Question

For each of the following reactions, identify the Lewis acid and the Lewis base. (a) \(\mathrm{AlCl}_{3}+\mathrm{Cl}^{-} \longrightarrow \mathrm{AlCl}_{4}^{-}\) (b) \(2 \mathrm{NH}_{3}+\mathrm{Ag}^{+} \longrightarrow \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}\) (c) \(\mathrm{SO}_{2}+\mathrm{OH} \longrightarrow \mathrm{HSO}_{3}^{-}\) (d) \(6 \mathrm{H}_{2} \mathrm{O}+\mathrm{Cr}^{3+} \longrightarrow \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}{ }^{3+}\)

Short answer

(a) Lewis acid: \( \mathrm{AlCl}_{3} \), base: \( \mathrm{Cl}^{-} \); (b) acid: \( \mathrm{Ag}^{+} \), base: \( \mathrm{NH}_{3} \); (c) acid: \( \mathrm{SO}_{2} \), base: \( \mathrm{OH}^{-} \); (d) acid: \( \mathrm{Cr}^{3+} \), base: \( \mathrm{H}_{2} \mathrm{O} \).

Step-by-step solution

Understand Lewis Acid-Base Theory

A Lewis acid is a substance that can accept a pair of electrons, while a Lewis base is a substance that can donate a pair of electrons. Understanding this concept helps identify acids and bases in the given reactions.

Analyze Reaction (a)

The reaction is \( \mathrm{AlCl}_{3}+\mathrm{Cl}^{-} \longrightarrow \mathrm{AlCl}_{4}^{-} \). Here, \( \mathrm{AlCl}_{3} \) accepts an electron pair from \( \mathrm{Cl}^{-} \), so \( \mathrm{AlCl}_{3} \) is the Lewis acid and \( \mathrm{Cl}^{-} \) is the Lewis base.

Analyze Reaction (b)

The reaction is \( 2 \mathrm{NH}_{3}+\mathrm{Ag}^{+} \longrightarrow \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+} \). \( \mathrm{Ag}^{+} \) accepts electron pairs from two \( \mathrm{NH}_{3} \) molecules, making \( \mathrm{Ag}^{+} \) the Lewis acid, while \( \mathrm{NH}_{3} \) is the Lewis base.

Analyze Reaction (c)

The reaction is \( \mathrm{SO}_{2}+\mathrm{OH}^- \longrightarrow \mathrm{HSO}_{3}^{-} \). \( \mathrm{SO}_{2} \) accepts an electron pair from \( \mathrm{OH}^{-} \), making \( \mathrm{SO}_{2} \) the Lewis acid and \( \mathrm{OH}^{-} \) the Lewis base.

Analyze Reaction (d)

The reaction is \( 6 \mathrm{H}_{2} \mathrm{O}+\mathrm{Cr}^{3+} \longrightarrow \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+} \). \( \mathrm{Cr}^{3+} \) accepts electron pairs from six water molecules, so \( \mathrm{Cr}^{3+} \) is the Lewis acid and \( \mathrm{H}_{2} \mathrm{O} \) is the Lewis base.

Key concepts

electron pair donor

The term "electron pair donor" is essential in understanding the role of Lewis bases in chemical reactions. In the context of Lewis Acid-Base Theory, a Lewis base is defined as any substance that can donate an electron pair. This capability allows it to interact with Lewis acids, which are species that accept electron pairs. By donating an electron pair, the Lewis base forms a coordinate covalent bond with a Lewis acid.

Consider the reaction \( \mathrm{AlCl}_{3} + \mathrm{Cl}^{-} \longrightarrow \mathrm{AlCl}_{4}^{-} \). Here, \( \mathrm{Cl}^{-} \) acts as the electron pair donor. It gives up its electron pair to bond with \( \mathrm{AlCl}_{3} \), a Lewis acid. This donation is crucial in forming a stable product, \( \mathrm{AlCl}_{4}^{-} \).

Similarly, in the reaction \( 2 \mathrm{NH}_{3} + \mathrm{Ag}^{+} \longrightarrow \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+} \), the \( \mathrm{NH}_{3} \) molecule is the electron pair donor. It donates a pair of electrons to \( \mathrm{Ag}^{+} \), facilitating the formation of the complex ion \( \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+} \).

Understanding which species acts as the electron pair donor gives insight into the direction and type of chemical interaction occurring.

electron pair acceptor

In Lewis Acid-Base Theory, an electron pair acceptor is commonly referred to as a Lewis acid. This is any substance that can accept an electron pair from a Lewis base. Accepting an electron pair is central to a Lewis acid's ability to form coordinate covalent bonds, which are key to many reactions and structures.

In the reaction \( \mathrm{SO}_{2} + \mathrm{OH}^{-} \longrightarrow \mathrm{HSO}_{3}^{-} \), the molecule \( \mathrm{SO}_{2} \) behaves as an electron pair acceptor. By accepting an electron pair from \( \mathrm{OH}^{-} \), it assists in the transformation to \( \mathrm{HSO}_{3}^{-} \).

Another example is in the reaction \( 6 \mathrm{H}_{2} \mathrm{O} + \mathrm{Cr}^{3+} \longrightarrow \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{3+} \). Here, \( \mathrm{Cr}^{3+} \) serves as the electron pair acceptor. By incorporating six electron pairs from water molecules, it forms a hydrated complex. This ability to accept electron pairs facilitates the stabilization of certain ions and molecules.

Recognizing electron pair acceptors is fundamental in predicting the course and products of chemical reactions.

chemical reactions analysis

Analyzing chemical reactions through the lens of electron pair donation and acceptance provides a structured approach to understanding both the process and outcome of reactions. This method of analysis is vital in predicting not only the products but also the interactions and mechanisms involved.

Let's break down a typical reaction analysis: first, identify the Lewis base, or the electron pair donor. Then, determine the Lewis acid, which is the electron pair acceptor. This identification can often reveal the direction of the electron flow and indicates which species will lose or gain electron pairs.

To illustrate, consider the reaction \( 2 \mathrm{NH}_{3} + \mathrm{Ag}^{+} \longrightarrow \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+} \). By recognizing \( \mathrm{NH}_{3} \) as the Lewis base and \( \mathrm{Ag}^{+} \) as the Lewis acid, we can predict the formation of a complex ion. The reaction proceeds as \( \mathrm{NH}_{3} \) donates electron pairs to \( \mathrm{Ag}^{+} \), forming \( \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+} \).

Efficiently analyzing chemical reactions using these parameters enables chemists to forecast possible reaction pathways and synthesize desired products.