is the Lewis base, because it donates the electron pair to BaSO3.

Step 2: Identifying the Lewis Acid and Base in Reaction (b)

In reaction (b), HgCl3- is interacting with Cl- to produce HgCl42-. In this reaction, Cl- (the Lewis base) donates its electron pair to HgCl3- (the Lewis acid) which accepts the electron pair. Thus, the Lewis base in this reaction is Cl-, while the Lewis acid is HgCl3-. The Cl- donates an electron pair and the HgCl3- accepts it.

Key Concepts

Lewis Acids

A Lewis Acid is a chemical species that can accept an electron pair. In the context of chemical reactions, Lewis acids usually contain an atom that is electron-deficient. This atom has an empty orbital that can accept an electron pair from a Lewis base.
In reaction to the above exercises, compounds like \(\mathrm{BaSO}_3\) and \(\mathrm{HgCl}_3^-\) act as Lewis acids. Why? Because they are able to accept electron pairs from other species. They make bonds by gaining electrons, which results in becoming a more stable configuration.

• They are typically electron-deficient.
• They can have a positive charge (like \(\mathrm{BaSO}_3\)).
• They can also be a neutral molecule with an empty orbital.

Knowing how they accept electron pairs helps us distinguish them from Lewis bases in reactions.

Lewis Bases

On the opposite side, we have Lewis Bases, which behave in an entirely different manner. A Lewis Base is a chemical species with the ability to donate an electron pair. It is equipped with a lone pair of electrons that it is willing to share with a suitable acceptor—a Lewis Acid.
In the exercises provided, the roles of Lewis bases are played by \(\mathrm{SOI}_2\) and \(\mathrm{Cl}^-\). These ions donate their electron pairs to the Lewis acids, allowing new bonds to form.

• They often have a negative charge or lone electron pairs.
• They can stabilize by donating electrons.
• Substances with excess electron pairs serve well as Lewis Bases.

Understanding how these bases donate electrons can significantly shift your understanding of chemical reactions.

Electron Pair Donor

In chemistry, an Electron Pair Donor is the functional aspect of a Lewis Base. This term specifically underlines the process where the species donates its electron pair to form a coordinate bond with the Lewis Acid. The donation facilitates the Lewis acid-base reaction, forming a new compound or complex.
In our reactions:

• \(\mathrm{SOI}_2\) acts as an electron pair donor, providing electrons to \(\mathrm{BaSO}_3\).
• \(\mathrm{Cl}^-\) donates its pair to \(\mathrm{HgCl}_3^-\) to form \(\mathrm{HgCl}_4^{2-}\).

Thus, when we see electron pair donation, it can lead to new molecules being formed, letting us understand the reaction's "giver" role, crucial for chemical bonding.

Electron Pair Acceptor

Complementary to the electron pair donor, an Electron Pair Acceptor describes the receiving role of a Lewis Acid. The molecule or ion with an empty orbital acts as the sink for these donated electrons, facilitating chemical bonds and stabilizing itself.
In the given reactions:

• \(\mathrm{BaSO}_3\) accepts an electron pair from \(\mathrm{SOI}_2\).
• \(\mathrm{HgCl}_3^-\) acts as an electron pair acceptor from \(\mathrm{Cl}^-\).

Understanding the role of electron pair acceptors in bonding reveals the "receiving" end of the process. It allows us to see how these molecules or ions stabilize by forming new bonds with electron-rich species.