Problem 126 - Chemistry A Molecular Approach

Question

Identify the Lewis acid and Lewis base from among the reactants in each equation. a. \(\mathrm{Ag}^{+}(a q)+2 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}(a q)\) b. \(\mathrm{AlBr}_{3}+\mathrm{NH}_{3} \rightleftharpoons \mathrm{H}_{3} \mathrm{NAlBr}_{3}\) c. \(\mathrm{F}^{-}(a q)+\mathrm{BF}_{3}(a q) \rightleftharpoons \mathrm{BF}_{4}^{-}(a q)\)

Step-by-Step Solution

Answer

For equation a, \(\mathrm{Ag}^{+}\) is the Lewis acid and \(\mathrm{NH}_{3}\) is the Lewis base. In equation b, \(\mathrm{AlBr}_{3}\) is the Lewis acid and \(\mathrm{NH}_{3}\) is the Lewis base. For equation c, \(\mathrm{BF}_{3}\) is the Lewis acid and \(\mathrm{F}^{-}\) is the Lewis base.

Step 1: Identify Lewis Acid and Base in Equation a

A Lewis acid is a species that can accept a pair of electrons, while a Lewis base is a species that can donate a pair of electrons. In equation a, \(\mathrm{Ag}^{+}\) is a Lewis acid because it accepts a pair of electrons from \(\mathrm{NH}_{3}\) to form \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}\). Therefore, \(\mathrm{NH}_{3}\) acts as a Lewis base as it donates a pair of electrons.

Step 2: Identify Lewis Acid and Base in Equation b

In equation b, \(\mathrm{AlBr}_{3}\) is the Lewis acid because it accepts an electron pair from \(\mathrm{NH}_{3}\) that results in the formation of \(\mathrm{H}_{3}\mathrm{NAlBr}_{3}\). The \(\mathrm{NH}_{3}\) molecule acts as a Lewis base as it donates an electron pair to \(\mathrm{AlBr}_{3}\).

Step 3: Identify Lewis Acid and Base in Equation c

In equation c, \(\mathrm{BF}_{3}\) is a Lewis acid as it accepts an electron pair from \(\mathrm{F}^{-}\) to form \(\mathrm{BF}_{4}^{-}\). The fluoride ion \(\mathrm{F}^{-}\) acts as a Lewis base since it donates an electron pair to \(\mathrm{BF}_{3}\).

Key Concepts

Electron Pair Acceptance

Understanding electron pair acceptance is crucial to identifying a Lewis acid during chemical reactions. A Lewis acid, by definition, is a chemical species that can accept an electron pair. This can be due to a lack of electrons in the valence shell of an atom, or an atom might have a positive charge which makes it electron-deficient and capable of accepting electron pairs.

For instance, in the chemical reactions provided, \(\mathrm{Ag}^{+}\), \(\mathrm{AlBr}_{3}\), and \(\mathrm{BF}_{3}\) each serve as a Lewis acid. These molecules or ions have available orbitals to accommodate electrons. \(\mathrm{Ag}^{+}\) accepts electron pairs from ammonia \(\mathrm{NH}_{3}\), \(\mathrm{AlBr}_{3}\) accepts an electron pair from \(\mathrm{NH}_{3}\), and \(\mathrm{BF}_{3}\) accepts an electron pair from the fluoride ion (\(\mathrm{F}^{-}\)). This electron pair acceptance is fundamental in advancing chemical processes.

Electron Pair Donation

Electron pair donation is the opposite concept and is key to understanding the role of a Lewis base. A Lewis base is a molecule or ion that donates a pair of electrons, often to a Lewis acid, forming a coordinate covalent bond. In other words, it's a species with available electrons that it can share to form new bonds.

In the given reactions, substances identified as Lewis bases are \(\mathrm{NH}_{3}\) and \(\mathrm{F}^{-}\). These bases donate electron pairs to their respective Lewis acids, which is characteristic of their role in chemical reactions. For instance, \(\mathrm{NH}_{3}\) is a common Lewis base because of its lone pair of electrons on nitrogen, which makes it adept at donating this pair to electron-deficient species like \(\mathrm{Ag}^{+}\) and \(\mathrm{AlBr}_{3}\). Similarly, the fluoride ion, \(\mathrm{F}^{-}\), donates its electrons to \(\mathrm{BF}_{3}\), proving its ability as a Lewis base.

Chemical Reactions

Chemical reactions involving Lewis acids and bases are central to the study of chemistry. These reactions typically result in the formation of a coordinate covalent bond, which occurs when one atom provides both electrons for the bond. This differs from a normal covalent bond, where each atom contributes one electron to the bond.

In the context of the provided chemical equations, the products, such as \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}\), \(\mathrm{H}_{3}\mathrm{NAlBr}_{3}\), and \(\mathrm{BF}_{4}^{-}\) demonstrate the creation of new coordinate covalent bonds. These compounds are the result of a Lewis base donating an electron pair to a Lewis acid. It's important to grasp that these reactions are pivotal in synthesis, catalysis, and even biological systems. When studying chemical reactions of this nature, identifying the Lewis acids and bases can provide insight into the reaction mechanisms and potential applications of the resulting compounds.